[GitHub] [tinkerpop] divijvaidya commented on a change in pull request #1414: TINKERPOP-2245 Added UnifiedChannelizer

[GitBox <gi...@apache.org>]

divijvaidya commented on a change in pull request #1414:
URL: https://github.com/apache/tinkerpop/pull/1414#discussion_r608578909



##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/WsAndHttpChannelizerHandler.java
##########
@@ -66,11 +66,11 @@ public void channelRead(final ChannelHandlerContext ctx, final Object obj) {
                 pipeline.remove(PIPELINE_REQUEST_HANDLER);
                 final ChannelHandler authenticator = pipeline.get(PIPELINE_AUTHENTICATOR);
                 pipeline.remove(PIPELINE_AUTHENTICATOR);
-                pipeline.addAfter(PIPELINE_HTTP_RESPONSE_ENCODER, PIPELINE_AUTHENTICATOR, authenticator);
+                pipeline.addAfter(PIPELINE_HTTP_AGGREGATOR, PIPELINE_AUTHENTICATOR, authenticator);

Review comment:
       please add documentation with how pipeline looks before and after execution of this function.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();
+    private ScheduledFuture<?> requestCancelFuture;
+    private Bindings bindings;
+    private final AtomicBoolean ending = new AtomicBoolean(false);
+    private final ScheduledExecutorService scheduledExecutorService;
+    private final GremlinScriptEngineManager scriptEngineManager;
+
+    MultiRexster(final Context gremlinContext, final String sessionId,
+                 final ConcurrentMap<String, Rexster> sessions) {
+        super(gremlinContext, sessionId, false, sessions);
+
+        // using a global function cache is cheaper than creating a new on per session especially if you have to
+        // create a lot of sessions. it will generate a ton of throw-away objects. mostly keeping the option open
+        // to not use it to preserve the ability to use the old functionality if wanted or if there is some specific
+        // use case with sessions that needs it. if we wanted this could eventually become a per-request option
+        // so that the client could control it as necessary and get scriptengine isolation if they need it.
+        if (gremlinContext.getSettings().useCommonEngineForSessions)
+            scriptEngineManager = gremlinContext.getGremlinExecutor().getScriptEngineManager();
+        else
+            scriptEngineManager = initializeGremlinExecutor(gremlinContext).getScriptEngineManager();
+
+        scheduledExecutorService = gremlinContext.getScheduledExecutorService();
+        addTask(gremlinContext);
+    }
+
+    @Override
+    public GremlinScriptEngine getScriptEngine(final Context gremlinContext, final String language) {
+        return scriptEngineManager.getEngineByName(language);
+    }
+
+    @Override
+    public boolean acceptingTasks() {

Review comment:
       nit
   
   s/acceptingTasks/isAcceptingTasks

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();
+    private ScheduledFuture<?> requestCancelFuture;
+    private Bindings bindings;
+    private final AtomicBoolean ending = new AtomicBoolean(false);
+    private final ScheduledExecutorService scheduledExecutorService;
+    private final GremlinScriptEngineManager scriptEngineManager;
+
+    MultiRexster(final Context gremlinContext, final String sessionId,
+                 final ConcurrentMap<String, Rexster> sessions) {
+        super(gremlinContext, sessionId, false, sessions);
+
+        // using a global function cache is cheaper than creating a new on per session especially if you have to
+        // create a lot of sessions. it will generate a ton of throw-away objects. mostly keeping the option open
+        // to not use it to preserve the ability to use the old functionality if wanted or if there is some specific
+        // use case with sessions that needs it. if we wanted this could eventually become a per-request option
+        // so that the client could control it as necessary and get scriptengine isolation if they need it.
+        if (gremlinContext.getSettings().useCommonEngineForSessions)
+            scriptEngineManager = gremlinContext.getGremlinExecutor().getScriptEngineManager();
+        else
+            scriptEngineManager = initializeGremlinExecutor(gremlinContext).getScriptEngineManager();
+
+        scheduledExecutorService = gremlinContext.getScheduledExecutorService();
+        addTask(gremlinContext);
+    }
+
+    @Override
+    public GremlinScriptEngine getScriptEngine(final Context gremlinContext, final String language) {
+        return scriptEngineManager.getEngineByName(language);
+    }
+
+    @Override
+    public boolean acceptingTasks() {
+        return !ending.get();
+    }
+
+    @Override
+    public void addTask(final Context gremlinContext) {
+        if (acceptingTasks())

Review comment:
       else? throw a meaningful error to the client.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {
+            if (closeReason.compareAndSet(null, CloseReason.CHANNEL_CLOSED)) {
+                // cancel session worker or it will keep waiting for items to appear in the session queue
+                cancel(true);
+                close();
+            }
+        });
+        this.sessions = sessions;
+        this.graphManager = gremlinContext.getGraphManager();
+    }
+
+    protected synchronized void cancel(final boolean mayInterruptIfRunning) {
+        final FutureTask<?> sf = (FutureTask) sessionFuture.get();
+        if (sf != null && !sf.isDone()) {
+            sf.cancel(mayInterruptIfRunning);
+        }
+    }
+
+    public boolean isTransactionManaged() {
+        return transactionManaged;
+    }
+
+    public String getSessionId() {
+        return sessionId;
+    }
+
+    public boolean isBoundTo(final Channel channel) {
+        return channel == initialChannel;
+    }
+
+    public long getActualTimeoutLengthWhenClosed() {
+        return actualTimeoutLengthWhenClosed;
+    }
+
+    public Optional<CloseReason> getCloseReason() {
+        return Optional.ofNullable(closeReason.get());
+    }
+
+    public GremlinScriptEngine getScriptEngine(final Context context, final String language) {
+        return context.getGremlinExecutor().getScriptEngineManager().getEngineByName(language);
+    }
+
+    public void setSessionThread(final Thread runner) {
+        this.sessionThread = runner;
+    }
+
+    @Override
+    public void setSessionCancelFuture(final ScheduledFuture<?> f) {
+        if (!sessionCancelFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session cancellation future is already set");
+    }
+
+    @Override
+    public void setSessionFuture(final Future<?> f) {
+        if (!sessionFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session future is already set");
+    }
+
+    @Override
+    public synchronized void triggerTimeout(final long timeout, final boolean causedBySession) {
+        // triggering timeout triggers the stop of the Rexster Runnable which will end in close()
+        // for final cleanup
+        final Future<?> f = sessionFuture.get();
+        if (f != null && !f.isDone()) {
+            if (closeReason.compareAndSet(null, causedBySession ? CloseReason.SESSION_TIMEOUT : CloseReason.REQUEST_TIMEOUT)) {
+                actualTimeoutLengthWhenClosed = timeout;
+
+                // if caused by a session timeout for a session OR if it is a request timeout for a sessionless
+                // request then we can just straight cancel() the Rexster instance
+                if (causedBySession || !sessionIdOnRequest)
+                    cancel(true);
+                else
+                    sessionThread.interrupt();
+            }
+        }
+    }
+
+    protected void process(final Context gremlinContext) throws RexsterException {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final Object gremlinToExecute = args.get(Tokens.ARGS_GREMLIN);
+
+        // for strict transactions track the aliases used so that we can commit them and only them on close()
+        if (gremlinContext.getSettings().strictTransactionManagement)
+            msg.optionalArgs(Tokens.ARGS_ALIASES).ifPresent(m -> aliasesUsedByRexster.addAll(((Map<String,String>) m).values()));
+
+        try {
+            // itty is optional as Bytecode could be a "graph operation" rather than a Traversal. graph operations
+            // don't need to be iterated and handle their own lifecycle
+            final Optional<Iterator<?>> itty = gremlinToExecute instanceof Bytecode ?
+                    fromBytecode(gremlinContext, (Bytecode) gremlinToExecute) :
+                    Optional.of(fromScript(gremlinContext, (String) gremlinToExecute));
+
+            processAuditLog(gremlinContext.getSettings(), gremlinContext.getChannelHandlerContext(), gremlinToExecute);
+
+            if (itty.isPresent())
+                handleIterator(gremlinContext, itty.get());
+        } catch (Exception ex) {
+            handleException(gremlinContext, ex);
+        }
+    }
+
+    protected void handleException(final Context gremlinContext, final Throwable t) throws RexsterException {
+        if (t instanceof RexsterException) throw (RexsterException) t;
+
+        final Optional<Throwable> possibleTemporaryException = determineIfTemporaryException(t);
+        if (possibleTemporaryException.isPresent()) {
+            final Throwable temporaryException = possibleTemporaryException.get();
+            throw new RexsterException(temporaryException.getMessage(), t,
+                    ResponseMessage.build(gremlinContext.getRequestMessage())
+                            .code(ResponseStatusCode.SERVER_ERROR_TEMPORARY)
+                            .statusMessage(temporaryException.getMessage())
+                            .statusAttributeException(temporaryException).create());
+        }
+
+        final Throwable root = ExceptionUtils.getRootCause(t);
+
+        if (root instanceof TimedInterruptTimeoutException) {
+            // occurs when the TimedInterruptCustomizerProvider is in play
+            final String msg = String.format("A timeout occurred within the script during evaluation of [%s] - consider increasing the limit given to TimedInterruptCustomizerProvider",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage("Timeout during script evaluation triggered by TimedInterruptCustomizerProvider")
+                    .create());
+        }
+
+        if (root instanceof TimeoutException) {
+            final String errorMessage = String.format("Script evaluation exceeded the configured threshold for request [%s]",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage(t.getMessage())
+                    .create());
+        }
+
+        if (root instanceof InterruptedException ||
+                root instanceof TraversalInterruptedException ||
+                root instanceof InterruptedIOException) {
+            String msg = "Processing interrupted but the reason why was not known";
+            switch (closeReason.get()) {
+                case CHANNEL_CLOSED:
+                    msg = "Processing interrupted because the channel was closed";
+                    break;
+                case SESSION_TIMEOUT:
+                    msg = String.format("Session closed - %s - sessionLifetimeTimeout of %s ms exceeded", sessionId, actualTimeoutLengthWhenClosed);
+                    break;
+                case REQUEST_TIMEOUT:
+                    msg = String.format("Evaluation exceeded timeout threshold of %s ms", actualTimeoutLengthWhenClosed);
+                    break;
+            }
+            final ResponseStatusCode code = closeReason.get() == CloseReason.SESSION_TIMEOUT || closeReason.get() == CloseReason.REQUEST_TIMEOUT ?
+                    ResponseStatusCode.SERVER_ERROR_TIMEOUT : ResponseStatusCode.SERVER_ERROR;
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(code)
+                    .statusMessage(msg).create());
+        }
+
+        if (root instanceof MultipleCompilationErrorsException && root.getMessage().contains("Method too large") &&
+                ((MultipleCompilationErrorsException) root).getErrorCollector().getErrorCount() == 1) {
+            final String errorMessage = String.format("The Gremlin statement that was submitted exceeds the maximum compilation size allowed by the JVM, please split it into multiple smaller statements - %s", trimMessage(gremlinContext.getRequestMessage()));
+            logger.warn(errorMessage);
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(errorMessage)
+                    .statusAttributeException(root).create());
+        }
+
+        // GroovyRuntimeException will hit a pretty wide range of eval type errors, like MissingPropertyException,
+        // CompilationFailedException, MissingMethodException, etc. If more specific handling is required then
+        // try to catch it earlier above.
+        if (root instanceof GroovyRuntimeException ||
+                root instanceof VerificationException ||
+                root instanceof ScriptException) {
+            throw new RexsterException(root.getMessage(), root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(root.getMessage())
+                    .statusAttributeException(root).create());
+        }
+
+        throw new RexsterException(root.getClass().getSimpleName() + ": " + root.getMessage(), root,
+                ResponseMessage.build(gremlinContext.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusAttributeException(root)
+                        .statusMessage(root.getMessage()).create());
+    }
+
+    /**
+     * Used to decrease the size of a Gremlin script that triggered a "method too large" exception so that it
+     * doesn't log a massive text string nor return a large error message.
+     */
+    private RequestMessage trimMessage(final RequestMessage msg) {
+        final RequestMessage trimmedMsg = RequestMessage.from(msg).create();
+        if (trimmedMsg.getArgs().containsKey(Tokens.ARGS_GREMLIN))
+            trimmedMsg.getArgs().put(Tokens.ARGS_GREMLIN, trimmedMsg.getArgs().get(Tokens.ARGS_GREMLIN).toString().substring(0, 1021) + "...");
+
+        return trimmedMsg;
+    }
+
+    /**
+     * Check if any exception in the chain is TemporaryException then we should respond with the right error code so
+     * that the client knows to retry.
+     */
+    protected Optional<Throwable> determineIfTemporaryException(final Throwable ex) {
+        return Stream.of(ExceptionUtils.getThrowables(ex)).
+                filter(i -> i instanceof TemporaryException).findFirst();
+    }
+
+    @Override
+    public synchronized void close() {
+        // already closing/closed
+        if (!sessions.containsKey(sessionId)) return;
+
+        sessions.remove(sessionId);
+
+        if (sessionCancelFuture.get() != null) {
+            final ScheduledFuture<?> f = sessionCancelFuture.get();
+            if (!f.isDone()) f.cancel(true);
+        }
+    }
+
+    protected Iterator<?> fromScript(final Context gremlinContext, final String script) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final String language = args.containsKey(Tokens.ARGS_LANGUAGE) ? (String) args.get(Tokens.ARGS_LANGUAGE) : "gremlin-groovy";
+        return IteratorUtils.asIterator(getScriptEngine(gremlinContext, language).eval(
+                script, mergeBindingsFromRequest(gremlinContext, getWorkerBindings())));
+    }
+
+    protected Optional<Iterator<?>> fromBytecode(final Context gremlinContext, final Bytecode bytecode) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+
+        final Traversal.Admin<?, ?> traversal;
+        final Map<String, String> aliases = (Map<String, String>) msg.optionalArgs(Tokens.ARGS_ALIASES).get();
+        final GraphManager graphManager = gremlinContext.getGraphManager();
+        final String traversalSourceName = aliases.entrySet().iterator().next().getValue();
+        final TraversalSource g = graphManager.getTraversalSource(traversalSourceName);
+
+        // handle bytecode based graph operations like commit/rollback commands
+        if (BytecodeHelper.isGraphOperation(bytecode)) {
+            handleGraphOperation(gremlinContext, bytecode, g.getGraph());
+            return Optional.empty();
+        } else {
+
+            final Optional<String> lambdaLanguage = BytecodeHelper.getLambdaLanguage(bytecode);
+            if (!lambdaLanguage.isPresent())
+                traversal = JavaTranslator.of(g).translate(bytecode);
+            else {
+                final SimpleBindings bindings = new SimpleBindings();
+                bindings.put(traversalSourceName, g);
+                traversal = gremlinContext.getGremlinExecutor().getScriptEngineManager().getEngineByName(lambdaLanguage.get()).eval(bytecode, bindings, traversalSourceName);
+            }
+
+            // compile the traversal - without it getEndStep() has nothing in it
+            traversal.applyStrategies();
+
+            return Optional.of(new TraverserIterator(traversal));
+        }
+    }
+
+    protected Bindings getWorkerBindings() throws RexsterException {
+        return new SimpleBindings(graphManager.getAsBindings());
+    }
+
+    protected Bindings mergeBindingsFromRequest(final Context gremlinContext, final Bindings bindings) throws RexsterException {

Review comment:
       are we still supporting bindings in 3.4.x series? I thought we removed them. No?

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {
+            if (closeReason.compareAndSet(null, CloseReason.CHANNEL_CLOSED)) {
+                // cancel session worker or it will keep waiting for items to appear in the session queue
+                cancel(true);
+                close();
+            }
+        });
+        this.sessions = sessions;
+        this.graphManager = gremlinContext.getGraphManager();
+    }
+
+    protected synchronized void cancel(final boolean mayInterruptIfRunning) {
+        final FutureTask<?> sf = (FutureTask) sessionFuture.get();
+        if (sf != null && !sf.isDone()) {
+            sf.cancel(mayInterruptIfRunning);
+        }
+    }
+
+    public boolean isTransactionManaged() {
+        return transactionManaged;
+    }
+
+    public String getSessionId() {
+        return sessionId;
+    }
+
+    public boolean isBoundTo(final Channel channel) {
+        return channel == initialChannel;
+    }
+
+    public long getActualTimeoutLengthWhenClosed() {
+        return actualTimeoutLengthWhenClosed;
+    }
+
+    public Optional<CloseReason> getCloseReason() {
+        return Optional.ofNullable(closeReason.get());
+    }
+
+    public GremlinScriptEngine getScriptEngine(final Context context, final String language) {
+        return context.getGremlinExecutor().getScriptEngineManager().getEngineByName(language);
+    }
+
+    public void setSessionThread(final Thread runner) {
+        this.sessionThread = runner;
+    }
+
+    @Override
+    public void setSessionCancelFuture(final ScheduledFuture<?> f) {
+        if (!sessionCancelFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session cancellation future is already set");
+    }
+
+    @Override
+    public void setSessionFuture(final Future<?> f) {
+        if (!sessionFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session future is already set");
+    }
+
+    @Override
+    public synchronized void triggerTimeout(final long timeout, final boolean causedBySession) {
+        // triggering timeout triggers the stop of the Rexster Runnable which will end in close()
+        // for final cleanup
+        final Future<?> f = sessionFuture.get();
+        if (f != null && !f.isDone()) {
+            if (closeReason.compareAndSet(null, causedBySession ? CloseReason.SESSION_TIMEOUT : CloseReason.REQUEST_TIMEOUT)) {
+                actualTimeoutLengthWhenClosed = timeout;
+
+                // if caused by a session timeout for a session OR if it is a request timeout for a sessionless
+                // request then we can just straight cancel() the Rexster instance
+                if (causedBySession || !sessionIdOnRequest)
+                    cancel(true);
+                else
+                    sessionThread.interrupt();
+            }
+        }
+    }
+
+    protected void process(final Context gremlinContext) throws RexsterException {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final Object gremlinToExecute = args.get(Tokens.ARGS_GREMLIN);
+
+        // for strict transactions track the aliases used so that we can commit them and only them on close()
+        if (gremlinContext.getSettings().strictTransactionManagement)
+            msg.optionalArgs(Tokens.ARGS_ALIASES).ifPresent(m -> aliasesUsedByRexster.addAll(((Map<String,String>) m).values()));
+
+        try {
+            // itty is optional as Bytecode could be a "graph operation" rather than a Traversal. graph operations
+            // don't need to be iterated and handle their own lifecycle
+            final Optional<Iterator<?>> itty = gremlinToExecute instanceof Bytecode ?
+                    fromBytecode(gremlinContext, (Bytecode) gremlinToExecute) :
+                    Optional.of(fromScript(gremlinContext, (String) gremlinToExecute));
+
+            processAuditLog(gremlinContext.getSettings(), gremlinContext.getChannelHandlerContext(), gremlinToExecute);
+
+            if (itty.isPresent())
+                handleIterator(gremlinContext, itty.get());
+        } catch (Exception ex) {
+            handleException(gremlinContext, ex);
+        }
+    }
+
+    protected void handleException(final Context gremlinContext, final Throwable t) throws RexsterException {
+        if (t instanceof RexsterException) throw (RexsterException) t;
+
+        final Optional<Throwable> possibleTemporaryException = determineIfTemporaryException(t);
+        if (possibleTemporaryException.isPresent()) {
+            final Throwable temporaryException = possibleTemporaryException.get();
+            throw new RexsterException(temporaryException.getMessage(), t,
+                    ResponseMessage.build(gremlinContext.getRequestMessage())
+                            .code(ResponseStatusCode.SERVER_ERROR_TEMPORARY)
+                            .statusMessage(temporaryException.getMessage())
+                            .statusAttributeException(temporaryException).create());
+        }
+
+        final Throwable root = ExceptionUtils.getRootCause(t);
+
+        if (root instanceof TimedInterruptTimeoutException) {
+            // occurs when the TimedInterruptCustomizerProvider is in play
+            final String msg = String.format("A timeout occurred within the script during evaluation of [%s] - consider increasing the limit given to TimedInterruptCustomizerProvider",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage("Timeout during script evaluation triggered by TimedInterruptCustomizerProvider")
+                    .create());
+        }
+
+        if (root instanceof TimeoutException) {
+            final String errorMessage = String.format("Script evaluation exceeded the configured threshold for request [%s]",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage(t.getMessage())
+                    .create());
+        }
+
+        if (root instanceof InterruptedException ||
+                root instanceof TraversalInterruptedException ||
+                root instanceof InterruptedIOException) {
+            String msg = "Processing interrupted but the reason why was not known";
+            switch (closeReason.get()) {
+                case CHANNEL_CLOSED:
+                    msg = "Processing interrupted because the channel was closed";
+                    break;
+                case SESSION_TIMEOUT:
+                    msg = String.format("Session closed - %s - sessionLifetimeTimeout of %s ms exceeded", sessionId, actualTimeoutLengthWhenClosed);
+                    break;
+                case REQUEST_TIMEOUT:
+                    msg = String.format("Evaluation exceeded timeout threshold of %s ms", actualTimeoutLengthWhenClosed);
+                    break;
+            }
+            final ResponseStatusCode code = closeReason.get() == CloseReason.SESSION_TIMEOUT || closeReason.get() == CloseReason.REQUEST_TIMEOUT ?
+                    ResponseStatusCode.SERVER_ERROR_TIMEOUT : ResponseStatusCode.SERVER_ERROR;
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(code)
+                    .statusMessage(msg).create());
+        }
+
+        if (root instanceof MultipleCompilationErrorsException && root.getMessage().contains("Method too large") &&
+                ((MultipleCompilationErrorsException) root).getErrorCollector().getErrorCount() == 1) {
+            final String errorMessage = String.format("The Gremlin statement that was submitted exceeds the maximum compilation size allowed by the JVM, please split it into multiple smaller statements - %s", trimMessage(gremlinContext.getRequestMessage()));
+            logger.warn(errorMessage);
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(errorMessage)
+                    .statusAttributeException(root).create());
+        }
+
+        // GroovyRuntimeException will hit a pretty wide range of eval type errors, like MissingPropertyException,
+        // CompilationFailedException, MissingMethodException, etc. If more specific handling is required then
+        // try to catch it earlier above.
+        if (root instanceof GroovyRuntimeException ||
+                root instanceof VerificationException ||
+                root instanceof ScriptException) {
+            throw new RexsterException(root.getMessage(), root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(root.getMessage())
+                    .statusAttributeException(root).create());
+        }
+
+        throw new RexsterException(root.getClass().getSimpleName() + ": " + root.getMessage(), root,
+                ResponseMessage.build(gremlinContext.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusAttributeException(root)
+                        .statusMessage(root.getMessage()).create());
+    }
+
+    /**
+     * Used to decrease the size of a Gremlin script that triggered a "method too large" exception so that it
+     * doesn't log a massive text string nor return a large error message.
+     */
+    private RequestMessage trimMessage(final RequestMessage msg) {
+        final RequestMessage trimmedMsg = RequestMessage.from(msg).create();
+        if (trimmedMsg.getArgs().containsKey(Tokens.ARGS_GREMLIN))
+            trimmedMsg.getArgs().put(Tokens.ARGS_GREMLIN, trimmedMsg.getArgs().get(Tokens.ARGS_GREMLIN).toString().substring(0, 1021) + "...");
+
+        return trimmedMsg;
+    }
+
+    /**
+     * Check if any exception in the chain is TemporaryException then we should respond with the right error code so
+     * that the client knows to retry.
+     */
+    protected Optional<Throwable> determineIfTemporaryException(final Throwable ex) {
+        return Stream.of(ExceptionUtils.getThrowables(ex)).
+                filter(i -> i instanceof TemporaryException).findFirst();
+    }
+
+    @Override
+    public synchronized void close() {
+        // already closing/closed
+        if (!sessions.containsKey(sessionId)) return;
+
+        sessions.remove(sessionId);
+
+        if (sessionCancelFuture.get() != null) {
+            final ScheduledFuture<?> f = sessionCancelFuture.get();
+            if (!f.isDone()) f.cancel(true);
+        }
+    }
+
+    protected Iterator<?> fromScript(final Context gremlinContext, final String script) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final String language = args.containsKey(Tokens.ARGS_LANGUAGE) ? (String) args.get(Tokens.ARGS_LANGUAGE) : "gremlin-groovy";
+        return IteratorUtils.asIterator(getScriptEngine(gremlinContext, language).eval(
+                script, mergeBindingsFromRequest(gremlinContext, getWorkerBindings())));
+    }
+
+    protected Optional<Iterator<?>> fromBytecode(final Context gremlinContext, final Bytecode bytecode) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+
+        final Traversal.Admin<?, ?> traversal;
+        final Map<String, String> aliases = (Map<String, String>) msg.optionalArgs(Tokens.ARGS_ALIASES).get();
+        final GraphManager graphManager = gremlinContext.getGraphManager();
+        final String traversalSourceName = aliases.entrySet().iterator().next().getValue();
+        final TraversalSource g = graphManager.getTraversalSource(traversalSourceName);
+
+        // handle bytecode based graph operations like commit/rollback commands
+        if (BytecodeHelper.isGraphOperation(bytecode)) {
+            handleGraphOperation(gremlinContext, bytecode, g.getGraph());
+            return Optional.empty();
+        } else {
+
+            final Optional<String> lambdaLanguage = BytecodeHelper.getLambdaLanguage(bytecode);
+            if (!lambdaLanguage.isPresent())
+                traversal = JavaTranslator.of(g).translate(bytecode);
+            else {
+                final SimpleBindings bindings = new SimpleBindings();
+                bindings.put(traversalSourceName, g);
+                traversal = gremlinContext.getGremlinExecutor().getScriptEngineManager().getEngineByName(lambdaLanguage.get()).eval(bytecode, bindings, traversalSourceName);
+            }
+
+            // compile the traversal - without it getEndStep() has nothing in it
+            traversal.applyStrategies();
+
+            return Optional.of(new TraverserIterator(traversal));
+        }
+    }
+
+    protected Bindings getWorkerBindings() throws RexsterException {
+        return new SimpleBindings(graphManager.getAsBindings());
+    }
+
+    protected Bindings mergeBindingsFromRequest(final Context gremlinContext, final Bindings bindings) throws RexsterException {
+        // alias any global bindings to a different variable.
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (msg.getArgs().containsKey(Tokens.ARGS_ALIASES)) {
+            final Map<String, String> aliases = (Map<String, String>) msg.getArgs().get(Tokens.ARGS_ALIASES);
+            for (Map.Entry<String,String> aliasKv : aliases.entrySet()) {
+                boolean found = false;
+
+                // first check if the alias refers to a Graph instance
+                final Graph graph = gremlinContext.getGraphManager().getGraph(aliasKv.getValue());
+                if (null != graph) {
+                    bindings.put(aliasKv.getKey(), graph);
+                    found = true;
+                }
+
+                // if the alias wasn't found as a Graph then perhaps it is a TraversalSource - it needs to be
+                // something
+                if (!found) {
+                    final TraversalSource ts = gremlinContext.getGraphManager().getTraversalSource(aliasKv.getValue());
+                    if (null != ts) {
+                        bindings.put(aliasKv.getKey(), ts);
+                        found = true;
+                    }
+                }
+
+                // this validation is important to calls to GraphManager.commit() and rollback() as they both
+                // expect that the aliases supplied are valid
+                if (!found) {
+                    final String error = String.format("Could not alias [%s] to [%s] as [%s] not in the Graph or TraversalSource global bindings",
+                            aliasKv.getKey(), aliasKv.getValue(), aliasKv.getValue());
+                    throw new RexsterException(error, ResponseMessage.build(msg)
+                            .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+                }
+            }
+        } else {
+            // there's no bindings so determine if that's ok with Gremlin Server
+            if (gremlinContext.getSettings().strictTransactionManagement) {
+                final String error = "Gremlin Server is configured with strictTransactionManagement as 'true' - the 'aliases' arguments must be provided";
+                throw new RexsterException(error, ResponseMessage.build(msg)
+                        .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+            }
+        }
+
+        // add any bindings to override any other supplied
+        Optional.ofNullable((Map<String, Object>) msg.getArgs().get(Tokens.ARGS_BINDINGS)).ifPresent(bindings::putAll);
+        return bindings;
+    }
+
+    /**
+     * Provides a generic way of iterating a result set back to the client.
+     *
+     * @param gremlinContext The Gremlin Server {@link Context} object containing settings, request message, etc.
+     * @param itty The result to iterator
+     */
+    protected void handleIterator(final Context gremlinContext, final Iterator<?> itty) throws InterruptedException {
+        final ChannelHandlerContext nettyContext = gremlinContext.getChannelHandlerContext();
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Settings settings = gremlinContext.getSettings();
+        boolean warnOnce = false;
+
+        // sessionless requests are always transaction managed, but in-session requests are configurable.
+        final boolean managedTransactionsForRequest = transactionManaged ?
+                true : (Boolean) msg.getArgs().getOrDefault(Tokens.ARGS_MANAGE_TRANSACTION, false);
+
+        // we have an empty iterator - happens on stuff like: g.V().iterate()
+        if (!itty.hasNext()) {
+            final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,ResponseStatusCode.NO_CONTENT, itty);
+            // as there is nothing left to iterate if we are transaction managed then we should execute a
+            // commit here before we send back a NO_CONTENT which implies success
+            if (managedTransactionsForRequest)
+                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+            gremlinContext.writeAndFlush(ResponseMessage.build(msg)
+                    .code(ResponseStatusCode.NO_CONTENT)
+                    .statusAttributes(attributes)
+                    .create());
+            return;
+        }
+
+        // the batch size can be overridden by the request
+        final int resultIterationBatchSize = (Integer) msg.optionalArgs(Tokens.ARGS_BATCH_SIZE)
+                .orElse(settings.resultIterationBatchSize);
+        List<Object> aggregate = new ArrayList<>(resultIterationBatchSize);
+
+        // use an external control to manage the loop as opposed to just checking hasNext() in the while.  this
+        // prevent situations where auto transactions create a new transaction after calls to commit() withing
+        // the loop on calls to hasNext().
+        boolean hasMore = itty.hasNext();
+
+        while (hasMore) {
+            if (Thread.interrupted()) throw new InterruptedException();
+
+            // check if an implementation needs to force flush the aggregated results before the iteration batch
+            // size is reached.
+            // todo: what implementation does this?! can we kill it going forward - seems always false
+            // final boolean forceFlush = isForceFlushed(nettyContext, msg, itty);
+            final boolean forceFlush = false;
+
+            // have to check the aggregate size because it is possible that the channel is not writeable (below)
+            // so iterating next() if the message is not written and flushed would bump the aggregate size beyond
+            // the expected resultIterationBatchSize.  Total serialization time for the response remains in
+            // effect so if the client is "slow" it may simply timeout.
+            //
+            // there is a need to check hasNext() on the iterator because if the channel is not writeable the
+            // previous pass through the while loop will have next()'d the iterator and if it is "done" then a
+            // NoSuchElementException will raise its head. also need a check to ensure that this iteration doesn't
+            // require a forced flush which can be forced by sub-classes.
+            //
+            // this could be placed inside the isWriteable() portion of the if-then below but it seems better to
+            // allow iteration to continue into a batch if that is possible rather than just doing nothing at all
+            // while waiting for the client to catch up
+            if (aggregate.size() < resultIterationBatchSize && itty.hasNext() && !forceFlush) aggregate.add(itty.next());
+
+            // Don't keep executor busy if client has already given up; there is no way to catch up if the channel is
+            // not active, and hence we should break the loop.
+            if (!nettyContext.channel().isActive()) {
+                if (managedTransactionsForRequest) {
+                    closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                }
+                break;
+            }
+
+            // send back a page of results if batch size is met or if it's the end of the results being iterated.
+            // also check writeability of the channel to prevent OOME for slow clients.
+            //
+            // clients might decide to close the Netty channel to the server with a CloseWebsocketFrame after errors
+            // like CorruptedFrameException. On the server, although the channel gets closed, there might be some
+            // executor threads waiting for watermark to clear which will not clear in these cases since client has
+            // already given up on these requests. This leads to these executors waiting for the client to consume
+            // results till the timeout. checking for isActive() should help prevent that.
+            if (nettyContext.channel().isActive() && nettyContext.channel().isWritable()) {
+                if (forceFlush || aggregate.size() == resultIterationBatchSize || !itty.hasNext()) {
+                    final ResponseStatusCode code = itty.hasNext() ? ResponseStatusCode.PARTIAL_CONTENT : ResponseStatusCode.SUCCESS;
+                    Frame frame = null;
+                    try {
+                        frame = makeFrame(gremlinContext, aggregate, code, itty);
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+
+                        // exception is handled in makeFrame() - serialization error gets written back to driver
+                        // at that point
+                        if (managedTransactionsForRequest)
+                            closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                        break;
+                    }
+
+                    // track whether there is anything left in the iterator because it needs to be accessed after
+                    // the transaction could be closed - in that case a call to hasNext() could open a new transaction
+                    // unintentionally
+                    final boolean moreInIterator = itty.hasNext();
+
+                    try {
+                        // only need to reset the aggregation list if there's more stuff to write
+                        if (moreInIterator)
+                            aggregate = new ArrayList<>(resultIterationBatchSize);
+                        else {
+                            // iteration and serialization are both complete which means this finished successfully. note that
+                            // errors internal to script eval or timeout will rollback given GremlinServer's global configurations.
+                            // local errors will get rolledback below because the exceptions aren't thrown in those cases to be
+                            // caught by the GremlinExecutor for global rollback logic. this only needs to be committed if
+                            // there are no more items to iterate and serialization is complete
+                            if (managedTransactionsForRequest)
+                                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+                            // exit the result iteration loop as there are no more results left.  using this external control
+                            // because of the above commit.  some graphs may open a new transaction on the call to
+                            // hasNext()
+                            hasMore = false;
+                        }
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+                        throw ex;
+                    }
+
+                    if (!moreInIterator) iterateComplete(gremlinContext, itty);
+
+                    // the flush is called after the commit has potentially occurred.  in this way, if a commit was
+                    // required then it will be 100% complete before the client receives it. the "frame" at this point
+                    // should have completely detached objects from the transaction (i.e. serialization has occurred)
+                    // so a new one should not be opened on the flush down the netty pipeline
+                    gremlinContext.writeAndFlush(code, frame);
+                }
+            } else {
+                // don't keep triggering this warning over and over again for the same request
+                if (!warnOnce) {
+                    logger.warn("Pausing response writing as writeBufferHighWaterMark exceeded on {} - writing will continue once client has caught up", msg);
+                    warnOnce = true;
+                }
+
+                // since the client is lagging we can hold here for a period of time for the client to catch up.
+                // this isn't blocking the IO thread - just a worker.
+                TimeUnit.MILLISECONDS.sleep(10);
+            }
+        }
+    }
+
+    /**
+     * If {@link Bytecode} is detected to contain a "graph operation" then it gets processed by this method.
+     */
+    protected void handleGraphOperation(final Context gremlinContext, final Bytecode bytecode, final Graph graph) throws Exception {

Review comment:
       Have we described what is a "graph operation" in the code somewhere? If not, perhaps have an enum (or interface) with some operations marked as GraphOperation. If yes, please link to that in javadoc.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {
+            if (closeReason.compareAndSet(null, CloseReason.CHANNEL_CLOSED)) {
+                // cancel session worker or it will keep waiting for items to appear in the session queue
+                cancel(true);
+                close();
+            }
+        });
+        this.sessions = sessions;
+        this.graphManager = gremlinContext.getGraphManager();
+    }
+
+    protected synchronized void cancel(final boolean mayInterruptIfRunning) {
+        final FutureTask<?> sf = (FutureTask) sessionFuture.get();
+        if (sf != null && !sf.isDone()) {
+            sf.cancel(mayInterruptIfRunning);
+        }
+    }
+
+    public boolean isTransactionManaged() {
+        return transactionManaged;
+    }
+
+    public String getSessionId() {
+        return sessionId;
+    }
+
+    public boolean isBoundTo(final Channel channel) {
+        return channel == initialChannel;
+    }
+
+    public long getActualTimeoutLengthWhenClosed() {
+        return actualTimeoutLengthWhenClosed;
+    }
+
+    public Optional<CloseReason> getCloseReason() {
+        return Optional.ofNullable(closeReason.get());
+    }
+
+    public GremlinScriptEngine getScriptEngine(final Context context, final String language) {
+        return context.getGremlinExecutor().getScriptEngineManager().getEngineByName(language);
+    }
+
+    public void setSessionThread(final Thread runner) {
+        this.sessionThread = runner;
+    }
+
+    @Override
+    public void setSessionCancelFuture(final ScheduledFuture<?> f) {
+        if (!sessionCancelFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session cancellation future is already set");
+    }
+
+    @Override
+    public void setSessionFuture(final Future<?> f) {
+        if (!sessionFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session future is already set");
+    }
+
+    @Override
+    public synchronized void triggerTimeout(final long timeout, final boolean causedBySession) {
+        // triggering timeout triggers the stop of the Rexster Runnable which will end in close()
+        // for final cleanup
+        final Future<?> f = sessionFuture.get();
+        if (f != null && !f.isDone()) {
+            if (closeReason.compareAndSet(null, causedBySession ? CloseReason.SESSION_TIMEOUT : CloseReason.REQUEST_TIMEOUT)) {
+                actualTimeoutLengthWhenClosed = timeout;
+
+                // if caused by a session timeout for a session OR if it is a request timeout for a sessionless
+                // request then we can just straight cancel() the Rexster instance
+                if (causedBySession || !sessionIdOnRequest)
+                    cancel(true);
+                else
+                    sessionThread.interrupt();
+            }
+        }
+    }
+
+    protected void process(final Context gremlinContext) throws RexsterException {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final Object gremlinToExecute = args.get(Tokens.ARGS_GREMLIN);
+
+        // for strict transactions track the aliases used so that we can commit them and only them on close()
+        if (gremlinContext.getSettings().strictTransactionManagement)
+            msg.optionalArgs(Tokens.ARGS_ALIASES).ifPresent(m -> aliasesUsedByRexster.addAll(((Map<String,String>) m).values()));
+
+        try {
+            // itty is optional as Bytecode could be a "graph operation" rather than a Traversal. graph operations
+            // don't need to be iterated and handle their own lifecycle
+            final Optional<Iterator<?>> itty = gremlinToExecute instanceof Bytecode ?
+                    fromBytecode(gremlinContext, (Bytecode) gremlinToExecute) :
+                    Optional.of(fromScript(gremlinContext, (String) gremlinToExecute));
+
+            processAuditLog(gremlinContext.getSettings(), gremlinContext.getChannelHandlerContext(), gremlinToExecute);
+
+            if (itty.isPresent())
+                handleIterator(gremlinContext, itty.get());
+        } catch (Exception ex) {
+            handleException(gremlinContext, ex);
+        }
+    }
+
+    protected void handleException(final Context gremlinContext, final Throwable t) throws RexsterException {
+        if (t instanceof RexsterException) throw (RexsterException) t;
+
+        final Optional<Throwable> possibleTemporaryException = determineIfTemporaryException(t);
+        if (possibleTemporaryException.isPresent()) {
+            final Throwable temporaryException = possibleTemporaryException.get();
+            throw new RexsterException(temporaryException.getMessage(), t,
+                    ResponseMessage.build(gremlinContext.getRequestMessage())
+                            .code(ResponseStatusCode.SERVER_ERROR_TEMPORARY)
+                            .statusMessage(temporaryException.getMessage())
+                            .statusAttributeException(temporaryException).create());
+        }
+
+        final Throwable root = ExceptionUtils.getRootCause(t);
+
+        if (root instanceof TimedInterruptTimeoutException) {
+            // occurs when the TimedInterruptCustomizerProvider is in play
+            final String msg = String.format("A timeout occurred within the script during evaluation of [%s] - consider increasing the limit given to TimedInterruptCustomizerProvider",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage("Timeout during script evaluation triggered by TimedInterruptCustomizerProvider")
+                    .create());
+        }
+
+        if (root instanceof TimeoutException) {
+            final String errorMessage = String.format("Script evaluation exceeded the configured threshold for request [%s]",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage(t.getMessage())
+                    .create());
+        }
+
+        if (root instanceof InterruptedException ||
+                root instanceof TraversalInterruptedException ||
+                root instanceof InterruptedIOException) {
+            String msg = "Processing interrupted but the reason why was not known";
+            switch (closeReason.get()) {
+                case CHANNEL_CLOSED:
+                    msg = "Processing interrupted because the channel was closed";
+                    break;
+                case SESSION_TIMEOUT:
+                    msg = String.format("Session closed - %s - sessionLifetimeTimeout of %s ms exceeded", sessionId, actualTimeoutLengthWhenClosed);
+                    break;
+                case REQUEST_TIMEOUT:
+                    msg = String.format("Evaluation exceeded timeout threshold of %s ms", actualTimeoutLengthWhenClosed);
+                    break;
+            }
+            final ResponseStatusCode code = closeReason.get() == CloseReason.SESSION_TIMEOUT || closeReason.get() == CloseReason.REQUEST_TIMEOUT ?
+                    ResponseStatusCode.SERVER_ERROR_TIMEOUT : ResponseStatusCode.SERVER_ERROR;
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(code)
+                    .statusMessage(msg).create());
+        }
+
+        if (root instanceof MultipleCompilationErrorsException && root.getMessage().contains("Method too large") &&
+                ((MultipleCompilationErrorsException) root).getErrorCollector().getErrorCount() == 1) {
+            final String errorMessage = String.format("The Gremlin statement that was submitted exceeds the maximum compilation size allowed by the JVM, please split it into multiple smaller statements - %s", trimMessage(gremlinContext.getRequestMessage()));
+            logger.warn(errorMessage);
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(errorMessage)
+                    .statusAttributeException(root).create());
+        }
+
+        // GroovyRuntimeException will hit a pretty wide range of eval type errors, like MissingPropertyException,
+        // CompilationFailedException, MissingMethodException, etc. If more specific handling is required then
+        // try to catch it earlier above.
+        if (root instanceof GroovyRuntimeException ||
+                root instanceof VerificationException ||
+                root instanceof ScriptException) {
+            throw new RexsterException(root.getMessage(), root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(root.getMessage())
+                    .statusAttributeException(root).create());
+        }
+
+        throw new RexsterException(root.getClass().getSimpleName() + ": " + root.getMessage(), root,
+                ResponseMessage.build(gremlinContext.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusAttributeException(root)
+                        .statusMessage(root.getMessage()).create());
+    }
+
+    /**
+     * Used to decrease the size of a Gremlin script that triggered a "method too large" exception so that it
+     * doesn't log a massive text string nor return a large error message.
+     */
+    private RequestMessage trimMessage(final RequestMessage msg) {
+        final RequestMessage trimmedMsg = RequestMessage.from(msg).create();
+        if (trimmedMsg.getArgs().containsKey(Tokens.ARGS_GREMLIN))
+            trimmedMsg.getArgs().put(Tokens.ARGS_GREMLIN, trimmedMsg.getArgs().get(Tokens.ARGS_GREMLIN).toString().substring(0, 1021) + "...");
+
+        return trimmedMsg;
+    }
+
+    /**
+     * Check if any exception in the chain is TemporaryException then we should respond with the right error code so
+     * that the client knows to retry.
+     */
+    protected Optional<Throwable> determineIfTemporaryException(final Throwable ex) {
+        return Stream.of(ExceptionUtils.getThrowables(ex)).
+                filter(i -> i instanceof TemporaryException).findFirst();
+    }
+
+    @Override
+    public synchronized void close() {
+        // already closing/closed
+        if (!sessions.containsKey(sessionId)) return;
+
+        sessions.remove(sessionId);
+
+        if (sessionCancelFuture.get() != null) {
+            final ScheduledFuture<?> f = sessionCancelFuture.get();
+            if (!f.isDone()) f.cancel(true);
+        }
+    }
+
+    protected Iterator<?> fromScript(final Context gremlinContext, final String script) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final String language = args.containsKey(Tokens.ARGS_LANGUAGE) ? (String) args.get(Tokens.ARGS_LANGUAGE) : "gremlin-groovy";
+        return IteratorUtils.asIterator(getScriptEngine(gremlinContext, language).eval(
+                script, mergeBindingsFromRequest(gremlinContext, getWorkerBindings())));
+    }
+
+    protected Optional<Iterator<?>> fromBytecode(final Context gremlinContext, final Bytecode bytecode) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+
+        final Traversal.Admin<?, ?> traversal;
+        final Map<String, String> aliases = (Map<String, String>) msg.optionalArgs(Tokens.ARGS_ALIASES).get();
+        final GraphManager graphManager = gremlinContext.getGraphManager();
+        final String traversalSourceName = aliases.entrySet().iterator().next().getValue();
+        final TraversalSource g = graphManager.getTraversalSource(traversalSourceName);
+
+        // handle bytecode based graph operations like commit/rollback commands
+        if (BytecodeHelper.isGraphOperation(bytecode)) {
+            handleGraphOperation(gremlinContext, bytecode, g.getGraph());
+            return Optional.empty();
+        } else {
+
+            final Optional<String> lambdaLanguage = BytecodeHelper.getLambdaLanguage(bytecode);
+            if (!lambdaLanguage.isPresent())
+                traversal = JavaTranslator.of(g).translate(bytecode);
+            else {
+                final SimpleBindings bindings = new SimpleBindings();
+                bindings.put(traversalSourceName, g);
+                traversal = gremlinContext.getGremlinExecutor().getScriptEngineManager().getEngineByName(lambdaLanguage.get()).eval(bytecode, bindings, traversalSourceName);
+            }
+
+            // compile the traversal - without it getEndStep() has nothing in it
+            traversal.applyStrategies();
+
+            return Optional.of(new TraverserIterator(traversal));
+        }
+    }
+
+    protected Bindings getWorkerBindings() throws RexsterException {
+        return new SimpleBindings(graphManager.getAsBindings());
+    }
+
+    protected Bindings mergeBindingsFromRequest(final Context gremlinContext, final Bindings bindings) throws RexsterException {
+        // alias any global bindings to a different variable.
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (msg.getArgs().containsKey(Tokens.ARGS_ALIASES)) {
+            final Map<String, String> aliases = (Map<String, String>) msg.getArgs().get(Tokens.ARGS_ALIASES);
+            for (Map.Entry<String,String> aliasKv : aliases.entrySet()) {
+                boolean found = false;
+
+                // first check if the alias refers to a Graph instance
+                final Graph graph = gremlinContext.getGraphManager().getGraph(aliasKv.getValue());
+                if (null != graph) {
+                    bindings.put(aliasKv.getKey(), graph);
+                    found = true;
+                }
+
+                // if the alias wasn't found as a Graph then perhaps it is a TraversalSource - it needs to be
+                // something
+                if (!found) {
+                    final TraversalSource ts = gremlinContext.getGraphManager().getTraversalSource(aliasKv.getValue());
+                    if (null != ts) {
+                        bindings.put(aliasKv.getKey(), ts);
+                        found = true;
+                    }
+                }
+
+                // this validation is important to calls to GraphManager.commit() and rollback() as they both
+                // expect that the aliases supplied are valid
+                if (!found) {
+                    final String error = String.format("Could not alias [%s] to [%s] as [%s] not in the Graph or TraversalSource global bindings",
+                            aliasKv.getKey(), aliasKv.getValue(), aliasKv.getValue());
+                    throw new RexsterException(error, ResponseMessage.build(msg)
+                            .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+                }
+            }
+        } else {
+            // there's no bindings so determine if that's ok with Gremlin Server
+            if (gremlinContext.getSettings().strictTransactionManagement) {
+                final String error = "Gremlin Server is configured with strictTransactionManagement as 'true' - the 'aliases' arguments must be provided";
+                throw new RexsterException(error, ResponseMessage.build(msg)
+                        .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+            }
+        }
+
+        // add any bindings to override any other supplied
+        Optional.ofNullable((Map<String, Object>) msg.getArgs().get(Tokens.ARGS_BINDINGS)).ifPresent(bindings::putAll);
+        return bindings;
+    }
+
+    /**
+     * Provides a generic way of iterating a result set back to the client.
+     *
+     * @param gremlinContext The Gremlin Server {@link Context} object containing settings, request message, etc.
+     * @param itty The result to iterator
+     */
+    protected void handleIterator(final Context gremlinContext, final Iterator<?> itty) throws InterruptedException {
+        final ChannelHandlerContext nettyContext = gremlinContext.getChannelHandlerContext();
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Settings settings = gremlinContext.getSettings();
+        boolean warnOnce = false;
+
+        // sessionless requests are always transaction managed, but in-session requests are configurable.
+        final boolean managedTransactionsForRequest = transactionManaged ?
+                true : (Boolean) msg.getArgs().getOrDefault(Tokens.ARGS_MANAGE_TRANSACTION, false);
+
+        // we have an empty iterator - happens on stuff like: g.V().iterate()
+        if (!itty.hasNext()) {
+            final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,ResponseStatusCode.NO_CONTENT, itty);
+            // as there is nothing left to iterate if we are transaction managed then we should execute a
+            // commit here before we send back a NO_CONTENT which implies success
+            if (managedTransactionsForRequest)
+                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+            gremlinContext.writeAndFlush(ResponseMessage.build(msg)
+                    .code(ResponseStatusCode.NO_CONTENT)
+                    .statusAttributes(attributes)
+                    .create());
+            return;
+        }
+
+        // the batch size can be overridden by the request
+        final int resultIterationBatchSize = (Integer) msg.optionalArgs(Tokens.ARGS_BATCH_SIZE)
+                .orElse(settings.resultIterationBatchSize);
+        List<Object> aggregate = new ArrayList<>(resultIterationBatchSize);
+
+        // use an external control to manage the loop as opposed to just checking hasNext() in the while.  this
+        // prevent situations where auto transactions create a new transaction after calls to commit() withing
+        // the loop on calls to hasNext().
+        boolean hasMore = itty.hasNext();
+
+        while (hasMore) {
+            if (Thread.interrupted()) throw new InterruptedException();
+
+            // check if an implementation needs to force flush the aggregated results before the iteration batch
+            // size is reached.
+            // todo: what implementation does this?! can we kill it going forward - seems always false
+            // final boolean forceFlush = isForceFlushed(nettyContext, msg, itty);
+            final boolean forceFlush = false;
+
+            // have to check the aggregate size because it is possible that the channel is not writeable (below)
+            // so iterating next() if the message is not written and flushed would bump the aggregate size beyond
+            // the expected resultIterationBatchSize.  Total serialization time for the response remains in
+            // effect so if the client is "slow" it may simply timeout.
+            //
+            // there is a need to check hasNext() on the iterator because if the channel is not writeable the
+            // previous pass through the while loop will have next()'d the iterator and if it is "done" then a
+            // NoSuchElementException will raise its head. also need a check to ensure that this iteration doesn't
+            // require a forced flush which can be forced by sub-classes.
+            //
+            // this could be placed inside the isWriteable() portion of the if-then below but it seems better to
+            // allow iteration to continue into a batch if that is possible rather than just doing nothing at all
+            // while waiting for the client to catch up
+            if (aggregate.size() < resultIterationBatchSize && itty.hasNext() && !forceFlush) aggregate.add(itty.next());
+
+            // Don't keep executor busy if client has already given up; there is no way to catch up if the channel is
+            // not active, and hence we should break the loop.
+            if (!nettyContext.channel().isActive()) {
+                if (managedTransactionsForRequest) {
+                    closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                }
+                break;
+            }
+
+            // send back a page of results if batch size is met or if it's the end of the results being iterated.
+            // also check writeability of the channel to prevent OOME for slow clients.
+            //
+            // clients might decide to close the Netty channel to the server with a CloseWebsocketFrame after errors
+            // like CorruptedFrameException. On the server, although the channel gets closed, there might be some
+            // executor threads waiting for watermark to clear which will not clear in these cases since client has
+            // already given up on these requests. This leads to these executors waiting for the client to consume
+            // results till the timeout. checking for isActive() should help prevent that.
+            if (nettyContext.channel().isActive() && nettyContext.channel().isWritable()) {
+                if (forceFlush || aggregate.size() == resultIterationBatchSize || !itty.hasNext()) {
+                    final ResponseStatusCode code = itty.hasNext() ? ResponseStatusCode.PARTIAL_CONTENT : ResponseStatusCode.SUCCESS;
+                    Frame frame = null;
+                    try {
+                        frame = makeFrame(gremlinContext, aggregate, code, itty);
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+
+                        // exception is handled in makeFrame() - serialization error gets written back to driver
+                        // at that point
+                        if (managedTransactionsForRequest)
+                            closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                        break;
+                    }
+
+                    // track whether there is anything left in the iterator because it needs to be accessed after
+                    // the transaction could be closed - in that case a call to hasNext() could open a new transaction
+                    // unintentionally
+                    final boolean moreInIterator = itty.hasNext();
+
+                    try {
+                        // only need to reset the aggregation list if there's more stuff to write
+                        if (moreInIterator)
+                            aggregate = new ArrayList<>(resultIterationBatchSize);
+                        else {
+                            // iteration and serialization are both complete which means this finished successfully. note that
+                            // errors internal to script eval or timeout will rollback given GremlinServer's global configurations.
+                            // local errors will get rolledback below because the exceptions aren't thrown in those cases to be
+                            // caught by the GremlinExecutor for global rollback logic. this only needs to be committed if
+                            // there are no more items to iterate and serialization is complete
+                            if (managedTransactionsForRequest)
+                                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+                            // exit the result iteration loop as there are no more results left.  using this external control
+                            // because of the above commit.  some graphs may open a new transaction on the call to
+                            // hasNext()
+                            hasMore = false;
+                        }
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+                        throw ex;
+                    }
+
+                    if (!moreInIterator) iterateComplete(gremlinContext, itty);
+
+                    // the flush is called after the commit has potentially occurred.  in this way, if a commit was
+                    // required then it will be 100% complete before the client receives it. the "frame" at this point
+                    // should have completely detached objects from the transaction (i.e. serialization has occurred)
+                    // so a new one should not be opened on the flush down the netty pipeline
+                    gremlinContext.writeAndFlush(code, frame);
+                }
+            } else {
+                // don't keep triggering this warning over and over again for the same request
+                if (!warnOnce) {
+                    logger.warn("Pausing response writing as writeBufferHighWaterMark exceeded on {} - writing will continue once client has caught up", msg);
+                    warnOnce = true;
+                }
+
+                // since the client is lagging we can hold here for a period of time for the client to catch up.
+                // this isn't blocking the IO thread - just a worker.
+                TimeUnit.MILLISECONDS.sleep(10);
+            }
+        }
+    }
+
+    /**
+     * If {@link Bytecode} is detected to contain a "graph operation" then it gets processed by this method.
+     */
+    protected void handleGraphOperation(final Context gremlinContext, final Bytecode bytecode, final Graph graph) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (graph.features().graph().supportsTransactions()) {
+            if (bytecode.equals(Bytecode.TX_COMMIT) || bytecode.equals(Bytecode.TX_ROLLBACK)) {
+                final boolean commit = bytecode.equals(Bytecode.TX_COMMIT);
+                closeTransaction(gremlinContext, commit ? Transaction.Status.COMMIT : Transaction.Status.ROLLBACK);
+
+                // write back a no-op for success
+                final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,
+                        ResponseStatusCode.NO_CONTENT, Collections.emptyIterator());
+                gremlinContext.writeAndFlush(ResponseMessage.build(msg)
+                            .code(ResponseStatusCode.NO_CONTENT)
+                            .statusAttributes(attributes)
+                            .create());
+            } else {
+                throw new IllegalStateException(String.format(
+                        "Bytecode in request is not a recognized graph operation: %s", bytecode.toString()));
+            }
+        }
+    }
+
+    /**
+     * Called when iteration within {@link #handleIterator(Context, Iterator)} is on its final pass and the final
+     * frame is about to be sent back to the client. This method only gets called on successful iteration of the
+     * entire result.
+     */
+    protected void iterateComplete(final Context gremlinContext, final Iterator<?> itty) {
+        // do nothing by default
+    }
+
+    /**
+     * Generates response status meta-data to put on a {@link ResponseMessage}.
+     *
+     * @param itty a reference to the current {@link Iterator} of results - it is not meant to be forwarded in
+     *             this method
+     */
+    protected Map<String, Object> generateStatusAttributes(final Context gremlinContext,
+                                                           final ResponseStatusCode code, final Iterator<?> itty) {
+        // only return server metadata on the last message
+        if (itty.hasNext()) return Collections.emptyMap();
+
+        final Map<String, Object> metaData = new HashMap<>();
+        metaData.put(Tokens.ARGS_HOST, gremlinContext.getChannelHandlerContext().channel().remoteAddress().toString());
+
+        return metaData;
+    }
+
+    /**
+     * Generates response result meta-data to put on a {@link ResponseMessage}.
+     *
+     * @param itty a reference to the current {@link Iterator} of results - it is not meant to be forwarded in
+     *             this method
+     */
+    protected Map<String, Object> generateResponseMetaData(final Context gremlinContext,
+                                                           final ResponseStatusCode code, final Iterator<?> itty) {
+        return Collections.emptyMap();
+    }
+
+    protected Frame makeFrame(final Context gremlinContext, final List<Object> aggregate,
+                              final ResponseStatusCode code, final Iterator<?> itty) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final ChannelHandlerContext nettyContext = gremlinContext.getChannelHandlerContext();
+        final MessageSerializer serializer = nettyContext.channel().attr(StateKey.SERIALIZER).get();
+        final boolean useBinary = nettyContext.channel().attr(StateKey.USE_BINARY).get();

Review comment:
       do we need this additional attribute? this decision could be made by looking at the serializer which is already stored per channel.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();

Review comment:
       Unbounded queue is dangerous and risks OOM. Perhaps have a limit and expose it via server settings?

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/channel/UnifiedChannelizer.java
##########
@@ -0,0 +1,77 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.channel;
+
+import io.netty.channel.ChannelPipeline;
+import org.apache.tinkerpop.gremlin.server.AbstractChannelizer;
+import org.apache.tinkerpop.gremlin.server.Channelizer;
+import org.apache.tinkerpop.gremlin.server.handler.HttpGremlinEndpointHandler;
+import org.apache.tinkerpop.gremlin.server.handler.UnifiedHandler;
+import org.apache.tinkerpop.gremlin.server.handler.WsAndHttpChannelizerHandler;
+import org.apache.tinkerpop.gremlin.server.util.ServerGremlinExecutor;
+
+/**
+ * A {@link Channelizer} that supports websocket and HTTP requests and does so with the most streamlined processing
+ * model for Gremlin Server introduced with 3.5.0.
+ */
+public class UnifiedChannelizer extends AbstractChannelizer {

Review comment:
       UnifiedChannelizer is a very ambiguous name. I would suggest renaming it to `WsAndHttpChannelizerV2`.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();
+    private ScheduledFuture<?> requestCancelFuture;

Review comment:
       nit please move all "final" members together

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/channel/UnifiedChannelizer.java
##########
@@ -0,0 +1,77 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.channel;
+
+import io.netty.channel.ChannelPipeline;
+import org.apache.tinkerpop.gremlin.server.AbstractChannelizer;
+import org.apache.tinkerpop.gremlin.server.Channelizer;
+import org.apache.tinkerpop.gremlin.server.handler.HttpGremlinEndpointHandler;
+import org.apache.tinkerpop.gremlin.server.handler.UnifiedHandler;
+import org.apache.tinkerpop.gremlin.server.handler.WsAndHttpChannelizerHandler;
+import org.apache.tinkerpop.gremlin.server.util.ServerGremlinExecutor;
+
+/**
+ * A {@link Channelizer} that supports websocket and HTTP requests and does so with the most streamlined processing
+ * model for Gremlin Server introduced with 3.5.0.
+ */
+public class UnifiedChannelizer extends AbstractChannelizer {
+
+    private WsAndHttpChannelizerHandler handler;
+    private UnifiedHandler unifiedHandler;
+    protected static final String PIPELINE_UNIFIED = "unified";
+
+    @Override
+    public void init(final ServerGremlinExecutor serverGremlinExecutor) {
+        super.init(serverGremlinExecutor);
+        handler = new WsAndHttpChannelizerHandler();
+        handler.init(serverGremlinExecutor, new HttpGremlinEndpointHandler(serializers, gremlinExecutor, graphManager, settings));
+
+        // these handlers don't share any state and can thus be initialized once per pipeline
+        unifiedHandler = new UnifiedHandler(settings, graphManager, gremlinExecutor, scheduledExecutorService, this);

Review comment:
       Same comment as earlier, `UnifiedHandler` is a very ambiguous class name. Suggest renaming to WsAndHttpV2Handler

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();
+    private ScheduledFuture<?> requestCancelFuture;
+    private Bindings bindings;
+    private final AtomicBoolean ending = new AtomicBoolean(false);
+    private final ScheduledExecutorService scheduledExecutorService;
+    private final GremlinScriptEngineManager scriptEngineManager;
+
+    MultiRexster(final Context gremlinContext, final String sessionId,
+                 final ConcurrentMap<String, Rexster> sessions) {
+        super(gremlinContext, sessionId, false, sessions);
+
+        // using a global function cache is cheaper than creating a new on per session especially if you have to
+        // create a lot of sessions. it will generate a ton of throw-away objects. mostly keeping the option open
+        // to not use it to preserve the ability to use the old functionality if wanted or if there is some specific
+        // use case with sessions that needs it. if we wanted this could eventually become a per-request option
+        // so that the client could control it as necessary and get scriptengine isolation if they need it.
+        if (gremlinContext.getSettings().useCommonEngineForSessions)
+            scriptEngineManager = gremlinContext.getGremlinExecutor().getScriptEngineManager();
+        else
+            scriptEngineManager = initializeGremlinExecutor(gremlinContext).getScriptEngineManager();
+
+        scheduledExecutorService = gremlinContext.getScheduledExecutorService();
+        addTask(gremlinContext);
+    }
+
+    @Override
+    public GremlinScriptEngine getScriptEngine(final Context gremlinContext, final String language) {
+        return scriptEngineManager.getEngineByName(language);
+    }
+
+    @Override
+    public boolean acceptingTasks() {
+        return !ending.get();
+    }
+
+    @Override
+    public void addTask(final Context gremlinContext) {
+        if (acceptingTasks())
+            queue.offer(gremlinContext);
+    }
+
+    @Override
+    public void run() {
+        // there must be one item in the queue at least since addTask() gets called before the worker
+        // is ever started
+        Context currentGremlinContext = queue.poll();
+        if (null == currentGremlinContext)
+            throw new IllegalStateException(String.format("Worker has no initial context for session: %s", getSessionId()));
+
+        try {
+            startTransaction(currentGremlinContext);
+            try {
+                while (true) {
+                    // schedule timeout for the current request from the queue
+                    final long seto = currentGremlinContext.getRequestTimeout();
+                    requestCancelFuture = scheduledExecutorService.schedule(
+                            () -> this.triggerTimeout(seto, false),
+                            seto, TimeUnit.MILLISECONDS);
+
+                    // only stop processing stuff in the queue if this Rexster isn't configured to hold state between
+                    // exceptions (i.e. the old OpProcessor way) or if this Rexster is closing down by certain death
+                    // (i.e. channel close or lifetime session timeout)
+                    try {
+                        process(currentGremlinContext);
+                    } catch (RexsterException ex) {
+                        if (!maintainStateAfterException || closeReason.get() == CloseReason.CHANNEL_CLOSED ||
+                            closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                            throw ex;
+                        }
+
+                        // reset the close reason as we are maintaining state
+                        closeReason.set(null);
+
+                        logger.warn(ex.getMessage(), ex);
+                        currentGremlinContext.writeAndFlush(ex.getResponseMessage());
+                    }
+
+                    // work is done within the timeout period so cancel it
+                    cancelRequestTimeout();
+
+                    currentGremlinContext = queue.take();
+                }
+            } catch (Exception ex) {
+                stopAcceptingRequests();
+
+                // the current context gets its exception handled...
+                handleException(currentGremlinContext, ex);
+            }
+        } catch (RexsterException rexex) {
+            // remaining work items in the queue are ignored since this worker is closing. must send
+            // back some sort of response to satisfy the client. writeAndFlush code is different than
+            // the ResponseMessage as we don't want the message to be "final" for the Context. that
+            // status must be reserved for the message that caused the error
+            for (Context gctx : queue) {
+                gctx.writeAndFlush(ResponseStatusCode.PARTIAL_CONTENT, ResponseMessage.build(gctx.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusMessage(String.format(
+                                "An earlier request [%s] failed prior to this one having a chance to execute",
+                                currentGremlinContext.getRequestMessage().getRequestId())).create());
+            }
+
+            // exception should trigger a rollback in the session. a more focused rollback may have occurred
+            // during process() and the related result iteration IF transaction management was enabled on
+            // the request
+            closeTransactionSafely(Transaction.Status.ROLLBACK);
+
+            // the current context could already be completed with SUCCESS and we're just waiting for another
+            // one to show up while a timeout occurs or the channel closes. in these cases, this would be a valid
+            // close in all likelihood so there's no reason to log or alert the client as the client already has
+            // the best answer
+            if (!currentGremlinContext.isFinalResponseWritten()) {
+                logger.warn(rexex.getMessage(), rexex);
+                currentGremlinContext.writeAndFlush(rexex.getResponseMessage());
+            }
+        } finally {
+            // if this is a normal end to the session or if the session life timeout is exceeded then the
+            // session needs to be removed and everything cleaned up
+            if (closeReason.compareAndSet(null, CloseReason.NORMAL) || closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                close();
+            }
+        }
+    }
+
+    @Override
+    public void close() {
+        ending.set(true);
+        cancelRequestTimeout();
+        super.close();
+        logger.debug("Session {} closed", getSessionId());
+    }
+
+    private void cancelRequestTimeout() {
+        if (requestCancelFuture != null && !requestCancelFuture.isDone())
+            requestCancelFuture.cancel(true);
+    }
+
+    private void stopAcceptingRequests() {
+        ending.set(true);

Review comment:
       plz. use test and set. If already set to true, don't execute the next line.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/Rexster.java
##########
@@ -0,0 +1,89 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import io.netty.channel.Channel;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.server.Context;
+
+import java.util.concurrent.Future;
+import java.util.concurrent.ScheduledFuture;
+
+/**
+ * The {@code Rexster} interface is essentially a form of "worker", named for Gremlin's trusty canine robot friend
+ * who represents the "server" aspect of TinkerPop. In the most generic sense, {@code Rexster} implementations take
+ * tasks, a Gremlin Server {@link Context} and process them, which typically means "execute some Gremlin".
+ * Implementations have a fair amount of flexibility in terms of how they go about doing this, but in most cases,
+ * the {@link SingleRexster} and the {@link MultiRexster} should handle most cases quite well and are extensible for
+ * providers.
+ */
+public interface Rexster extends Runnable {

Review comment:
       I understand the historical concept here but for the sake of code readability for new folks being introduced to the code here, a generic or more recognizable name would be beneficial. Possible alternatives, `GremlinWorker` (this aligns closely with the thread pool naming convention since each rexter is executed by a thread from "worker pool") or `QueryExecutorService` (aligns with concept of ExecutorService in Java). 

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();
+    private ScheduledFuture<?> requestCancelFuture;
+    private Bindings bindings;
+    private final AtomicBoolean ending = new AtomicBoolean(false);
+    private final ScheduledExecutorService scheduledExecutorService;
+    private final GremlinScriptEngineManager scriptEngineManager;
+
+    MultiRexster(final Context gremlinContext, final String sessionId,

Review comment:
       javadoc

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/GremlinServer.java
##########
@@ -284,8 +285,10 @@ private static Channelizer createChannelizer(final Settings settings) throws Exc
             }
 
             try {
-                if (gremlinExecutorService != null)
-                    gremlinExecutorService.awaitTermination(30000, TimeUnit.MILLISECONDS);
+                if (gremlinExecutorService != null) {
+                    if (!gremlinExecutorService.awaitTermination(30000, TimeUnit.MILLISECONDS))

Review comment:
       nit: please surround even the single line if statements with {} 

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();
+    private ScheduledFuture<?> requestCancelFuture;
+    private Bindings bindings;
+    private final AtomicBoolean ending = new AtomicBoolean(false);
+    private final ScheduledExecutorService scheduledExecutorService;
+    private final GremlinScriptEngineManager scriptEngineManager;
+
+    MultiRexster(final Context gremlinContext, final String sessionId,
+                 final ConcurrentMap<String, Rexster> sessions) {
+        super(gremlinContext, sessionId, false, sessions);
+
+        // using a global function cache is cheaper than creating a new on per session especially if you have to
+        // create a lot of sessions. it will generate a ton of throw-away objects. mostly keeping the option open
+        // to not use it to preserve the ability to use the old functionality if wanted or if there is some specific
+        // use case with sessions that needs it. if we wanted this could eventually become a per-request option
+        // so that the client could control it as necessary and get scriptengine isolation if they need it.
+        if (gremlinContext.getSettings().useCommonEngineForSessions)
+            scriptEngineManager = gremlinContext.getGremlinExecutor().getScriptEngineManager();
+        else
+            scriptEngineManager = initializeGremlinExecutor(gremlinContext).getScriptEngineManager();
+
+        scheduledExecutorService = gremlinContext.getScheduledExecutorService();
+        addTask(gremlinContext);
+    }
+
+    @Override
+    public GremlinScriptEngine getScriptEngine(final Context gremlinContext, final String language) {
+        return scriptEngineManager.getEngineByName(language);
+    }
+
+    @Override
+    public boolean acceptingTasks() {
+        return !ending.get();
+    }
+
+    @Override
+    public void addTask(final Context gremlinContext) {
+        if (acceptingTasks())
+            queue.offer(gremlinContext);
+    }
+
+    @Override
+    public void run() {
+        // there must be one item in the queue at least since addTask() gets called before the worker
+        // is ever started
+        Context currentGremlinContext = queue.poll();
+        if (null == currentGremlinContext)
+            throw new IllegalStateException(String.format("Worker has no initial context for session: %s", getSessionId()));
+
+        try {
+            startTransaction(currentGremlinContext);
+            try {
+                while (true) {
+                    // schedule timeout for the current request from the queue
+                    final long seto = currentGremlinContext.getRequestTimeout();
+                    requestCancelFuture = scheduledExecutorService.schedule(
+                            () -> this.triggerTimeout(seto, false),
+                            seto, TimeUnit.MILLISECONDS);
+
+                    // only stop processing stuff in the queue if this Rexster isn't configured to hold state between
+                    // exceptions (i.e. the old OpProcessor way) or if this Rexster is closing down by certain death
+                    // (i.e. channel close or lifetime session timeout)
+                    try {
+                        process(currentGremlinContext);
+                    } catch (RexsterException ex) {
+                        if (!maintainStateAfterException || closeReason.get() == CloseReason.CHANNEL_CLOSED ||
+                            closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                            throw ex;
+                        }
+
+                        // reset the close reason as we are maintaining state
+                        closeReason.set(null);
+
+                        logger.warn(ex.getMessage(), ex);
+                        currentGremlinContext.writeAndFlush(ex.getResponseMessage());
+                    }
+
+                    // work is done within the timeout period so cancel it
+                    cancelRequestTimeout();
+
+                    currentGremlinContext = queue.take();
+                }
+            } catch (Exception ex) {
+                stopAcceptingRequests();
+
+                // the current context gets its exception handled...
+                handleException(currentGremlinContext, ex);
+            }
+        } catch (RexsterException rexex) {
+            // remaining work items in the queue are ignored since this worker is closing. must send
+            // back some sort of response to satisfy the client. writeAndFlush code is different than
+            // the ResponseMessage as we don't want the message to be "final" for the Context. that
+            // status must be reserved for the message that caused the error
+            for (Context gctx : queue) {
+                gctx.writeAndFlush(ResponseStatusCode.PARTIAL_CONTENT, ResponseMessage.build(gctx.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusMessage(String.format(
+                                "An earlier request [%s] failed prior to this one having a chance to execute",
+                                currentGremlinContext.getRequestMessage().getRequestId())).create());
+            }
+
+            // exception should trigger a rollback in the session. a more focused rollback may have occurred
+            // during process() and the related result iteration IF transaction management was enabled on
+            // the request
+            closeTransactionSafely(Transaction.Status.ROLLBACK);
+
+            // the current context could already be completed with SUCCESS and we're just waiting for another
+            // one to show up while a timeout occurs or the channel closes. in these cases, this would be a valid
+            // close in all likelihood so there's no reason to log or alert the client as the client already has
+            // the best answer
+            if (!currentGremlinContext.isFinalResponseWritten()) {
+                logger.warn(rexex.getMessage(), rexex);
+                currentGremlinContext.writeAndFlush(rexex.getResponseMessage());
+            }
+        } finally {
+            // if this is a normal end to the session or if the session life timeout is exceeded then the
+            // session needs to be removed and everything cleaned up
+            if (closeReason.compareAndSet(null, CloseReason.NORMAL) || closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                close();
+            }
+        }
+    }
+
+    @Override
+    public void close() {
+        ending.set(true);

Review comment:
       plz. use test and set. If already set to true, don't execute the next line.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/Rexster.java
##########
@@ -0,0 +1,89 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;

Review comment:
       should this be in handler? This isn't a Netty handler.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();
+    private ScheduledFuture<?> requestCancelFuture;
+    private Bindings bindings;
+    private final AtomicBoolean ending = new AtomicBoolean(false);
+    private final ScheduledExecutorService scheduledExecutorService;
+    private final GremlinScriptEngineManager scriptEngineManager;
+
+    MultiRexster(final Context gremlinContext, final String sessionId,
+                 final ConcurrentMap<String, Rexster> sessions) {
+        super(gremlinContext, sessionId, false, sessions);
+
+        // using a global function cache is cheaper than creating a new on per session especially if you have to
+        // create a lot of sessions. it will generate a ton of throw-away objects. mostly keeping the option open
+        // to not use it to preserve the ability to use the old functionality if wanted or if there is some specific
+        // use case with sessions that needs it. if we wanted this could eventually become a per-request option
+        // so that the client could control it as necessary and get scriptengine isolation if they need it.
+        if (gremlinContext.getSettings().useCommonEngineForSessions)
+            scriptEngineManager = gremlinContext.getGremlinExecutor().getScriptEngineManager();
+        else
+            scriptEngineManager = initializeGremlinExecutor(gremlinContext).getScriptEngineManager();
+
+        scheduledExecutorService = gremlinContext.getScheduledExecutorService();
+        addTask(gremlinContext);
+    }
+
+    @Override
+    public GremlinScriptEngine getScriptEngine(final Context gremlinContext, final String language) {
+        return scriptEngineManager.getEngineByName(language);
+    }
+
+    @Override
+    public boolean acceptingTasks() {
+        return !ending.get();
+    }
+
+    @Override
+    public void addTask(final Context gremlinContext) {
+        if (acceptingTasks())
+            queue.offer(gremlinContext);
+    }
+
+    @Override
+    public void run() {
+        // there must be one item in the queue at least since addTask() gets called before the worker
+        // is ever started
+        Context currentGremlinContext = queue.poll();
+        if (null == currentGremlinContext)
+            throw new IllegalStateException(String.format("Worker has no initial context for session: %s", getSessionId()));
+
+        try {
+            startTransaction(currentGremlinContext);
+            try {
+                while (true) {
+                    // schedule timeout for the current request from the queue
+                    final long seto = currentGremlinContext.getRequestTimeout();
+                    requestCancelFuture = scheduledExecutorService.schedule(
+                            () -> this.triggerTimeout(seto, false),
+                            seto, TimeUnit.MILLISECONDS);
+
+                    // only stop processing stuff in the queue if this Rexster isn't configured to hold state between
+                    // exceptions (i.e. the old OpProcessor way) or if this Rexster is closing down by certain death
+                    // (i.e. channel close or lifetime session timeout)
+                    try {
+                        process(currentGremlinContext);
+                    } catch (RexsterException ex) {
+                        if (!maintainStateAfterException || closeReason.get() == CloseReason.CHANNEL_CLOSED ||
+                            closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                            throw ex;
+                        }
+
+                        // reset the close reason as we are maintaining state
+                        closeReason.set(null);
+
+                        logger.warn(ex.getMessage(), ex);
+                        currentGremlinContext.writeAndFlush(ex.getResponseMessage());
+                    }
+
+                    // work is done within the timeout period so cancel it
+                    cancelRequestTimeout();
+
+                    currentGremlinContext = queue.take();
+                }
+            } catch (Exception ex) {
+                stopAcceptingRequests();
+
+                // the current context gets its exception handled...
+                handleException(currentGremlinContext, ex);
+            }
+        } catch (RexsterException rexex) {
+            // remaining work items in the queue are ignored since this worker is closing. must send
+            // back some sort of response to satisfy the client. writeAndFlush code is different than
+            // the ResponseMessage as we don't want the message to be "final" for the Context. that
+            // status must be reserved for the message that caused the error
+            for (Context gctx : queue) {
+                gctx.writeAndFlush(ResponseStatusCode.PARTIAL_CONTENT, ResponseMessage.build(gctx.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusMessage(String.format(
+                                "An earlier request [%s] failed prior to this one having a chance to execute",
+                                currentGremlinContext.getRequestMessage().getRequestId())).create());
+            }
+
+            // exception should trigger a rollback in the session. a more focused rollback may have occurred
+            // during process() and the related result iteration IF transaction management was enabled on
+            // the request
+            closeTransactionSafely(Transaction.Status.ROLLBACK);
+
+            // the current context could already be completed with SUCCESS and we're just waiting for another
+            // one to show up while a timeout occurs or the channel closes. in these cases, this would be a valid
+            // close in all likelihood so there's no reason to log or alert the client as the client already has
+            // the best answer
+            if (!currentGremlinContext.isFinalResponseWritten()) {
+                logger.warn(rexex.getMessage(), rexex);
+                currentGremlinContext.writeAndFlush(rexex.getResponseMessage());
+            }
+        } finally {
+            // if this is a normal end to the session or if the session life timeout is exceeded then the
+            // session needs to be removed and everything cleaned up
+            if (closeReason.compareAndSet(null, CloseReason.NORMAL) || closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                close();
+            }
+        }
+    }
+
+    @Override
+    public void close() {
+        ending.set(true);
+        cancelRequestTimeout();
+        super.close();
+        logger.debug("Session {} closed", getSessionId());
+    }
+
+    private void cancelRequestTimeout() {
+        if (requestCancelFuture != null && !requestCancelFuture.isDone())

Review comment:
       else? perhaps a debug log saying, cannot cancel since already cancelled or something of that sort.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/channel/UnifiedChannelizer.java
##########
@@ -0,0 +1,77 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.channel;
+
+import io.netty.channel.ChannelPipeline;
+import org.apache.tinkerpop.gremlin.server.AbstractChannelizer;
+import org.apache.tinkerpop.gremlin.server.Channelizer;
+import org.apache.tinkerpop.gremlin.server.handler.HttpGremlinEndpointHandler;
+import org.apache.tinkerpop.gremlin.server.handler.UnifiedHandler;
+import org.apache.tinkerpop.gremlin.server.handler.WsAndHttpChannelizerHandler;
+import org.apache.tinkerpop.gremlin.server.util.ServerGremlinExecutor;
+
+/**
+ * A {@link Channelizer} that supports websocket and HTTP requests and does so with the most streamlined processing
+ * model for Gremlin Server introduced with 3.5.0.
+ */
+public class UnifiedChannelizer extends AbstractChannelizer {
+
+    private WsAndHttpChannelizerHandler handler;
+    private UnifiedHandler unifiedHandler;
+    protected static final String PIPELINE_UNIFIED = "unified";
+
+    @Override
+    public void init(final ServerGremlinExecutor serverGremlinExecutor) {
+        super.init(serverGremlinExecutor);
+        handler = new WsAndHttpChannelizerHandler();
+        handler.init(serverGremlinExecutor, new HttpGremlinEndpointHandler(serializers, gremlinExecutor, graphManager, settings));
+
+        // these handlers don't share any state and can thus be initialized once per pipeline
+        unifiedHandler = new UnifiedHandler(settings, graphManager, gremlinExecutor, scheduledExecutorService, this);
+    }
+
+    @Override
+    public void configure(final ChannelPipeline pipeline) {
+        handler.configure(pipeline);
+        pipeline.addAfter(PIPELINE_HTTP_REQUEST_DECODER, "WsAndHttpChannelizerHandler", handler);
+    }
+
+    @Override
+    public void finalize(final ChannelPipeline pipeline) {
+        super.finalize(pipeline);
+        pipeline.remove(PIPELINE_OP_SELECTOR);

Review comment:
       Please add a comment here on why are we removing these handlers. Perhaps add a TODO with a JIRA link to change code in 3.5.0 so that we don't add these handlers by default for all in AbstractChannelizer.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();
+    private ScheduledFuture<?> requestCancelFuture;
+    private Bindings bindings;
+    private final AtomicBoolean ending = new AtomicBoolean(false);
+    private final ScheduledExecutorService scheduledExecutorService;
+    private final GremlinScriptEngineManager scriptEngineManager;
+
+    MultiRexster(final Context gremlinContext, final String sessionId,
+                 final ConcurrentMap<String, Rexster> sessions) {
+        super(gremlinContext, sessionId, false, sessions);
+
+        // using a global function cache is cheaper than creating a new on per session especially if you have to
+        // create a lot of sessions. it will generate a ton of throw-away objects. mostly keeping the option open
+        // to not use it to preserve the ability to use the old functionality if wanted or if there is some specific
+        // use case with sessions that needs it. if we wanted this could eventually become a per-request option
+        // so that the client could control it as necessary and get scriptengine isolation if they need it.
+        if (gremlinContext.getSettings().useCommonEngineForSessions)
+            scriptEngineManager = gremlinContext.getGremlinExecutor().getScriptEngineManager();
+        else
+            scriptEngineManager = initializeGremlinExecutor(gremlinContext).getScriptEngineManager();
+
+        scheduledExecutorService = gremlinContext.getScheduledExecutorService();
+        addTask(gremlinContext);
+    }
+
+    @Override
+    public GremlinScriptEngine getScriptEngine(final Context gremlinContext, final String language) {
+        return scriptEngineManager.getEngineByName(language);
+    }
+
+    @Override
+    public boolean acceptingTasks() {
+        return !ending.get();
+    }
+
+    @Override
+    public void addTask(final Context gremlinContext) {
+        if (acceptingTasks())
+            queue.offer(gremlinContext);
+    }
+
+    @Override
+    public void run() {
+        // there must be one item in the queue at least since addTask() gets called before the worker
+        // is ever started
+        Context currentGremlinContext = queue.poll();
+        if (null == currentGremlinContext)
+            throw new IllegalStateException(String.format("Worker has no initial context for session: %s", getSessionId()));
+
+        try {
+            startTransaction(currentGremlinContext);
+            try {
+                while (true) {
+                    // schedule timeout for the current request from the queue
+                    final long seto = currentGremlinContext.getRequestTimeout();
+                    requestCancelFuture = scheduledExecutorService.schedule(
+                            () -> this.triggerTimeout(seto, false),
+                            seto, TimeUnit.MILLISECONDS);
+
+                    // only stop processing stuff in the queue if this Rexster isn't configured to hold state between
+                    // exceptions (i.e. the old OpProcessor way) or if this Rexster is closing down by certain death
+                    // (i.e. channel close or lifetime session timeout)
+                    try {
+                        process(currentGremlinContext);
+                    } catch (RexsterException ex) {
+                        if (!maintainStateAfterException || closeReason.get() == CloseReason.CHANNEL_CLOSED ||
+                            closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                            throw ex;
+                        }
+
+                        // reset the close reason as we are maintaining state
+                        closeReason.set(null);
+
+                        logger.warn(ex.getMessage(), ex);
+                        currentGremlinContext.writeAndFlush(ex.getResponseMessage());
+                    }
+
+                    // work is done within the timeout period so cancel it
+                    cancelRequestTimeout();
+
+                    currentGremlinContext = queue.take();
+                }
+            } catch (Exception ex) {
+                stopAcceptingRequests();
+
+                // the current context gets its exception handled...
+                handleException(currentGremlinContext, ex);
+            }
+        } catch (RexsterException rexex) {
+            // remaining work items in the queue are ignored since this worker is closing. must send
+            // back some sort of response to satisfy the client. writeAndFlush code is different than
+            // the ResponseMessage as we don't want the message to be "final" for the Context. that
+            // status must be reserved for the message that caused the error
+            for (Context gctx : queue) {
+                gctx.writeAndFlush(ResponseStatusCode.PARTIAL_CONTENT, ResponseMessage.build(gctx.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusMessage(String.format(
+                                "An earlier request [%s] failed prior to this one having a chance to execute",
+                                currentGremlinContext.getRequestMessage().getRequestId())).create());
+            }
+
+            // exception should trigger a rollback in the session. a more focused rollback may have occurred
+            // during process() and the related result iteration IF transaction management was enabled on
+            // the request
+            closeTransactionSafely(Transaction.Status.ROLLBACK);
+
+            // the current context could already be completed with SUCCESS and we're just waiting for another
+            // one to show up while a timeout occurs or the channel closes. in these cases, this would be a valid
+            // close in all likelihood so there's no reason to log or alert the client as the client already has
+            // the best answer
+            if (!currentGremlinContext.isFinalResponseWritten()) {
+                logger.warn(rexex.getMessage(), rexex);
+                currentGremlinContext.writeAndFlush(rexex.getResponseMessage());
+            }
+        } finally {
+            // if this is a normal end to the session or if the session life timeout is exceeded then the
+            // session needs to be removed and everything cleaned up
+            if (closeReason.compareAndSet(null, CloseReason.NORMAL) || closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                close();
+            }
+        }
+    }
+
+    @Override
+    public void close() {

Review comment:
       javadoc specifying what are the things that are cleaned up on close. what happens to existing requests and whether this method is completely sync i.e. all resources are cleared successfully before we return.
   
   We should also create an async variation to speed up the server shutdown time.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/MultiRexster.java
##########
@@ -0,0 +1,236 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.GroovyCompilerGremlinPlugin;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngineManager;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor.CONFIG_GLOBAL_FUNCTION_CACHE_ENABLED;
+
+/**
+ * A {@link Rexster} implementation that queues tasks given to it and executes them in a serial fashion within the
+ * same thread which thus allows multiple tasks to be executed in the same transaction.
+ */
+public class MultiRexster extends AbstractRexster {
+    private static final Logger logger = LoggerFactory.getLogger(MultiRexster.class);
+    protected final BlockingQueue<Context> queue = new LinkedBlockingQueue<>();
+    private ScheduledFuture<?> requestCancelFuture;
+    private Bindings bindings;
+    private final AtomicBoolean ending = new AtomicBoolean(false);
+    private final ScheduledExecutorService scheduledExecutorService;
+    private final GremlinScriptEngineManager scriptEngineManager;
+
+    MultiRexster(final Context gremlinContext, final String sessionId,
+                 final ConcurrentMap<String, Rexster> sessions) {
+        super(gremlinContext, sessionId, false, sessions);
+
+        // using a global function cache is cheaper than creating a new on per session especially if you have to
+        // create a lot of sessions. it will generate a ton of throw-away objects. mostly keeping the option open
+        // to not use it to preserve the ability to use the old functionality if wanted or if there is some specific
+        // use case with sessions that needs it. if we wanted this could eventually become a per-request option
+        // so that the client could control it as necessary and get scriptengine isolation if they need it.
+        if (gremlinContext.getSettings().useCommonEngineForSessions)
+            scriptEngineManager = gremlinContext.getGremlinExecutor().getScriptEngineManager();
+        else
+            scriptEngineManager = initializeGremlinExecutor(gremlinContext).getScriptEngineManager();
+
+        scheduledExecutorService = gremlinContext.getScheduledExecutorService();
+        addTask(gremlinContext);
+    }
+
+    @Override
+    public GremlinScriptEngine getScriptEngine(final Context gremlinContext, final String language) {
+        return scriptEngineManager.getEngineByName(language);
+    }
+
+    @Override
+    public boolean acceptingTasks() {
+        return !ending.get();
+    }
+
+    @Override
+    public void addTask(final Context gremlinContext) {
+        if (acceptingTasks())
+            queue.offer(gremlinContext);
+    }
+
+    @Override
+    public void run() {
+        // there must be one item in the queue at least since addTask() gets called before the worker
+        // is ever started
+        Context currentGremlinContext = queue.poll();
+        if (null == currentGremlinContext)
+            throw new IllegalStateException(String.format("Worker has no initial context for session: %s", getSessionId()));
+
+        try {
+            startTransaction(currentGremlinContext);
+            try {
+                while (true) {
+                    // schedule timeout for the current request from the queue
+                    final long seto = currentGremlinContext.getRequestTimeout();
+                    requestCancelFuture = scheduledExecutorService.schedule(
+                            () -> this.triggerTimeout(seto, false),
+                            seto, TimeUnit.MILLISECONDS);
+
+                    // only stop processing stuff in the queue if this Rexster isn't configured to hold state between
+                    // exceptions (i.e. the old OpProcessor way) or if this Rexster is closing down by certain death
+                    // (i.e. channel close or lifetime session timeout)
+                    try {
+                        process(currentGremlinContext);
+                    } catch (RexsterException ex) {
+                        if (!maintainStateAfterException || closeReason.get() == CloseReason.CHANNEL_CLOSED ||
+                            closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                            throw ex;
+                        }
+
+                        // reset the close reason as we are maintaining state
+                        closeReason.set(null);
+
+                        logger.warn(ex.getMessage(), ex);
+                        currentGremlinContext.writeAndFlush(ex.getResponseMessage());
+                    }
+
+                    // work is done within the timeout period so cancel it
+                    cancelRequestTimeout();
+
+                    currentGremlinContext = queue.take();
+                }
+            } catch (Exception ex) {
+                stopAcceptingRequests();
+
+                // the current context gets its exception handled...
+                handleException(currentGremlinContext, ex);
+            }
+        } catch (RexsterException rexex) {
+            // remaining work items in the queue are ignored since this worker is closing. must send
+            // back some sort of response to satisfy the client. writeAndFlush code is different than
+            // the ResponseMessage as we don't want the message to be "final" for the Context. that
+            // status must be reserved for the message that caused the error
+            for (Context gctx : queue) {
+                gctx.writeAndFlush(ResponseStatusCode.PARTIAL_CONTENT, ResponseMessage.build(gctx.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusMessage(String.format(
+                                "An earlier request [%s] failed prior to this one having a chance to execute",
+                                currentGremlinContext.getRequestMessage().getRequestId())).create());
+            }
+
+            // exception should trigger a rollback in the session. a more focused rollback may have occurred
+            // during process() and the related result iteration IF transaction management was enabled on
+            // the request
+            closeTransactionSafely(Transaction.Status.ROLLBACK);
+
+            // the current context could already be completed with SUCCESS and we're just waiting for another
+            // one to show up while a timeout occurs or the channel closes. in these cases, this would be a valid
+            // close in all likelihood so there's no reason to log or alert the client as the client already has
+            // the best answer
+            if (!currentGremlinContext.isFinalResponseWritten()) {
+                logger.warn(rexex.getMessage(), rexex);
+                currentGremlinContext.writeAndFlush(rexex.getResponseMessage());
+            }
+        } finally {
+            // if this is a normal end to the session or if the session life timeout is exceeded then the
+            // session needs to be removed and everything cleaned up
+            if (closeReason.compareAndSet(null, CloseReason.NORMAL) || closeReason.get() == CloseReason.SESSION_TIMEOUT) {
+                close();
+            }
+        }
+    }
+
+    @Override
+    public void close() {

Review comment:
       should be synchronous method?

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }

Review comment:
       Nice!! please document each of them. e.g. what does normal mean? client initiated close before completion or close after completion?

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/util/RexsterExecutorService.java
##########
@@ -0,0 +1,50 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.util;
+
+import org.apache.tinkerpop.gremlin.server.handler.Rexster;
+
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.RunnableFuture;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * A special {@code ThreadPoolExecutor} which will construct {@link RexsterFutureTask} instances and inject the
+ * current running thread into a {@link Rexster} instance if one is present.
+ */
+public class RexsterExecutorService extends ThreadPoolExecutor {
+
+    public RexsterExecutorService(final int nThreads, final ThreadFactory threadFactory) {
+        super(nThreads, nThreads,0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<>(), threadFactory);
+    }
+
+    @Override
+    protected <T> RunnableFuture<T> newTaskFor(final Runnable runnable, final T value) {
+        return new RexsterFutureTask<>(runnable, value);
+    }
+
+
+    @Override
+    protected void beforeExecute(final Thread t, final Runnable r) {
+        if (r instanceof RexsterFutureTask)

Review comment:
       else? what are the other types of tasks that we are expecting here?

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {
+            if (closeReason.compareAndSet(null, CloseReason.CHANNEL_CLOSED)) {
+                // cancel session worker or it will keep waiting for items to appear in the session queue
+                cancel(true);
+                close();
+            }
+        });
+        this.sessions = sessions;
+        this.graphManager = gremlinContext.getGraphManager();
+    }
+
+    protected synchronized void cancel(final boolean mayInterruptIfRunning) {
+        final FutureTask<?> sf = (FutureTask) sessionFuture.get();
+        if (sf != null && !sf.isDone()) {
+            sf.cancel(mayInterruptIfRunning);
+        }
+    }
+
+    public boolean isTransactionManaged() {
+        return transactionManaged;
+    }
+
+    public String getSessionId() {
+        return sessionId;
+    }
+
+    public boolean isBoundTo(final Channel channel) {
+        return channel == initialChannel;
+    }
+
+    public long getActualTimeoutLengthWhenClosed() {
+        return actualTimeoutLengthWhenClosed;
+    }
+
+    public Optional<CloseReason> getCloseReason() {
+        return Optional.ofNullable(closeReason.get());
+    }
+
+    public GremlinScriptEngine getScriptEngine(final Context context, final String language) {
+        return context.getGremlinExecutor().getScriptEngineManager().getEngineByName(language);
+    }
+
+    public void setSessionThread(final Thread runner) {
+        this.sessionThread = runner;
+    }
+
+    @Override
+    public void setSessionCancelFuture(final ScheduledFuture<?> f) {
+        if (!sessionCancelFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session cancellation future is already set");
+    }
+
+    @Override
+    public void setSessionFuture(final Future<?> f) {
+        if (!sessionFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session future is already set");
+    }
+
+    @Override
+    public synchronized void triggerTimeout(final long timeout, final boolean causedBySession) {
+        // triggering timeout triggers the stop of the Rexster Runnable which will end in close()
+        // for final cleanup
+        final Future<?> f = sessionFuture.get();
+        if (f != null && !f.isDone()) {
+            if (closeReason.compareAndSet(null, causedBySession ? CloseReason.SESSION_TIMEOUT : CloseReason.REQUEST_TIMEOUT)) {
+                actualTimeoutLengthWhenClosed = timeout;
+
+                // if caused by a session timeout for a session OR if it is a request timeout for a sessionless
+                // request then we can just straight cancel() the Rexster instance
+                if (causedBySession || !sessionIdOnRequest)
+                    cancel(true);
+                else
+                    sessionThread.interrupt();
+            }
+        }
+    }
+
+    protected void process(final Context gremlinContext) throws RexsterException {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final Object gremlinToExecute = args.get(Tokens.ARGS_GREMLIN);
+
+        // for strict transactions track the aliases used so that we can commit them and only them on close()
+        if (gremlinContext.getSettings().strictTransactionManagement)
+            msg.optionalArgs(Tokens.ARGS_ALIASES).ifPresent(m -> aliasesUsedByRexster.addAll(((Map<String,String>) m).values()));
+
+        try {
+            // itty is optional as Bytecode could be a "graph operation" rather than a Traversal. graph operations
+            // don't need to be iterated and handle their own lifecycle
+            final Optional<Iterator<?>> itty = gremlinToExecute instanceof Bytecode ?
+                    fromBytecode(gremlinContext, (Bytecode) gremlinToExecute) :
+                    Optional.of(fromScript(gremlinContext, (String) gremlinToExecute));
+
+            processAuditLog(gremlinContext.getSettings(), gremlinContext.getChannelHandlerContext(), gremlinToExecute);
+
+            if (itty.isPresent())
+                handleIterator(gremlinContext, itty.get());
+        } catch (Exception ex) {
+            handleException(gremlinContext, ex);
+        }
+    }
+
+    protected void handleException(final Context gremlinContext, final Throwable t) throws RexsterException {
+        if (t instanceof RexsterException) throw (RexsterException) t;
+
+        final Optional<Throwable> possibleTemporaryException = determineIfTemporaryException(t);
+        if (possibleTemporaryException.isPresent()) {
+            final Throwable temporaryException = possibleTemporaryException.get();
+            throw new RexsterException(temporaryException.getMessage(), t,
+                    ResponseMessage.build(gremlinContext.getRequestMessage())
+                            .code(ResponseStatusCode.SERVER_ERROR_TEMPORARY)
+                            .statusMessage(temporaryException.getMessage())
+                            .statusAttributeException(temporaryException).create());
+        }
+
+        final Throwable root = ExceptionUtils.getRootCause(t);
+
+        if (root instanceof TimedInterruptTimeoutException) {
+            // occurs when the TimedInterruptCustomizerProvider is in play
+            final String msg = String.format("A timeout occurred within the script during evaluation of [%s] - consider increasing the limit given to TimedInterruptCustomizerProvider",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage("Timeout during script evaluation triggered by TimedInterruptCustomizerProvider")
+                    .create());
+        }
+
+        if (root instanceof TimeoutException) {
+            final String errorMessage = String.format("Script evaluation exceeded the configured threshold for request [%s]",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage(t.getMessage())
+                    .create());
+        }
+
+        if (root instanceof InterruptedException ||
+                root instanceof TraversalInterruptedException ||
+                root instanceof InterruptedIOException) {
+            String msg = "Processing interrupted but the reason why was not known";
+            switch (closeReason.get()) {
+                case CHANNEL_CLOSED:
+                    msg = "Processing interrupted because the channel was closed";
+                    break;
+                case SESSION_TIMEOUT:
+                    msg = String.format("Session closed - %s - sessionLifetimeTimeout of %s ms exceeded", sessionId, actualTimeoutLengthWhenClosed);
+                    break;
+                case REQUEST_TIMEOUT:
+                    msg = String.format("Evaluation exceeded timeout threshold of %s ms", actualTimeoutLengthWhenClosed);
+                    break;
+            }
+            final ResponseStatusCode code = closeReason.get() == CloseReason.SESSION_TIMEOUT || closeReason.get() == CloseReason.REQUEST_TIMEOUT ?
+                    ResponseStatusCode.SERVER_ERROR_TIMEOUT : ResponseStatusCode.SERVER_ERROR;
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(code)
+                    .statusMessage(msg).create());
+        }
+
+        if (root instanceof MultipleCompilationErrorsException && root.getMessage().contains("Method too large") &&
+                ((MultipleCompilationErrorsException) root).getErrorCollector().getErrorCount() == 1) {
+            final String errorMessage = String.format("The Gremlin statement that was submitted exceeds the maximum compilation size allowed by the JVM, please split it into multiple smaller statements - %s", trimMessage(gremlinContext.getRequestMessage()));
+            logger.warn(errorMessage);
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(errorMessage)
+                    .statusAttributeException(root).create());
+        }
+
+        // GroovyRuntimeException will hit a pretty wide range of eval type errors, like MissingPropertyException,
+        // CompilationFailedException, MissingMethodException, etc. If more specific handling is required then
+        // try to catch it earlier above.
+        if (root instanceof GroovyRuntimeException ||
+                root instanceof VerificationException ||
+                root instanceof ScriptException) {
+            throw new RexsterException(root.getMessage(), root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(root.getMessage())
+                    .statusAttributeException(root).create());
+        }
+
+        throw new RexsterException(root.getClass().getSimpleName() + ": " + root.getMessage(), root,
+                ResponseMessage.build(gremlinContext.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusAttributeException(root)
+                        .statusMessage(root.getMessage()).create());
+    }
+
+    /**
+     * Used to decrease the size of a Gremlin script that triggered a "method too large" exception so that it
+     * doesn't log a massive text string nor return a large error message.
+     */
+    private RequestMessage trimMessage(final RequestMessage msg) {
+        final RequestMessage trimmedMsg = RequestMessage.from(msg).create();
+        if (trimmedMsg.getArgs().containsKey(Tokens.ARGS_GREMLIN))
+            trimmedMsg.getArgs().put(Tokens.ARGS_GREMLIN, trimmedMsg.getArgs().get(Tokens.ARGS_GREMLIN).toString().substring(0, 1021) + "...");
+
+        return trimmedMsg;
+    }
+
+    /**
+     * Check if any exception in the chain is TemporaryException then we should respond with the right error code so
+     * that the client knows to retry.
+     */
+    protected Optional<Throwable> determineIfTemporaryException(final Throwable ex) {
+        return Stream.of(ExceptionUtils.getThrowables(ex)).
+                filter(i -> i instanceof TemporaryException).findFirst();
+    }
+
+    @Override
+    public synchronized void close() {
+        // already closing/closed
+        if (!sessions.containsKey(sessionId)) return;
+
+        sessions.remove(sessionId);
+
+        if (sessionCancelFuture.get() != null) {
+            final ScheduledFuture<?> f = sessionCancelFuture.get();
+            if (!f.isDone()) f.cancel(true);
+        }
+    }
+
+    protected Iterator<?> fromScript(final Context gremlinContext, final String script) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final String language = args.containsKey(Tokens.ARGS_LANGUAGE) ? (String) args.get(Tokens.ARGS_LANGUAGE) : "gremlin-groovy";
+        return IteratorUtils.asIterator(getScriptEngine(gremlinContext, language).eval(
+                script, mergeBindingsFromRequest(gremlinContext, getWorkerBindings())));
+    }
+
+    protected Optional<Iterator<?>> fromBytecode(final Context gremlinContext, final Bytecode bytecode) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+
+        final Traversal.Admin<?, ?> traversal;
+        final Map<String, String> aliases = (Map<String, String>) msg.optionalArgs(Tokens.ARGS_ALIASES).get();
+        final GraphManager graphManager = gremlinContext.getGraphManager();
+        final String traversalSourceName = aliases.entrySet().iterator().next().getValue();
+        final TraversalSource g = graphManager.getTraversalSource(traversalSourceName);
+
+        // handle bytecode based graph operations like commit/rollback commands
+        if (BytecodeHelper.isGraphOperation(bytecode)) {
+            handleGraphOperation(gremlinContext, bytecode, g.getGraph());
+            return Optional.empty();
+        } else {
+
+            final Optional<String> lambdaLanguage = BytecodeHelper.getLambdaLanguage(bytecode);
+            if (!lambdaLanguage.isPresent())
+                traversal = JavaTranslator.of(g).translate(bytecode);
+            else {
+                final SimpleBindings bindings = new SimpleBindings();
+                bindings.put(traversalSourceName, g);
+                traversal = gremlinContext.getGremlinExecutor().getScriptEngineManager().getEngineByName(lambdaLanguage.get()).eval(bytecode, bindings, traversalSourceName);
+            }
+
+            // compile the traversal - without it getEndStep() has nothing in it
+            traversal.applyStrategies();
+
+            return Optional.of(new TraverserIterator(traversal));
+        }
+    }
+
+    protected Bindings getWorkerBindings() throws RexsterException {
+        return new SimpleBindings(graphManager.getAsBindings());
+    }
+
+    protected Bindings mergeBindingsFromRequest(final Context gremlinContext, final Bindings bindings) throws RexsterException {
+        // alias any global bindings to a different variable.
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (msg.getArgs().containsKey(Tokens.ARGS_ALIASES)) {
+            final Map<String, String> aliases = (Map<String, String>) msg.getArgs().get(Tokens.ARGS_ALIASES);
+            for (Map.Entry<String,String> aliasKv : aliases.entrySet()) {
+                boolean found = false;
+
+                // first check if the alias refers to a Graph instance
+                final Graph graph = gremlinContext.getGraphManager().getGraph(aliasKv.getValue());
+                if (null != graph) {
+                    bindings.put(aliasKv.getKey(), graph);
+                    found = true;
+                }
+
+                // if the alias wasn't found as a Graph then perhaps it is a TraversalSource - it needs to be
+                // something
+                if (!found) {
+                    final TraversalSource ts = gremlinContext.getGraphManager().getTraversalSource(aliasKv.getValue());
+                    if (null != ts) {
+                        bindings.put(aliasKv.getKey(), ts);
+                        found = true;
+                    }
+                }
+
+                // this validation is important to calls to GraphManager.commit() and rollback() as they both
+                // expect that the aliases supplied are valid
+                if (!found) {
+                    final String error = String.format("Could not alias [%s] to [%s] as [%s] not in the Graph or TraversalSource global bindings",
+                            aliasKv.getKey(), aliasKv.getValue(), aliasKv.getValue());
+                    throw new RexsterException(error, ResponseMessage.build(msg)
+                            .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+                }
+            }
+        } else {
+            // there's no bindings so determine if that's ok with Gremlin Server
+            if (gremlinContext.getSettings().strictTransactionManagement) {
+                final String error = "Gremlin Server is configured with strictTransactionManagement as 'true' - the 'aliases' arguments must be provided";
+                throw new RexsterException(error, ResponseMessage.build(msg)
+                        .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+            }
+        }
+
+        // add any bindings to override any other supplied
+        Optional.ofNullable((Map<String, Object>) msg.getArgs().get(Tokens.ARGS_BINDINGS)).ifPresent(bindings::putAll);
+        return bindings;
+    }
+
+    /**
+     * Provides a generic way of iterating a result set back to the client.
+     *
+     * @param gremlinContext The Gremlin Server {@link Context} object containing settings, request message, etc.
+     * @param itty The result to iterator
+     */
+    protected void handleIterator(final Context gremlinContext, final Iterator<?> itty) throws InterruptedException {
+        final ChannelHandlerContext nettyContext = gremlinContext.getChannelHandlerContext();
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Settings settings = gremlinContext.getSettings();
+        boolean warnOnce = false;
+
+        // sessionless requests are always transaction managed, but in-session requests are configurable.
+        final boolean managedTransactionsForRequest = transactionManaged ?
+                true : (Boolean) msg.getArgs().getOrDefault(Tokens.ARGS_MANAGE_TRANSACTION, false);
+
+        // we have an empty iterator - happens on stuff like: g.V().iterate()
+        if (!itty.hasNext()) {
+            final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,ResponseStatusCode.NO_CONTENT, itty);
+            // as there is nothing left to iterate if we are transaction managed then we should execute a
+            // commit here before we send back a NO_CONTENT which implies success
+            if (managedTransactionsForRequest)
+                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+            gremlinContext.writeAndFlush(ResponseMessage.build(msg)
+                    .code(ResponseStatusCode.NO_CONTENT)
+                    .statusAttributes(attributes)
+                    .create());
+            return;
+        }
+
+        // the batch size can be overridden by the request
+        final int resultIterationBatchSize = (Integer) msg.optionalArgs(Tokens.ARGS_BATCH_SIZE)
+                .orElse(settings.resultIterationBatchSize);
+        List<Object> aggregate = new ArrayList<>(resultIterationBatchSize);
+
+        // use an external control to manage the loop as opposed to just checking hasNext() in the while.  this
+        // prevent situations where auto transactions create a new transaction after calls to commit() withing
+        // the loop on calls to hasNext().
+        boolean hasMore = itty.hasNext();
+
+        while (hasMore) {
+            if (Thread.interrupted()) throw new InterruptedException();
+
+            // check if an implementation needs to force flush the aggregated results before the iteration batch
+            // size is reached.
+            // todo: what implementation does this?! can we kill it going forward - seems always false
+            // final boolean forceFlush = isForceFlushed(nettyContext, msg, itty);
+            final boolean forceFlush = false;
+
+            // have to check the aggregate size because it is possible that the channel is not writeable (below)
+            // so iterating next() if the message is not written and flushed would bump the aggregate size beyond
+            // the expected resultIterationBatchSize.  Total serialization time for the response remains in
+            // effect so if the client is "slow" it may simply timeout.
+            //
+            // there is a need to check hasNext() on the iterator because if the channel is not writeable the
+            // previous pass through the while loop will have next()'d the iterator and if it is "done" then a
+            // NoSuchElementException will raise its head. also need a check to ensure that this iteration doesn't
+            // require a forced flush which can be forced by sub-classes.
+            //
+            // this could be placed inside the isWriteable() portion of the if-then below but it seems better to
+            // allow iteration to continue into a batch if that is possible rather than just doing nothing at all
+            // while waiting for the client to catch up
+            if (aggregate.size() < resultIterationBatchSize && itty.hasNext() && !forceFlush) aggregate.add(itty.next());
+
+            // Don't keep executor busy if client has already given up; there is no way to catch up if the channel is
+            // not active, and hence we should break the loop.
+            if (!nettyContext.channel().isActive()) {
+                if (managedTransactionsForRequest) {
+                    closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                }
+                break;
+            }
+
+            // send back a page of results if batch size is met or if it's the end of the results being iterated.
+            // also check writeability of the channel to prevent OOME for slow clients.
+            //
+            // clients might decide to close the Netty channel to the server with a CloseWebsocketFrame after errors
+            // like CorruptedFrameException. On the server, although the channel gets closed, there might be some
+            // executor threads waiting for watermark to clear which will not clear in these cases since client has
+            // already given up on these requests. This leads to these executors waiting for the client to consume
+            // results till the timeout. checking for isActive() should help prevent that.
+            if (nettyContext.channel().isActive() && nettyContext.channel().isWritable()) {
+                if (forceFlush || aggregate.size() == resultIterationBatchSize || !itty.hasNext()) {
+                    final ResponseStatusCode code = itty.hasNext() ? ResponseStatusCode.PARTIAL_CONTENT : ResponseStatusCode.SUCCESS;
+                    Frame frame = null;
+                    try {
+                        frame = makeFrame(gremlinContext, aggregate, code, itty);
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+
+                        // exception is handled in makeFrame() - serialization error gets written back to driver
+                        // at that point
+                        if (managedTransactionsForRequest)
+                            closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                        break;
+                    }
+
+                    // track whether there is anything left in the iterator because it needs to be accessed after
+                    // the transaction could be closed - in that case a call to hasNext() could open a new transaction
+                    // unintentionally
+                    final boolean moreInIterator = itty.hasNext();
+
+                    try {
+                        // only need to reset the aggregation list if there's more stuff to write
+                        if (moreInIterator)
+                            aggregate = new ArrayList<>(resultIterationBatchSize);
+                        else {
+                            // iteration and serialization are both complete which means this finished successfully. note that
+                            // errors internal to script eval or timeout will rollback given GremlinServer's global configurations.
+                            // local errors will get rolledback below because the exceptions aren't thrown in those cases to be
+                            // caught by the GremlinExecutor for global rollback logic. this only needs to be committed if
+                            // there are no more items to iterate and serialization is complete
+                            if (managedTransactionsForRequest)
+                                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+                            // exit the result iteration loop as there are no more results left.  using this external control
+                            // because of the above commit.  some graphs may open a new transaction on the call to
+                            // hasNext()
+                            hasMore = false;
+                        }
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+                        throw ex;
+                    }
+
+                    if (!moreInIterator) iterateComplete(gremlinContext, itty);
+
+                    // the flush is called after the commit has potentially occurred.  in this way, if a commit was
+                    // required then it will be 100% complete before the client receives it. the "frame" at this point
+                    // should have completely detached objects from the transaction (i.e. serialization has occurred)
+                    // so a new one should not be opened on the flush down the netty pipeline
+                    gremlinContext.writeAndFlush(code, frame);
+                }
+            } else {
+                // don't keep triggering this warning over and over again for the same request
+                if (!warnOnce) {
+                    logger.warn("Pausing response writing as writeBufferHighWaterMark exceeded on {} - writing will continue once client has caught up", msg);
+                    warnOnce = true;
+                }
+
+                // since the client is lagging we can hold here for a period of time for the client to catch up.
+                // this isn't blocking the IO thread - just a worker.
+                TimeUnit.MILLISECONDS.sleep(10);
+            }
+        }
+    }
+
+    /**
+     * If {@link Bytecode} is detected to contain a "graph operation" then it gets processed by this method.
+     */
+    protected void handleGraphOperation(final Context gremlinContext, final Bytecode bytecode, final Graph graph) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (graph.features().graph().supportsTransactions()) {
+            if (bytecode.equals(Bytecode.TX_COMMIT) || bytecode.equals(Bytecode.TX_ROLLBACK)) {
+                final boolean commit = bytecode.equals(Bytecode.TX_COMMIT);
+                closeTransaction(gremlinContext, commit ? Transaction.Status.COMMIT : Transaction.Status.ROLLBACK);
+
+                // write back a no-op for success
+                final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,
+                        ResponseStatusCode.NO_CONTENT, Collections.emptyIterator());
+                gremlinContext.writeAndFlush(ResponseMessage.build(msg)

Review comment:
       what happens when a channel is closed at this time and writeAndFlush fails? this will happen when client is sends a message for commit and closes immediately without waiting for ack from server. Perhaps we should eat up the exception here and add a warn that we weren't able to send ack to the client. 

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/Rexster.java
##########
@@ -0,0 +1,89 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import io.netty.channel.Channel;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.server.Context;
+
+import java.util.concurrent.Future;
+import java.util.concurrent.ScheduledFuture;
+
+/**
+ * The {@code Rexster} interface is essentially a form of "worker", named for Gremlin's trusty canine robot friend

Review comment:
       Is it a form of "worker" or form of "work"? Since it implements Runnable, it seems like Rexter is a unit of work that is processed by RexterExecutorService.
   
   This implements runnable (which signifies that it is a standalone task) and yet it's implementations have a addTask() method and contains executorService as it's member?
   
   Here's a suggestion, let me know what you think about it:
   - GremlinOpExecutor is a service backed which executes a GremlinOpTask. 
   - GremlinOp can either be TransactionOp or QueryOp etc.
   - GremlinOpExecutor has lifecycle states which begins at server start up and ends on server shutdown.
   - GremlinOpTask to GremlinOpExecutor are provided from a Netty Event loop thread pool (which we shouldn't block) and hence, any underlying implementation should delegate to it's own thread pool ASAP.
   - Interface SessionExecutor containing methods like setSessionThread etc.
   - The concrete implementation, MultiTaskGremlinOpExecutor can execute multiple GremlinOps at the same time (using a queue and worker thread pool underneath) and it also implements SessionExecutor. 
   - GremlinOpExecutor should have methods such as:
      - Future executeGremlinOpAsync
      
      
   I am suggesting the above model so that we can get some unification in vocabulary used in the code base. Currently we have terms such as task, job, rexter, worker, executor, graph operation etc. sued across the code base without any concrete meaning to them which makes it difficult to read code.
      

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {
+            if (closeReason.compareAndSet(null, CloseReason.CHANNEL_CLOSED)) {
+                // cancel session worker or it will keep waiting for items to appear in the session queue
+                cancel(true);
+                close();
+            }
+        });
+        this.sessions = sessions;
+        this.graphManager = gremlinContext.getGraphManager();
+    }
+
+    protected synchronized void cancel(final boolean mayInterruptIfRunning) {
+        final FutureTask<?> sf = (FutureTask) sessionFuture.get();
+        if (sf != null && !sf.isDone()) {
+            sf.cancel(mayInterruptIfRunning);
+        }
+    }
+
+    public boolean isTransactionManaged() {
+        return transactionManaged;
+    }
+
+    public String getSessionId() {
+        return sessionId;
+    }
+
+    public boolean isBoundTo(final Channel channel) {
+        return channel == initialChannel;
+    }
+
+    public long getActualTimeoutLengthWhenClosed() {
+        return actualTimeoutLengthWhenClosed;
+    }
+
+    public Optional<CloseReason> getCloseReason() {
+        return Optional.ofNullable(closeReason.get());
+    }
+
+    public GremlinScriptEngine getScriptEngine(final Context context, final String language) {
+        return context.getGremlinExecutor().getScriptEngineManager().getEngineByName(language);
+    }
+
+    public void setSessionThread(final Thread runner) {
+        this.sessionThread = runner;
+    }
+
+    @Override
+    public void setSessionCancelFuture(final ScheduledFuture<?> f) {
+        if (!sessionCancelFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session cancellation future is already set");
+    }
+
+    @Override
+    public void setSessionFuture(final Future<?> f) {
+        if (!sessionFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session future is already set");
+    }
+
+    @Override
+    public synchronized void triggerTimeout(final long timeout, final boolean causedBySession) {
+        // triggering timeout triggers the stop of the Rexster Runnable which will end in close()
+        // for final cleanup
+        final Future<?> f = sessionFuture.get();
+        if (f != null && !f.isDone()) {
+            if (closeReason.compareAndSet(null, causedBySession ? CloseReason.SESSION_TIMEOUT : CloseReason.REQUEST_TIMEOUT)) {
+                actualTimeoutLengthWhenClosed = timeout;
+
+                // if caused by a session timeout for a session OR if it is a request timeout for a sessionless
+                // request then we can just straight cancel() the Rexster instance
+                if (causedBySession || !sessionIdOnRequest)
+                    cancel(true);
+                else
+                    sessionThread.interrupt();
+            }
+        }
+    }
+
+    protected void process(final Context gremlinContext) throws RexsterException {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final Object gremlinToExecute = args.get(Tokens.ARGS_GREMLIN);
+
+        // for strict transactions track the aliases used so that we can commit them and only them on close()
+        if (gremlinContext.getSettings().strictTransactionManagement)
+            msg.optionalArgs(Tokens.ARGS_ALIASES).ifPresent(m -> aliasesUsedByRexster.addAll(((Map<String,String>) m).values()));
+
+        try {
+            // itty is optional as Bytecode could be a "graph operation" rather than a Traversal. graph operations
+            // don't need to be iterated and handle their own lifecycle
+            final Optional<Iterator<?>> itty = gremlinToExecute instanceof Bytecode ?
+                    fromBytecode(gremlinContext, (Bytecode) gremlinToExecute) :
+                    Optional.of(fromScript(gremlinContext, (String) gremlinToExecute));
+
+            processAuditLog(gremlinContext.getSettings(), gremlinContext.getChannelHandlerContext(), gremlinToExecute);
+
+            if (itty.isPresent())
+                handleIterator(gremlinContext, itty.get());
+        } catch (Exception ex) {
+            handleException(gremlinContext, ex);
+        }
+    }
+
+    protected void handleException(final Context gremlinContext, final Throwable t) throws RexsterException {
+        if (t instanceof RexsterException) throw (RexsterException) t;
+
+        final Optional<Throwable> possibleTemporaryException = determineIfTemporaryException(t);
+        if (possibleTemporaryException.isPresent()) {
+            final Throwable temporaryException = possibleTemporaryException.get();
+            throw new RexsterException(temporaryException.getMessage(), t,
+                    ResponseMessage.build(gremlinContext.getRequestMessage())
+                            .code(ResponseStatusCode.SERVER_ERROR_TEMPORARY)
+                            .statusMessage(temporaryException.getMessage())
+                            .statusAttributeException(temporaryException).create());
+        }
+
+        final Throwable root = ExceptionUtils.getRootCause(t);
+
+        if (root instanceof TimedInterruptTimeoutException) {
+            // occurs when the TimedInterruptCustomizerProvider is in play
+            final String msg = String.format("A timeout occurred within the script during evaluation of [%s] - consider increasing the limit given to TimedInterruptCustomizerProvider",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage("Timeout during script evaluation triggered by TimedInterruptCustomizerProvider")
+                    .create());
+        }
+
+        if (root instanceof TimeoutException) {
+            final String errorMessage = String.format("Script evaluation exceeded the configured threshold for request [%s]",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage(t.getMessage())
+                    .create());
+        }
+
+        if (root instanceof InterruptedException ||
+                root instanceof TraversalInterruptedException ||
+                root instanceof InterruptedIOException) {
+            String msg = "Processing interrupted but the reason why was not known";
+            switch (closeReason.get()) {
+                case CHANNEL_CLOSED:
+                    msg = "Processing interrupted because the channel was closed";
+                    break;
+                case SESSION_TIMEOUT:
+                    msg = String.format("Session closed - %s - sessionLifetimeTimeout of %s ms exceeded", sessionId, actualTimeoutLengthWhenClosed);
+                    break;
+                case REQUEST_TIMEOUT:
+                    msg = String.format("Evaluation exceeded timeout threshold of %s ms", actualTimeoutLengthWhenClosed);
+                    break;
+            }
+            final ResponseStatusCode code = closeReason.get() == CloseReason.SESSION_TIMEOUT || closeReason.get() == CloseReason.REQUEST_TIMEOUT ?
+                    ResponseStatusCode.SERVER_ERROR_TIMEOUT : ResponseStatusCode.SERVER_ERROR;
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(code)
+                    .statusMessage(msg).create());
+        }
+
+        if (root instanceof MultipleCompilationErrorsException && root.getMessage().contains("Method too large") &&
+                ((MultipleCompilationErrorsException) root).getErrorCollector().getErrorCount() == 1) {
+            final String errorMessage = String.format("The Gremlin statement that was submitted exceeds the maximum compilation size allowed by the JVM, please split it into multiple smaller statements - %s", trimMessage(gremlinContext.getRequestMessage()));
+            logger.warn(errorMessage);
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(errorMessage)
+                    .statusAttributeException(root).create());
+        }
+
+        // GroovyRuntimeException will hit a pretty wide range of eval type errors, like MissingPropertyException,
+        // CompilationFailedException, MissingMethodException, etc. If more specific handling is required then
+        // try to catch it earlier above.
+        if (root instanceof GroovyRuntimeException ||
+                root instanceof VerificationException ||
+                root instanceof ScriptException) {
+            throw new RexsterException(root.getMessage(), root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(root.getMessage())
+                    .statusAttributeException(root).create());
+        }
+
+        throw new RexsterException(root.getClass().getSimpleName() + ": " + root.getMessage(), root,
+                ResponseMessage.build(gremlinContext.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusAttributeException(root)
+                        .statusMessage(root.getMessage()).create());
+    }
+
+    /**
+     * Used to decrease the size of a Gremlin script that triggered a "method too large" exception so that it
+     * doesn't log a massive text string nor return a large error message.
+     */
+    private RequestMessage trimMessage(final RequestMessage msg) {
+        final RequestMessage trimmedMsg = RequestMessage.from(msg).create();
+        if (trimmedMsg.getArgs().containsKey(Tokens.ARGS_GREMLIN))
+            trimmedMsg.getArgs().put(Tokens.ARGS_GREMLIN, trimmedMsg.getArgs().get(Tokens.ARGS_GREMLIN).toString().substring(0, 1021) + "...");
+
+        return trimmedMsg;
+    }
+
+    /**
+     * Check if any exception in the chain is TemporaryException then we should respond with the right error code so
+     * that the client knows to retry.
+     */
+    protected Optional<Throwable> determineIfTemporaryException(final Throwable ex) {
+        return Stream.of(ExceptionUtils.getThrowables(ex)).
+                filter(i -> i instanceof TemporaryException).findFirst();
+    }
+
+    @Override
+    public synchronized void close() {
+        // already closing/closed
+        if (!sessions.containsKey(sessionId)) return;
+
+        sessions.remove(sessionId);
+
+        if (sessionCancelFuture.get() != null) {
+            final ScheduledFuture<?> f = sessionCancelFuture.get();
+            if (!f.isDone()) f.cancel(true);
+        }
+    }
+
+    protected Iterator<?> fromScript(final Context gremlinContext, final String script) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final String language = args.containsKey(Tokens.ARGS_LANGUAGE) ? (String) args.get(Tokens.ARGS_LANGUAGE) : "gremlin-groovy";
+        return IteratorUtils.asIterator(getScriptEngine(gremlinContext, language).eval(
+                script, mergeBindingsFromRequest(gremlinContext, getWorkerBindings())));
+    }
+
+    protected Optional<Iterator<?>> fromBytecode(final Context gremlinContext, final Bytecode bytecode) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+
+        final Traversal.Admin<?, ?> traversal;
+        final Map<String, String> aliases = (Map<String, String>) msg.optionalArgs(Tokens.ARGS_ALIASES).get();
+        final GraphManager graphManager = gremlinContext.getGraphManager();
+        final String traversalSourceName = aliases.entrySet().iterator().next().getValue();
+        final TraversalSource g = graphManager.getTraversalSource(traversalSourceName);
+
+        // handle bytecode based graph operations like commit/rollback commands
+        if (BytecodeHelper.isGraphOperation(bytecode)) {
+            handleGraphOperation(gremlinContext, bytecode, g.getGraph());
+            return Optional.empty();
+        } else {
+
+            final Optional<String> lambdaLanguage = BytecodeHelper.getLambdaLanguage(bytecode);
+            if (!lambdaLanguage.isPresent())
+                traversal = JavaTranslator.of(g).translate(bytecode);
+            else {
+                final SimpleBindings bindings = new SimpleBindings();
+                bindings.put(traversalSourceName, g);
+                traversal = gremlinContext.getGremlinExecutor().getScriptEngineManager().getEngineByName(lambdaLanguage.get()).eval(bytecode, bindings, traversalSourceName);
+            }
+
+            // compile the traversal - without it getEndStep() has nothing in it
+            traversal.applyStrategies();
+
+            return Optional.of(new TraverserIterator(traversal));
+        }
+    }
+
+    protected Bindings getWorkerBindings() throws RexsterException {
+        return new SimpleBindings(graphManager.getAsBindings());
+    }
+
+    protected Bindings mergeBindingsFromRequest(final Context gremlinContext, final Bindings bindings) throws RexsterException {
+        // alias any global bindings to a different variable.
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (msg.getArgs().containsKey(Tokens.ARGS_ALIASES)) {
+            final Map<String, String> aliases = (Map<String, String>) msg.getArgs().get(Tokens.ARGS_ALIASES);
+            for (Map.Entry<String,String> aliasKv : aliases.entrySet()) {
+                boolean found = false;
+
+                // first check if the alias refers to a Graph instance
+                final Graph graph = gremlinContext.getGraphManager().getGraph(aliasKv.getValue());
+                if (null != graph) {
+                    bindings.put(aliasKv.getKey(), graph);
+                    found = true;
+                }
+
+                // if the alias wasn't found as a Graph then perhaps it is a TraversalSource - it needs to be
+                // something
+                if (!found) {
+                    final TraversalSource ts = gremlinContext.getGraphManager().getTraversalSource(aliasKv.getValue());
+                    if (null != ts) {
+                        bindings.put(aliasKv.getKey(), ts);
+                        found = true;
+                    }
+                }
+
+                // this validation is important to calls to GraphManager.commit() and rollback() as they both
+                // expect that the aliases supplied are valid
+                if (!found) {
+                    final String error = String.format("Could not alias [%s] to [%s] as [%s] not in the Graph or TraversalSource global bindings",
+                            aliasKv.getKey(), aliasKv.getValue(), aliasKv.getValue());
+                    throw new RexsterException(error, ResponseMessage.build(msg)
+                            .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+                }
+            }
+        } else {
+            // there's no bindings so determine if that's ok with Gremlin Server
+            if (gremlinContext.getSettings().strictTransactionManagement) {
+                final String error = "Gremlin Server is configured with strictTransactionManagement as 'true' - the 'aliases' arguments must be provided";
+                throw new RexsterException(error, ResponseMessage.build(msg)
+                        .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+            }
+        }
+
+        // add any bindings to override any other supplied
+        Optional.ofNullable((Map<String, Object>) msg.getArgs().get(Tokens.ARGS_BINDINGS)).ifPresent(bindings::putAll);
+        return bindings;
+    }
+
+    /**
+     * Provides a generic way of iterating a result set back to the client.
+     *
+     * @param gremlinContext The Gremlin Server {@link Context} object containing settings, request message, etc.
+     * @param itty The result to iterator
+     */
+    protected void handleIterator(final Context gremlinContext, final Iterator<?> itty) throws InterruptedException {
+        final ChannelHandlerContext nettyContext = gremlinContext.getChannelHandlerContext();
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Settings settings = gremlinContext.getSettings();
+        boolean warnOnce = false;
+
+        // sessionless requests are always transaction managed, but in-session requests are configurable.
+        final boolean managedTransactionsForRequest = transactionManaged ?
+                true : (Boolean) msg.getArgs().getOrDefault(Tokens.ARGS_MANAGE_TRANSACTION, false);
+
+        // we have an empty iterator - happens on stuff like: g.V().iterate()
+        if (!itty.hasNext()) {
+            final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,ResponseStatusCode.NO_CONTENT, itty);
+            // as there is nothing left to iterate if we are transaction managed then we should execute a
+            // commit here before we send back a NO_CONTENT which implies success
+            if (managedTransactionsForRequest)
+                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+            gremlinContext.writeAndFlush(ResponseMessage.build(msg)
+                    .code(ResponseStatusCode.NO_CONTENT)
+                    .statusAttributes(attributes)
+                    .create());
+            return;
+        }
+
+        // the batch size can be overridden by the request
+        final int resultIterationBatchSize = (Integer) msg.optionalArgs(Tokens.ARGS_BATCH_SIZE)
+                .orElse(settings.resultIterationBatchSize);
+        List<Object> aggregate = new ArrayList<>(resultIterationBatchSize);
+
+        // use an external control to manage the loop as opposed to just checking hasNext() in the while.  this
+        // prevent situations where auto transactions create a new transaction after calls to commit() withing
+        // the loop on calls to hasNext().
+        boolean hasMore = itty.hasNext();
+
+        while (hasMore) {
+            if (Thread.interrupted()) throw new InterruptedException();
+
+            // check if an implementation needs to force flush the aggregated results before the iteration batch
+            // size is reached.
+            // todo: what implementation does this?! can we kill it going forward - seems always false
+            // final boolean forceFlush = isForceFlushed(nettyContext, msg, itty);
+            final boolean forceFlush = false;
+
+            // have to check the aggregate size because it is possible that the channel is not writeable (below)
+            // so iterating next() if the message is not written and flushed would bump the aggregate size beyond
+            // the expected resultIterationBatchSize.  Total serialization time for the response remains in
+            // effect so if the client is "slow" it may simply timeout.
+            //
+            // there is a need to check hasNext() on the iterator because if the channel is not writeable the
+            // previous pass through the while loop will have next()'d the iterator and if it is "done" then a
+            // NoSuchElementException will raise its head. also need a check to ensure that this iteration doesn't
+            // require a forced flush which can be forced by sub-classes.
+            //
+            // this could be placed inside the isWriteable() portion of the if-then below but it seems better to
+            // allow iteration to continue into a batch if that is possible rather than just doing nothing at all
+            // while waiting for the client to catch up
+            if (aggregate.size() < resultIterationBatchSize && itty.hasNext() && !forceFlush) aggregate.add(itty.next());
+
+            // Don't keep executor busy if client has already given up; there is no way to catch up if the channel is
+            // not active, and hence we should break the loop.
+            if (!nettyContext.channel().isActive()) {
+                if (managedTransactionsForRequest) {
+                    closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                }
+                break;
+            }
+
+            // send back a page of results if batch size is met or if it's the end of the results being iterated.
+            // also check writeability of the channel to prevent OOME for slow clients.
+            //
+            // clients might decide to close the Netty channel to the server with a CloseWebsocketFrame after errors
+            // like CorruptedFrameException. On the server, although the channel gets closed, there might be some
+            // executor threads waiting for watermark to clear which will not clear in these cases since client has
+            // already given up on these requests. This leads to these executors waiting for the client to consume
+            // results till the timeout. checking for isActive() should help prevent that.
+            if (nettyContext.channel().isActive() && nettyContext.channel().isWritable()) {
+                if (forceFlush || aggregate.size() == resultIterationBatchSize || !itty.hasNext()) {
+                    final ResponseStatusCode code = itty.hasNext() ? ResponseStatusCode.PARTIAL_CONTENT : ResponseStatusCode.SUCCESS;
+                    Frame frame = null;
+                    try {
+                        frame = makeFrame(gremlinContext, aggregate, code, itty);
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+
+                        // exception is handled in makeFrame() - serialization error gets written back to driver
+                        // at that point
+                        if (managedTransactionsForRequest)
+                            closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                        break;
+                    }
+
+                    // track whether there is anything left in the iterator because it needs to be accessed after
+                    // the transaction could be closed - in that case a call to hasNext() could open a new transaction
+                    // unintentionally
+                    final boolean moreInIterator = itty.hasNext();
+
+                    try {
+                        // only need to reset the aggregation list if there's more stuff to write
+                        if (moreInIterator)
+                            aggregate = new ArrayList<>(resultIterationBatchSize);
+                        else {
+                            // iteration and serialization are both complete which means this finished successfully. note that
+                            // errors internal to script eval or timeout will rollback given GremlinServer's global configurations.
+                            // local errors will get rolledback below because the exceptions aren't thrown in those cases to be
+                            // caught by the GremlinExecutor for global rollback logic. this only needs to be committed if
+                            // there are no more items to iterate and serialization is complete
+                            if (managedTransactionsForRequest)
+                                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+                            // exit the result iteration loop as there are no more results left.  using this external control
+                            // because of the above commit.  some graphs may open a new transaction on the call to
+                            // hasNext()
+                            hasMore = false;
+                        }
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+                        throw ex;
+                    }
+
+                    if (!moreInIterator) iterateComplete(gremlinContext, itty);
+
+                    // the flush is called after the commit has potentially occurred.  in this way, if a commit was
+                    // required then it will be 100% complete before the client receives it. the "frame" at this point
+                    // should have completely detached objects from the transaction (i.e. serialization has occurred)
+                    // so a new one should not be opened on the flush down the netty pipeline
+                    gremlinContext.writeAndFlush(code, frame);
+                }
+            } else {
+                // don't keep triggering this warning over and over again for the same request
+                if (!warnOnce) {
+                    logger.warn("Pausing response writing as writeBufferHighWaterMark exceeded on {} - writing will continue once client has caught up", msg);
+                    warnOnce = true;
+                }
+
+                // since the client is lagging we can hold here for a period of time for the client to catch up.
+                // this isn't blocking the IO thread - just a worker.
+                TimeUnit.MILLISECONDS.sleep(10);
+            }
+        }
+    }
+
+    /**
+     * If {@link Bytecode} is detected to contain a "graph operation" then it gets processed by this method.
+     */
+    protected void handleGraphOperation(final Context gremlinContext, final Bytecode bytecode, final Graph graph) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (graph.features().graph().supportsTransactions()) {
+            if (bytecode.equals(Bytecode.TX_COMMIT) || bytecode.equals(Bytecode.TX_ROLLBACK)) {
+                final boolean commit = bytecode.equals(Bytecode.TX_COMMIT);
+                closeTransaction(gremlinContext, commit ? Transaction.Status.COMMIT : Transaction.Status.ROLLBACK);
+
+                // write back a no-op for success
+                final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,
+                        ResponseStatusCode.NO_CONTENT, Collections.emptyIterator());
+                gremlinContext.writeAndFlush(ResponseMessage.build(msg)
+                            .code(ResponseStatusCode.NO_CONTENT)

Review comment:
       Why aren't we sending 200 i.e. ResponseStatusCode.SUCCESS?

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {

Review comment:
       When we are iterating for results, we do something similar where we try to see if channel is inactive, then we throw an exception. would closing the rexter over here interrupt the underlying execution as well? do we handle the race condition between the exception I mentioned above vs. close here?

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {
+            if (closeReason.compareAndSet(null, CloseReason.CHANNEL_CLOSED)) {
+                // cancel session worker or it will keep waiting for items to appear in the session queue
+                cancel(true);
+                close();
+            }
+        });
+        this.sessions = sessions;
+        this.graphManager = gremlinContext.getGraphManager();
+    }
+
+    protected synchronized void cancel(final boolean mayInterruptIfRunning) {
+        final FutureTask<?> sf = (FutureTask) sessionFuture.get();
+        if (sf != null && !sf.isDone()) {
+            sf.cancel(mayInterruptIfRunning);
+        }
+    }
+
+    public boolean isTransactionManaged() {
+        return transactionManaged;
+    }
+
+    public String getSessionId() {
+        return sessionId;
+    }
+
+    public boolean isBoundTo(final Channel channel) {
+        return channel == initialChannel;
+    }
+
+    public long getActualTimeoutLengthWhenClosed() {
+        return actualTimeoutLengthWhenClosed;
+    }
+
+    public Optional<CloseReason> getCloseReason() {
+        return Optional.ofNullable(closeReason.get());
+    }
+
+    public GremlinScriptEngine getScriptEngine(final Context context, final String language) {
+        return context.getGremlinExecutor().getScriptEngineManager().getEngineByName(language);
+    }
+
+    public void setSessionThread(final Thread runner) {
+        this.sessionThread = runner;
+    }
+
+    @Override
+    public void setSessionCancelFuture(final ScheduledFuture<?> f) {
+        if (!sessionCancelFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session cancellation future is already set");
+    }
+
+    @Override
+    public void setSessionFuture(final Future<?> f) {
+        if (!sessionFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session future is already set");
+    }
+
+    @Override
+    public synchronized void triggerTimeout(final long timeout, final boolean causedBySession) {
+        // triggering timeout triggers the stop of the Rexster Runnable which will end in close()
+        // for final cleanup
+        final Future<?> f = sessionFuture.get();
+        if (f != null && !f.isDone()) {
+            if (closeReason.compareAndSet(null, causedBySession ? CloseReason.SESSION_TIMEOUT : CloseReason.REQUEST_TIMEOUT)) {
+                actualTimeoutLengthWhenClosed = timeout;
+
+                // if caused by a session timeout for a session OR if it is a request timeout for a sessionless
+                // request then we can just straight cancel() the Rexster instance
+                if (causedBySession || !sessionIdOnRequest)
+                    cancel(true);
+                else
+                    sessionThread.interrupt();
+            }
+        }
+    }
+
+    protected void process(final Context gremlinContext) throws RexsterException {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final Object gremlinToExecute = args.get(Tokens.ARGS_GREMLIN);
+
+        // for strict transactions track the aliases used so that we can commit them and only them on close()
+        if (gremlinContext.getSettings().strictTransactionManagement)
+            msg.optionalArgs(Tokens.ARGS_ALIASES).ifPresent(m -> aliasesUsedByRexster.addAll(((Map<String,String>) m).values()));
+
+        try {
+            // itty is optional as Bytecode could be a "graph operation" rather than a Traversal. graph operations
+            // don't need to be iterated and handle their own lifecycle
+            final Optional<Iterator<?>> itty = gremlinToExecute instanceof Bytecode ?
+                    fromBytecode(gremlinContext, (Bytecode) gremlinToExecute) :
+                    Optional.of(fromScript(gremlinContext, (String) gremlinToExecute));
+
+            processAuditLog(gremlinContext.getSettings(), gremlinContext.getChannelHandlerContext(), gremlinToExecute);
+
+            if (itty.isPresent())
+                handleIterator(gremlinContext, itty.get());
+        } catch (Exception ex) {
+            handleException(gremlinContext, ex);
+        }
+    }
+
+    protected void handleException(final Context gremlinContext, final Throwable t) throws RexsterException {
+        if (t instanceof RexsterException) throw (RexsterException) t;
+
+        final Optional<Throwable> possibleTemporaryException = determineIfTemporaryException(t);
+        if (possibleTemporaryException.isPresent()) {
+            final Throwable temporaryException = possibleTemporaryException.get();
+            throw new RexsterException(temporaryException.getMessage(), t,
+                    ResponseMessage.build(gremlinContext.getRequestMessage())
+                            .code(ResponseStatusCode.SERVER_ERROR_TEMPORARY)
+                            .statusMessage(temporaryException.getMessage())
+                            .statusAttributeException(temporaryException).create());
+        }
+
+        final Throwable root = ExceptionUtils.getRootCause(t);
+
+        if (root instanceof TimedInterruptTimeoutException) {
+            // occurs when the TimedInterruptCustomizerProvider is in play
+            final String msg = String.format("A timeout occurred within the script during evaluation of [%s] - consider increasing the limit given to TimedInterruptCustomizerProvider",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage("Timeout during script evaluation triggered by TimedInterruptCustomizerProvider")
+                    .create());
+        }
+
+        if (root instanceof TimeoutException) {
+            final String errorMessage = String.format("Script evaluation exceeded the configured threshold for request [%s]",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage(t.getMessage())
+                    .create());
+        }
+
+        if (root instanceof InterruptedException ||
+                root instanceof TraversalInterruptedException ||
+                root instanceof InterruptedIOException) {
+            String msg = "Processing interrupted but the reason why was not known";
+            switch (closeReason.get()) {
+                case CHANNEL_CLOSED:
+                    msg = "Processing interrupted because the channel was closed";
+                    break;
+                case SESSION_TIMEOUT:
+                    msg = String.format("Session closed - %s - sessionLifetimeTimeout of %s ms exceeded", sessionId, actualTimeoutLengthWhenClosed);
+                    break;
+                case REQUEST_TIMEOUT:
+                    msg = String.format("Evaluation exceeded timeout threshold of %s ms", actualTimeoutLengthWhenClosed);
+                    break;
+            }
+            final ResponseStatusCode code = closeReason.get() == CloseReason.SESSION_TIMEOUT || closeReason.get() == CloseReason.REQUEST_TIMEOUT ?
+                    ResponseStatusCode.SERVER_ERROR_TIMEOUT : ResponseStatusCode.SERVER_ERROR;
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(code)
+                    .statusMessage(msg).create());
+        }
+
+        if (root instanceof MultipleCompilationErrorsException && root.getMessage().contains("Method too large") &&
+                ((MultipleCompilationErrorsException) root).getErrorCollector().getErrorCount() == 1) {
+            final String errorMessage = String.format("The Gremlin statement that was submitted exceeds the maximum compilation size allowed by the JVM, please split it into multiple smaller statements - %s", trimMessage(gremlinContext.getRequestMessage()));
+            logger.warn(errorMessage);
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(errorMessage)
+                    .statusAttributeException(root).create());
+        }
+
+        // GroovyRuntimeException will hit a pretty wide range of eval type errors, like MissingPropertyException,
+        // CompilationFailedException, MissingMethodException, etc. If more specific handling is required then
+        // try to catch it earlier above.
+        if (root instanceof GroovyRuntimeException ||
+                root instanceof VerificationException ||
+                root instanceof ScriptException) {
+            throw new RexsterException(root.getMessage(), root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(root.getMessage())
+                    .statusAttributeException(root).create());
+        }
+
+        throw new RexsterException(root.getClass().getSimpleName() + ": " + root.getMessage(), root,
+                ResponseMessage.build(gremlinContext.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusAttributeException(root)
+                        .statusMessage(root.getMessage()).create());
+    }
+
+    /**
+     * Used to decrease the size of a Gremlin script that triggered a "method too large" exception so that it
+     * doesn't log a massive text string nor return a large error message.
+     */
+    private RequestMessage trimMessage(final RequestMessage msg) {
+        final RequestMessage trimmedMsg = RequestMessage.from(msg).create();
+        if (trimmedMsg.getArgs().containsKey(Tokens.ARGS_GREMLIN))
+            trimmedMsg.getArgs().put(Tokens.ARGS_GREMLIN, trimmedMsg.getArgs().get(Tokens.ARGS_GREMLIN).toString().substring(0, 1021) + "...");
+
+        return trimmedMsg;
+    }
+
+    /**
+     * Check if any exception in the chain is TemporaryException then we should respond with the right error code so
+     * that the client knows to retry.
+     */
+    protected Optional<Throwable> determineIfTemporaryException(final Throwable ex) {
+        return Stream.of(ExceptionUtils.getThrowables(ex)).
+                filter(i -> i instanceof TemporaryException).findFirst();
+    }
+
+    @Override
+    public synchronized void close() {
+        // already closing/closed
+        if (!sessions.containsKey(sessionId)) return;
+
+        sessions.remove(sessionId);
+
+        if (sessionCancelFuture.get() != null) {
+            final ScheduledFuture<?> f = sessionCancelFuture.get();
+            if (!f.isDone()) f.cancel(true);
+        }
+    }
+
+    protected Iterator<?> fromScript(final Context gremlinContext, final String script) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final String language = args.containsKey(Tokens.ARGS_LANGUAGE) ? (String) args.get(Tokens.ARGS_LANGUAGE) : "gremlin-groovy";
+        return IteratorUtils.asIterator(getScriptEngine(gremlinContext, language).eval(
+                script, mergeBindingsFromRequest(gremlinContext, getWorkerBindings())));
+    }
+
+    protected Optional<Iterator<?>> fromBytecode(final Context gremlinContext, final Bytecode bytecode) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+
+        final Traversal.Admin<?, ?> traversal;
+        final Map<String, String> aliases = (Map<String, String>) msg.optionalArgs(Tokens.ARGS_ALIASES).get();
+        final GraphManager graphManager = gremlinContext.getGraphManager();
+        final String traversalSourceName = aliases.entrySet().iterator().next().getValue();
+        final TraversalSource g = graphManager.getTraversalSource(traversalSourceName);
+
+        // handle bytecode based graph operations like commit/rollback commands
+        if (BytecodeHelper.isGraphOperation(bytecode)) {
+            handleGraphOperation(gremlinContext, bytecode, g.getGraph());
+            return Optional.empty();
+        } else {
+
+            final Optional<String> lambdaLanguage = BytecodeHelper.getLambdaLanguage(bytecode);
+            if (!lambdaLanguage.isPresent())
+                traversal = JavaTranslator.of(g).translate(bytecode);
+            else {
+                final SimpleBindings bindings = new SimpleBindings();
+                bindings.put(traversalSourceName, g);
+                traversal = gremlinContext.getGremlinExecutor().getScriptEngineManager().getEngineByName(lambdaLanguage.get()).eval(bytecode, bindings, traversalSourceName);
+            }
+
+            // compile the traversal - without it getEndStep() has nothing in it
+            traversal.applyStrategies();
+
+            return Optional.of(new TraverserIterator(traversal));
+        }
+    }
+
+    protected Bindings getWorkerBindings() throws RexsterException {
+        return new SimpleBindings(graphManager.getAsBindings());
+    }
+
+    protected Bindings mergeBindingsFromRequest(final Context gremlinContext, final Bindings bindings) throws RexsterException {
+        // alias any global bindings to a different variable.
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (msg.getArgs().containsKey(Tokens.ARGS_ALIASES)) {
+            final Map<String, String> aliases = (Map<String, String>) msg.getArgs().get(Tokens.ARGS_ALIASES);
+            for (Map.Entry<String,String> aliasKv : aliases.entrySet()) {
+                boolean found = false;
+
+                // first check if the alias refers to a Graph instance
+                final Graph graph = gremlinContext.getGraphManager().getGraph(aliasKv.getValue());
+                if (null != graph) {
+                    bindings.put(aliasKv.getKey(), graph);
+                    found = true;
+                }
+
+                // if the alias wasn't found as a Graph then perhaps it is a TraversalSource - it needs to be
+                // something
+                if (!found) {
+                    final TraversalSource ts = gremlinContext.getGraphManager().getTraversalSource(aliasKv.getValue());
+                    if (null != ts) {
+                        bindings.put(aliasKv.getKey(), ts);
+                        found = true;
+                    }
+                }
+
+                // this validation is important to calls to GraphManager.commit() and rollback() as they both
+                // expect that the aliases supplied are valid
+                if (!found) {
+                    final String error = String.format("Could not alias [%s] to [%s] as [%s] not in the Graph or TraversalSource global bindings",
+                            aliasKv.getKey(), aliasKv.getValue(), aliasKv.getValue());
+                    throw new RexsterException(error, ResponseMessage.build(msg)
+                            .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+                }
+            }
+        } else {
+            // there's no bindings so determine if that's ok with Gremlin Server
+            if (gremlinContext.getSettings().strictTransactionManagement) {
+                final String error = "Gremlin Server is configured with strictTransactionManagement as 'true' - the 'aliases' arguments must be provided";
+                throw new RexsterException(error, ResponseMessage.build(msg)
+                        .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+            }
+        }
+
+        // add any bindings to override any other supplied
+        Optional.ofNullable((Map<String, Object>) msg.getArgs().get(Tokens.ARGS_BINDINGS)).ifPresent(bindings::putAll);
+        return bindings;
+    }
+
+    /**
+     * Provides a generic way of iterating a result set back to the client.
+     *
+     * @param gremlinContext The Gremlin Server {@link Context} object containing settings, request message, etc.
+     * @param itty The result to iterator
+     */
+    protected void handleIterator(final Context gremlinContext, final Iterator<?> itty) throws InterruptedException {
+        final ChannelHandlerContext nettyContext = gremlinContext.getChannelHandlerContext();
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Settings settings = gremlinContext.getSettings();
+        boolean warnOnce = false;
+
+        // sessionless requests are always transaction managed, but in-session requests are configurable.
+        final boolean managedTransactionsForRequest = transactionManaged ?
+                true : (Boolean) msg.getArgs().getOrDefault(Tokens.ARGS_MANAGE_TRANSACTION, false);
+
+        // we have an empty iterator - happens on stuff like: g.V().iterate()
+        if (!itty.hasNext()) {
+            final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,ResponseStatusCode.NO_CONTENT, itty);
+            // as there is nothing left to iterate if we are transaction managed then we should execute a
+            // commit here before we send back a NO_CONTENT which implies success
+            if (managedTransactionsForRequest)
+                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+            gremlinContext.writeAndFlush(ResponseMessage.build(msg)
+                    .code(ResponseStatusCode.NO_CONTENT)
+                    .statusAttributes(attributes)
+                    .create());
+            return;
+        }
+
+        // the batch size can be overridden by the request
+        final int resultIterationBatchSize = (Integer) msg.optionalArgs(Tokens.ARGS_BATCH_SIZE)
+                .orElse(settings.resultIterationBatchSize);
+        List<Object> aggregate = new ArrayList<>(resultIterationBatchSize);
+
+        // use an external control to manage the loop as opposed to just checking hasNext() in the while.  this
+        // prevent situations where auto transactions create a new transaction after calls to commit() withing
+        // the loop on calls to hasNext().
+        boolean hasMore = itty.hasNext();
+
+        while (hasMore) {
+            if (Thread.interrupted()) throw new InterruptedException();
+
+            // check if an implementation needs to force flush the aggregated results before the iteration batch
+            // size is reached.
+            // todo: what implementation does this?! can we kill it going forward - seems always false
+            // final boolean forceFlush = isForceFlushed(nettyContext, msg, itty);
+            final boolean forceFlush = false;
+
+            // have to check the aggregate size because it is possible that the channel is not writeable (below)
+            // so iterating next() if the message is not written and flushed would bump the aggregate size beyond
+            // the expected resultIterationBatchSize.  Total serialization time for the response remains in
+            // effect so if the client is "slow" it may simply timeout.
+            //
+            // there is a need to check hasNext() on the iterator because if the channel is not writeable the
+            // previous pass through the while loop will have next()'d the iterator and if it is "done" then a
+            // NoSuchElementException will raise its head. also need a check to ensure that this iteration doesn't
+            // require a forced flush which can be forced by sub-classes.
+            //
+            // this could be placed inside the isWriteable() portion of the if-then below but it seems better to
+            // allow iteration to continue into a batch if that is possible rather than just doing nothing at all
+            // while waiting for the client to catch up
+            if (aggregate.size() < resultIterationBatchSize && itty.hasNext() && !forceFlush) aggregate.add(itty.next());
+
+            // Don't keep executor busy if client has already given up; there is no way to catch up if the channel is
+            // not active, and hence we should break the loop.
+            if (!nettyContext.channel().isActive()) {
+                if (managedTransactionsForRequest) {
+                    closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);

Review comment:
       should log at debug since we are silently taking an action without any prompt from client. same case if we are breaking the sending of results.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {
+            if (closeReason.compareAndSet(null, CloseReason.CHANNEL_CLOSED)) {
+                // cancel session worker or it will keep waiting for items to appear in the session queue
+                cancel(true);

Review comment:
       cancel is already called from inside close

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {
+            if (closeReason.compareAndSet(null, CloseReason.CHANNEL_CLOSED)) {
+                // cancel session worker or it will keep waiting for items to appear in the session queue
+                cancel(true);
+                close();
+            }
+        });
+        this.sessions = sessions;
+        this.graphManager = gremlinContext.getGraphManager();
+    }
+
+    protected synchronized void cancel(final boolean mayInterruptIfRunning) {
+        final FutureTask<?> sf = (FutureTask) sessionFuture.get();
+        if (sf != null && !sf.isDone()) {
+            sf.cancel(mayInterruptIfRunning);
+        }
+    }
+
+    public boolean isTransactionManaged() {
+        return transactionManaged;
+    }
+
+    public String getSessionId() {
+        return sessionId;
+    }
+
+    public boolean isBoundTo(final Channel channel) {
+        return channel == initialChannel;
+    }
+
+    public long getActualTimeoutLengthWhenClosed() {
+        return actualTimeoutLengthWhenClosed;
+    }
+
+    public Optional<CloseReason> getCloseReason() {
+        return Optional.ofNullable(closeReason.get());
+    }
+
+    public GremlinScriptEngine getScriptEngine(final Context context, final String language) {
+        return context.getGremlinExecutor().getScriptEngineManager().getEngineByName(language);
+    }
+
+    public void setSessionThread(final Thread runner) {
+        this.sessionThread = runner;
+    }
+
+    @Override
+    public void setSessionCancelFuture(final ScheduledFuture<?> f) {
+        if (!sessionCancelFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session cancellation future is already set");
+    }
+
+    @Override
+    public void setSessionFuture(final Future<?> f) {
+        if (!sessionFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session future is already set");
+    }
+
+    @Override
+    public synchronized void triggerTimeout(final long timeout, final boolean causedBySession) {
+        // triggering timeout triggers the stop of the Rexster Runnable which will end in close()
+        // for final cleanup
+        final Future<?> f = sessionFuture.get();
+        if (f != null && !f.isDone()) {
+            if (closeReason.compareAndSet(null, causedBySession ? CloseReason.SESSION_TIMEOUT : CloseReason.REQUEST_TIMEOUT)) {
+                actualTimeoutLengthWhenClosed = timeout;
+
+                // if caused by a session timeout for a session OR if it is a request timeout for a sessionless
+                // request then we can just straight cancel() the Rexster instance
+                if (causedBySession || !sessionIdOnRequest)
+                    cancel(true);
+                else
+                    sessionThread.interrupt();
+            }
+        }
+    }
+
+    protected void process(final Context gremlinContext) throws RexsterException {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final Object gremlinToExecute = args.get(Tokens.ARGS_GREMLIN);
+
+        // for strict transactions track the aliases used so that we can commit them and only them on close()
+        if (gremlinContext.getSettings().strictTransactionManagement)
+            msg.optionalArgs(Tokens.ARGS_ALIASES).ifPresent(m -> aliasesUsedByRexster.addAll(((Map<String,String>) m).values()));
+
+        try {
+            // itty is optional as Bytecode could be a "graph operation" rather than a Traversal. graph operations
+            // don't need to be iterated and handle their own lifecycle
+            final Optional<Iterator<?>> itty = gremlinToExecute instanceof Bytecode ?
+                    fromBytecode(gremlinContext, (Bytecode) gremlinToExecute) :
+                    Optional.of(fromScript(gremlinContext, (String) gremlinToExecute));
+
+            processAuditLog(gremlinContext.getSettings(), gremlinContext.getChannelHandlerContext(), gremlinToExecute);
+
+            if (itty.isPresent())
+                handleIterator(gremlinContext, itty.get());
+        } catch (Exception ex) {
+            handleException(gremlinContext, ex);
+        }
+    }
+
+    protected void handleException(final Context gremlinContext, final Throwable t) throws RexsterException {
+        if (t instanceof RexsterException) throw (RexsterException) t;
+
+        final Optional<Throwable> possibleTemporaryException = determineIfTemporaryException(t);
+        if (possibleTemporaryException.isPresent()) {
+            final Throwable temporaryException = possibleTemporaryException.get();
+            throw new RexsterException(temporaryException.getMessage(), t,
+                    ResponseMessage.build(gremlinContext.getRequestMessage())
+                            .code(ResponseStatusCode.SERVER_ERROR_TEMPORARY)
+                            .statusMessage(temporaryException.getMessage())
+                            .statusAttributeException(temporaryException).create());
+        }
+
+        final Throwable root = ExceptionUtils.getRootCause(t);
+
+        if (root instanceof TimedInterruptTimeoutException) {
+            // occurs when the TimedInterruptCustomizerProvider is in play
+            final String msg = String.format("A timeout occurred within the script during evaluation of [%s] - consider increasing the limit given to TimedInterruptCustomizerProvider",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage("Timeout during script evaluation triggered by TimedInterruptCustomizerProvider")
+                    .create());
+        }
+
+        if (root instanceof TimeoutException) {
+            final String errorMessage = String.format("Script evaluation exceeded the configured threshold for request [%s]",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage(t.getMessage())
+                    .create());
+        }
+
+        if (root instanceof InterruptedException ||
+                root instanceof TraversalInterruptedException ||
+                root instanceof InterruptedIOException) {
+            String msg = "Processing interrupted but the reason why was not known";
+            switch (closeReason.get()) {
+                case CHANNEL_CLOSED:
+                    msg = "Processing interrupted because the channel was closed";
+                    break;
+                case SESSION_TIMEOUT:
+                    msg = String.format("Session closed - %s - sessionLifetimeTimeout of %s ms exceeded", sessionId, actualTimeoutLengthWhenClosed);
+                    break;
+                case REQUEST_TIMEOUT:
+                    msg = String.format("Evaluation exceeded timeout threshold of %s ms", actualTimeoutLengthWhenClosed);
+                    break;
+            }
+            final ResponseStatusCode code = closeReason.get() == CloseReason.SESSION_TIMEOUT || closeReason.get() == CloseReason.REQUEST_TIMEOUT ?
+                    ResponseStatusCode.SERVER_ERROR_TIMEOUT : ResponseStatusCode.SERVER_ERROR;
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(code)
+                    .statusMessage(msg).create());
+        }
+
+        if (root instanceof MultipleCompilationErrorsException && root.getMessage().contains("Method too large") &&
+                ((MultipleCompilationErrorsException) root).getErrorCollector().getErrorCount() == 1) {
+            final String errorMessage = String.format("The Gremlin statement that was submitted exceeds the maximum compilation size allowed by the JVM, please split it into multiple smaller statements - %s", trimMessage(gremlinContext.getRequestMessage()));
+            logger.warn(errorMessage);
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(errorMessage)
+                    .statusAttributeException(root).create());
+        }
+
+        // GroovyRuntimeException will hit a pretty wide range of eval type errors, like MissingPropertyException,
+        // CompilationFailedException, MissingMethodException, etc. If more specific handling is required then
+        // try to catch it earlier above.
+        if (root instanceof GroovyRuntimeException ||
+                root instanceof VerificationException ||
+                root instanceof ScriptException) {
+            throw new RexsterException(root.getMessage(), root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(root.getMessage())
+                    .statusAttributeException(root).create());
+        }
+
+        throw new RexsterException(root.getClass().getSimpleName() + ": " + root.getMessage(), root,
+                ResponseMessage.build(gremlinContext.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusAttributeException(root)
+                        .statusMessage(root.getMessage()).create());
+    }
+
+    /**
+     * Used to decrease the size of a Gremlin script that triggered a "method too large" exception so that it
+     * doesn't log a massive text string nor return a large error message.
+     */
+    private RequestMessage trimMessage(final RequestMessage msg) {
+        final RequestMessage trimmedMsg = RequestMessage.from(msg).create();
+        if (trimmedMsg.getArgs().containsKey(Tokens.ARGS_GREMLIN))
+            trimmedMsg.getArgs().put(Tokens.ARGS_GREMLIN, trimmedMsg.getArgs().get(Tokens.ARGS_GREMLIN).toString().substring(0, 1021) + "...");
+
+        return trimmedMsg;
+    }
+
+    /**
+     * Check if any exception in the chain is TemporaryException then we should respond with the right error code so
+     * that the client knows to retry.
+     */
+    protected Optional<Throwable> determineIfTemporaryException(final Throwable ex) {
+        return Stream.of(ExceptionUtils.getThrowables(ex)).
+                filter(i -> i instanceof TemporaryException).findFirst();
+    }
+
+    @Override
+    public synchronized void close() {
+        // already closing/closed
+        if (!sessions.containsKey(sessionId)) return;
+
+        sessions.remove(sessionId);
+
+        if (sessionCancelFuture.get() != null) {
+            final ScheduledFuture<?> f = sessionCancelFuture.get();
+            if (!f.isDone()) f.cancel(true);
+        }
+    }
+
+    protected Iterator<?> fromScript(final Context gremlinContext, final String script) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final String language = args.containsKey(Tokens.ARGS_LANGUAGE) ? (String) args.get(Tokens.ARGS_LANGUAGE) : "gremlin-groovy";
+        return IteratorUtils.asIterator(getScriptEngine(gremlinContext, language).eval(
+                script, mergeBindingsFromRequest(gremlinContext, getWorkerBindings())));
+    }
+
+    protected Optional<Iterator<?>> fromBytecode(final Context gremlinContext, final Bytecode bytecode) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+
+        final Traversal.Admin<?, ?> traversal;
+        final Map<String, String> aliases = (Map<String, String>) msg.optionalArgs(Tokens.ARGS_ALIASES).get();
+        final GraphManager graphManager = gremlinContext.getGraphManager();
+        final String traversalSourceName = aliases.entrySet().iterator().next().getValue();
+        final TraversalSource g = graphManager.getTraversalSource(traversalSourceName);
+
+        // handle bytecode based graph operations like commit/rollback commands
+        if (BytecodeHelper.isGraphOperation(bytecode)) {
+            handleGraphOperation(gremlinContext, bytecode, g.getGraph());
+            return Optional.empty();
+        } else {
+
+            final Optional<String> lambdaLanguage = BytecodeHelper.getLambdaLanguage(bytecode);
+            if (!lambdaLanguage.isPresent())
+                traversal = JavaTranslator.of(g).translate(bytecode);
+            else {
+                final SimpleBindings bindings = new SimpleBindings();
+                bindings.put(traversalSourceName, g);
+                traversal = gremlinContext.getGremlinExecutor().getScriptEngineManager().getEngineByName(lambdaLanguage.get()).eval(bytecode, bindings, traversalSourceName);
+            }
+
+            // compile the traversal - without it getEndStep() has nothing in it
+            traversal.applyStrategies();
+
+            return Optional.of(new TraverserIterator(traversal));
+        }
+    }
+
+    protected Bindings getWorkerBindings() throws RexsterException {
+        return new SimpleBindings(graphManager.getAsBindings());
+    }
+
+    protected Bindings mergeBindingsFromRequest(final Context gremlinContext, final Bindings bindings) throws RexsterException {
+        // alias any global bindings to a different variable.
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (msg.getArgs().containsKey(Tokens.ARGS_ALIASES)) {
+            final Map<String, String> aliases = (Map<String, String>) msg.getArgs().get(Tokens.ARGS_ALIASES);
+            for (Map.Entry<String,String> aliasKv : aliases.entrySet()) {
+                boolean found = false;
+
+                // first check if the alias refers to a Graph instance
+                final Graph graph = gremlinContext.getGraphManager().getGraph(aliasKv.getValue());
+                if (null != graph) {
+                    bindings.put(aliasKv.getKey(), graph);
+                    found = true;
+                }
+
+                // if the alias wasn't found as a Graph then perhaps it is a TraversalSource - it needs to be
+                // something
+                if (!found) {
+                    final TraversalSource ts = gremlinContext.getGraphManager().getTraversalSource(aliasKv.getValue());
+                    if (null != ts) {
+                        bindings.put(aliasKv.getKey(), ts);
+                        found = true;
+                    }
+                }
+
+                // this validation is important to calls to GraphManager.commit() and rollback() as they both
+                // expect that the aliases supplied are valid
+                if (!found) {
+                    final String error = String.format("Could not alias [%s] to [%s] as [%s] not in the Graph or TraversalSource global bindings",
+                            aliasKv.getKey(), aliasKv.getValue(), aliasKv.getValue());
+                    throw new RexsterException(error, ResponseMessage.build(msg)
+                            .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+                }
+            }
+        } else {
+            // there's no bindings so determine if that's ok with Gremlin Server
+            if (gremlinContext.getSettings().strictTransactionManagement) {
+                final String error = "Gremlin Server is configured with strictTransactionManagement as 'true' - the 'aliases' arguments must be provided";
+                throw new RexsterException(error, ResponseMessage.build(msg)
+                        .code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(error).create());
+            }
+        }
+
+        // add any bindings to override any other supplied
+        Optional.ofNullable((Map<String, Object>) msg.getArgs().get(Tokens.ARGS_BINDINGS)).ifPresent(bindings::putAll);
+        return bindings;
+    }
+
+    /**
+     * Provides a generic way of iterating a result set back to the client.
+     *
+     * @param gremlinContext The Gremlin Server {@link Context} object containing settings, request message, etc.
+     * @param itty The result to iterator
+     */
+    protected void handleIterator(final Context gremlinContext, final Iterator<?> itty) throws InterruptedException {
+        final ChannelHandlerContext nettyContext = gremlinContext.getChannelHandlerContext();
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Settings settings = gremlinContext.getSettings();
+        boolean warnOnce = false;
+
+        // sessionless requests are always transaction managed, but in-session requests are configurable.
+        final boolean managedTransactionsForRequest = transactionManaged ?
+                true : (Boolean) msg.getArgs().getOrDefault(Tokens.ARGS_MANAGE_TRANSACTION, false);
+
+        // we have an empty iterator - happens on stuff like: g.V().iterate()
+        if (!itty.hasNext()) {
+            final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,ResponseStatusCode.NO_CONTENT, itty);
+            // as there is nothing left to iterate if we are transaction managed then we should execute a
+            // commit here before we send back a NO_CONTENT which implies success
+            if (managedTransactionsForRequest)
+                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+            gremlinContext.writeAndFlush(ResponseMessage.build(msg)
+                    .code(ResponseStatusCode.NO_CONTENT)
+                    .statusAttributes(attributes)
+                    .create());
+            return;
+        }
+
+        // the batch size can be overridden by the request
+        final int resultIterationBatchSize = (Integer) msg.optionalArgs(Tokens.ARGS_BATCH_SIZE)
+                .orElse(settings.resultIterationBatchSize);
+        List<Object> aggregate = new ArrayList<>(resultIterationBatchSize);
+
+        // use an external control to manage the loop as opposed to just checking hasNext() in the while.  this
+        // prevent situations where auto transactions create a new transaction after calls to commit() withing
+        // the loop on calls to hasNext().
+        boolean hasMore = itty.hasNext();
+
+        while (hasMore) {
+            if (Thread.interrupted()) throw new InterruptedException();
+
+            // check if an implementation needs to force flush the aggregated results before the iteration batch
+            // size is reached.
+            // todo: what implementation does this?! can we kill it going forward - seems always false
+            // final boolean forceFlush = isForceFlushed(nettyContext, msg, itty);
+            final boolean forceFlush = false;
+
+            // have to check the aggregate size because it is possible that the channel is not writeable (below)
+            // so iterating next() if the message is not written and flushed would bump the aggregate size beyond
+            // the expected resultIterationBatchSize.  Total serialization time for the response remains in
+            // effect so if the client is "slow" it may simply timeout.
+            //
+            // there is a need to check hasNext() on the iterator because if the channel is not writeable the
+            // previous pass through the while loop will have next()'d the iterator and if it is "done" then a
+            // NoSuchElementException will raise its head. also need a check to ensure that this iteration doesn't
+            // require a forced flush which can be forced by sub-classes.
+            //
+            // this could be placed inside the isWriteable() portion of the if-then below but it seems better to
+            // allow iteration to continue into a batch if that is possible rather than just doing nothing at all
+            // while waiting for the client to catch up
+            if (aggregate.size() < resultIterationBatchSize && itty.hasNext() && !forceFlush) aggregate.add(itty.next());
+
+            // Don't keep executor busy if client has already given up; there is no way to catch up if the channel is
+            // not active, and hence we should break the loop.
+            if (!nettyContext.channel().isActive()) {
+                if (managedTransactionsForRequest) {
+                    closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                }
+                break;
+            }
+
+            // send back a page of results if batch size is met or if it's the end of the results being iterated.
+            // also check writeability of the channel to prevent OOME for slow clients.
+            //
+            // clients might decide to close the Netty channel to the server with a CloseWebsocketFrame after errors
+            // like CorruptedFrameException. On the server, although the channel gets closed, there might be some
+            // executor threads waiting for watermark to clear which will not clear in these cases since client has
+            // already given up on these requests. This leads to these executors waiting for the client to consume
+            // results till the timeout. checking for isActive() should help prevent that.
+            if (nettyContext.channel().isActive() && nettyContext.channel().isWritable()) {
+                if (forceFlush || aggregate.size() == resultIterationBatchSize || !itty.hasNext()) {
+                    final ResponseStatusCode code = itty.hasNext() ? ResponseStatusCode.PARTIAL_CONTENT : ResponseStatusCode.SUCCESS;
+                    Frame frame = null;
+                    try {
+                        frame = makeFrame(gremlinContext, aggregate, code, itty);
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+
+                        // exception is handled in makeFrame() - serialization error gets written back to driver
+                        // at that point
+                        if (managedTransactionsForRequest)
+                            closeTransaction(gremlinContext, Transaction.Status.ROLLBACK);
+                        break;
+                    }
+
+                    // track whether there is anything left in the iterator because it needs to be accessed after
+                    // the transaction could be closed - in that case a call to hasNext() could open a new transaction
+                    // unintentionally
+                    final boolean moreInIterator = itty.hasNext();
+
+                    try {
+                        // only need to reset the aggregation list if there's more stuff to write
+                        if (moreInIterator)
+                            aggregate = new ArrayList<>(resultIterationBatchSize);
+                        else {
+                            // iteration and serialization are both complete which means this finished successfully. note that
+                            // errors internal to script eval or timeout will rollback given GremlinServer's global configurations.
+                            // local errors will get rolledback below because the exceptions aren't thrown in those cases to be
+                            // caught by the GremlinExecutor for global rollback logic. this only needs to be committed if
+                            // there are no more items to iterate and serialization is complete
+                            if (managedTransactionsForRequest)
+                                closeTransaction(gremlinContext, Transaction.Status.COMMIT);
+
+                            // exit the result iteration loop as there are no more results left.  using this external control
+                            // because of the above commit.  some graphs may open a new transaction on the call to
+                            // hasNext()
+                            hasMore = false;
+                        }
+                    } catch (Exception ex) {
+                        // a frame may use a Bytebuf which is a countable release - if it does not get written
+                        // downstream it needs to be released here
+                        if (frame != null) frame.tryRelease();
+                        throw ex;
+                    }
+
+                    if (!moreInIterator) iterateComplete(gremlinContext, itty);
+
+                    // the flush is called after the commit has potentially occurred.  in this way, if a commit was
+                    // required then it will be 100% complete before the client receives it. the "frame" at this point
+                    // should have completely detached objects from the transaction (i.e. serialization has occurred)
+                    // so a new one should not be opened on the flush down the netty pipeline
+                    gremlinContext.writeAndFlush(code, frame);
+                }
+            } else {
+                // don't keep triggering this warning over and over again for the same request
+                if (!warnOnce) {
+                    logger.warn("Pausing response writing as writeBufferHighWaterMark exceeded on {} - writing will continue once client has caught up", msg);
+                    warnOnce = true;
+                }
+
+                // since the client is lagging we can hold here for a period of time for the client to catch up.
+                // this isn't blocking the IO thread - just a worker.
+                TimeUnit.MILLISECONDS.sleep(10);
+            }
+        }
+    }
+
+    /**
+     * If {@link Bytecode} is detected to contain a "graph operation" then it gets processed by this method.
+     */
+    protected void handleGraphOperation(final Context gremlinContext, final Bytecode bytecode, final Graph graph) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        if (graph.features().graph().supportsTransactions()) {
+            if (bytecode.equals(Bytecode.TX_COMMIT) || bytecode.equals(Bytecode.TX_ROLLBACK)) {
+                final boolean commit = bytecode.equals(Bytecode.TX_COMMIT);
+                closeTransaction(gremlinContext, commit ? Transaction.Status.COMMIT : Transaction.Status.ROLLBACK);
+
+                // write back a no-op for success
+                final Map<String, Object> attributes = generateStatusAttributes(gremlinContext,
+                        ResponseStatusCode.NO_CONTENT, Collections.emptyIterator());
+                gremlinContext.writeAndFlush(ResponseMessage.build(msg)
+                            .code(ResponseStatusCode.NO_CONTENT)
+                            .statusAttributes(attributes)
+                            .create());
+            } else {
+                throw new IllegalStateException(String.format(
+                        "Bytecode in request is not a recognized graph operation: %s", bytecode.toString()));
+            }
+        }
+    }
+
+    /**
+     * Called when iteration within {@link #handleIterator(Context, Iterator)} is on its final pass and the final
+     * frame is about to be sent back to the client. This method only gets called on successful iteration of the
+     * entire result.
+     */
+    protected void iterateComplete(final Context gremlinContext, final Iterator<?> itty) {
+        // do nothing by default
+    }
+
+    /**
+     * Generates response status meta-data to put on a {@link ResponseMessage}.
+     *
+     * @param itty a reference to the current {@link Iterator} of results - it is not meant to be forwarded in
+     *             this method
+     */
+    protected Map<String, Object> generateStatusAttributes(final Context gremlinContext,
+                                                           final ResponseStatusCode code, final Iterator<?> itty) {
+        // only return server metadata on the last message
+        if (itty.hasNext()) return Collections.emptyMap();
+
+        final Map<String, Object> metaData = new HashMap<>();
+        metaData.put(Tokens.ARGS_HOST, gremlinContext.getChannelHandlerContext().channel().remoteAddress().toString());
+
+        return metaData;
+    }
+
+    /**
+     * Generates response result meta-data to put on a {@link ResponseMessage}.
+     *
+     * @param itty a reference to the current {@link Iterator} of results - it is not meant to be forwarded in
+     *             this method
+     */
+    protected Map<String, Object> generateResponseMetaData(final Context gremlinContext,
+                                                           final ResponseStatusCode code, final Iterator<?> itty) {
+        return Collections.emptyMap();
+    }
+
+    protected Frame makeFrame(final Context gremlinContext, final List<Object> aggregate,
+                              final ResponseStatusCode code, final Iterator<?> itty) throws Exception {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final ChannelHandlerContext nettyContext = gremlinContext.getChannelHandlerContext();
+        final MessageSerializer serializer = nettyContext.channel().attr(StateKey.SERIALIZER).get();
+        final boolean useBinary = nettyContext.channel().attr(StateKey.USE_BINARY).get();
+
+        final Map<String, Object> responseMetaData = generateResponseMetaData(gremlinContext, code, itty);
+        final Map<String, Object> statusAttributes = generateStatusAttributes(gremlinContext, code, itty);
+        try {
+            if (useBinary) {
+                return new Frame(serializer.serializeResponseAsBinary(ResponseMessage.build(msg)
+                        .code(code)
+                        .statusAttributes(statusAttributes)
+                        .responseMetaData(responseMetaData)
+                        .result(aggregate).create(), nettyContext.alloc()));
+            } else {
+                // the expectation is that the GremlinTextRequestDecoder will have placed a MessageTextSerializer
+                // instance on the channel.
+                final MessageTextSerializer textSerializer = (MessageTextSerializer) serializer;
+                return new Frame(textSerializer.serializeResponseAsString(ResponseMessage.build(msg)
+                        .code(code)
+                        .statusAttributes(statusAttributes)
+                        .responseMetaData(responseMetaData)
+                        .result(aggregate).create()));
+            }
+        } catch (Exception ex) {
+            logger.warn("The result [{}] in the request {} could not be serialized and returned.", aggregate, msg.getRequestId(), ex);
+            final String errorMessage = String.format("Error during serialization: %s", ExceptionHelper.getMessageFromExceptionOrCause(ex));
+            final ResponseMessage error = ResponseMessage.build(msg.getRequestId())
+                    .statusMessage(errorMessage)
+                    .statusAttributeException(ex)
+                    .code(ResponseStatusCode.SERVER_ERROR_SERIALIZATION).create();
+            gremlinContext.writeAndFlush(error);
+            throw ex;
+        }
+    }
+
+    /**
+     * Called right before a transaction starts within {@link #run()}.
+     */
+    protected void startTransaction(final Context gremlinContext) {
+        // check if transactions are open and rollback first to ensure a fresh start.
+        graphManager.rollbackAll();
+    }
+
+    /**
+     * Close the transaction without a {@link Context} which supplies {@code null} to that argument for
+     * {@link #closeTransaction(Context, Transaction.Status)}. This method is idempotent.
+     */
+    protected void closeTransaction(final Transaction.Status status) {
+        closeTransaction(null, status);
+    }
+
+    /**
+     * Tries to close the transaction but will catch exceptions and log them. This method is idempotent.
+     */
+    protected void closeTransactionSafely(final Transaction.Status status) {
+        closeTransactionSafely(null, status);
+    }
+
+    /**
+     * Tries to close the transaction but will catch exceptions and log them. This method is idempotent.
+     */
+    protected void closeTransactionSafely(final Context gremlinContext, final Transaction.Status status) {
+        try {
+            closeTransaction(gremlinContext, status);
+        } catch (Exception ex) {
+            logger.error("Failed to close transaction", ex);
+        }
+    }
+
+    /**
+     * Closes a transaction with commit or rollback. Strict transaction management settings are observed when
+     * configured as such in {@link Settings#strictTransactionManagement} and when aliases are present on the
+     * request in the current {@link Context}. If the supplied {@link Context} is {@code null} then "strict" is
+     * bypassed so this form must be called with care. Bypassing is often useful to ensure that all transactions
+     * are cleaned up when multiple graphs are referenced. Prefer calling {@link #closeTransaction(Transaction.Status)}
+     * in this case instead. This method is idempotent.
+     */
+    protected void closeTransaction(final Context gremlinContext, final Transaction.Status status) {
+        if (status != Transaction.Status.COMMIT && status != Transaction.Status.ROLLBACK)
+            throw new IllegalStateException(String.format("Transaction.Status not supported: %s", status));
+
+        final boolean commit = status == Transaction.Status.COMMIT;
+        final boolean strict = gremlinContext != null && gremlinContext.getSettings().strictTransactionManagement;
+
+        if (strict) {
+            if (commit)
+                graphManager.commit(new HashSet<>(aliasesUsedByRexster));
+            else
+                graphManager.rollback(new HashSet<>(aliasesUsedByRexster));
+        } else {
+            if (commit)
+                graphManager.commitAll();
+            else
+                graphManager.rollbackAll();
+        }
+    }
+
+    private void processAuditLog(final Settings settings, final ChannelHandlerContext ctx, final Object gremlinToExecute) {

Review comment:
       This should probably be a method in a class call AuditLogger.

##########
File path: gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/AbstractRexster.java
##########
@@ -0,0 +1,758 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.tinkerpop.gremlin.server.handler;
+
+import groovy.lang.GroovyRuntimeException;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.apache.tinkerpop.gremlin.driver.MessageSerializer;
+import org.apache.tinkerpop.gremlin.driver.Tokens;
+import org.apache.tinkerpop.gremlin.driver.message.RequestMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseMessage;
+import org.apache.tinkerpop.gremlin.driver.message.ResponseStatusCode;
+import org.apache.tinkerpop.gremlin.driver.ser.MessageTextSerializer;
+import org.apache.tinkerpop.gremlin.groovy.jsr223.TimedInterruptTimeoutException;
+import org.apache.tinkerpop.gremlin.jsr223.GremlinScriptEngine;
+import org.apache.tinkerpop.gremlin.jsr223.JavaTranslator;
+import org.apache.tinkerpop.gremlin.process.traversal.Bytecode;
+import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
+import org.apache.tinkerpop.gremlin.process.traversal.TraversalSource;
+import org.apache.tinkerpop.gremlin.process.traversal.strategy.verification.VerificationException;
+import org.apache.tinkerpop.gremlin.process.traversal.util.BytecodeHelper;
+import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalInterruptedException;
+import org.apache.tinkerpop.gremlin.server.Context;
+import org.apache.tinkerpop.gremlin.server.GraphManager;
+import org.apache.tinkerpop.gremlin.server.GremlinServer;
+import org.apache.tinkerpop.gremlin.server.Settings;
+import org.apache.tinkerpop.gremlin.server.auth.AuthenticatedUser;
+import org.apache.tinkerpop.gremlin.server.util.ExceptionHelper;
+import org.apache.tinkerpop.gremlin.server.util.TraverserIterator;
+import org.apache.tinkerpop.gremlin.structure.Graph;
+import org.apache.tinkerpop.gremlin.structure.Transaction;
+import org.apache.tinkerpop.gremlin.structure.util.TemporaryException;
+import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;
+import org.codehaus.groovy.control.MultipleCompilationErrorsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.script.Bindings;
+import javax.script.ScriptException;
+import javax.script.SimpleBindings;
+import java.io.InterruptedIOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Stream;
+
+/**
+ * A base implementation of {@link Rexster} which offers some common functionality that matches typical Gremlin Server
+ * request response expectations for script, bytecode and graph operations. The class is designed to be extended but
+ * take care in understanding the way that different methods are called as they do depend on one another a bit. It
+ * maybe best to examine the source code to determine how best to use this class or to extend from the higher order
+ * classes of {@link SingleRexster} or {@link MultiRexster}.
+ */
+public abstract class AbstractRexster implements Rexster, AutoCloseable {
+    private static final Logger logger = LoggerFactory.getLogger(AbstractRexster.class);
+    private static final Logger auditLogger = LoggerFactory.getLogger(GremlinServer.AUDIT_LOGGER_NAME);
+
+    private final boolean sessionIdOnRequest;
+    private final Channel initialChannel;
+    private final boolean transactionManaged;
+    private final String sessionId;
+    private final AtomicReference<ScheduledFuture<?>> sessionCancelFuture = new AtomicReference<>();
+    private final AtomicReference<Future<?>> sessionFuture = new AtomicReference<>();
+    private long actualTimeoutLengthWhenClosed = 0;
+    private Thread sessionThread;
+    protected final boolean maintainStateAfterException;
+    protected final AtomicReference<CloseReason> closeReason = new AtomicReference<>();
+    protected final GraphManager graphManager;
+    protected final ConcurrentMap<String, Rexster> sessions;
+    protected final Set<String> aliasesUsedByRexster = new HashSet<>();
+
+    protected enum CloseReason { UNDETERMINED, CHANNEL_CLOSED, SESSION_TIMEOUT, REQUEST_TIMEOUT, NORMAL }
+
+    AbstractRexster(final Context gremlinContext, final String sessionId,
+                    final boolean transactionManaged,
+                    final ConcurrentMap<String, Rexster> sessions) {
+        // this only applies to sessions
+        this.maintainStateAfterException = (boolean) gremlinContext.getRequestMessage().
+                optionalArgs(Tokens.ARGS_MAINTAIN_STATE_AFTER_EXCEPTION).orElse(false);
+        this.sessionIdOnRequest = gremlinContext.getRequestMessage().optionalArgs(Tokens.ARGS_SESSION).isPresent();
+        this.transactionManaged = transactionManaged;
+        this.sessionId = sessionId;
+        this.initialChannel = gremlinContext.getChannelHandlerContext().channel();
+
+        // close Rexster if the channel closes to cleanup and close transactions
+        this.initialChannel.closeFuture().addListener(f -> {
+            if (closeReason.compareAndSet(null, CloseReason.CHANNEL_CLOSED)) {
+                // cancel session worker or it will keep waiting for items to appear in the session queue
+                cancel(true);
+                close();
+            }
+        });
+        this.sessions = sessions;
+        this.graphManager = gremlinContext.getGraphManager();
+    }
+
+    protected synchronized void cancel(final boolean mayInterruptIfRunning) {
+        final FutureTask<?> sf = (FutureTask) sessionFuture.get();
+        if (sf != null && !sf.isDone()) {
+            sf.cancel(mayInterruptIfRunning);
+        }
+    }
+
+    public boolean isTransactionManaged() {
+        return transactionManaged;
+    }
+
+    public String getSessionId() {
+        return sessionId;
+    }
+
+    public boolean isBoundTo(final Channel channel) {
+        return channel == initialChannel;
+    }
+
+    public long getActualTimeoutLengthWhenClosed() {
+        return actualTimeoutLengthWhenClosed;
+    }
+
+    public Optional<CloseReason> getCloseReason() {
+        return Optional.ofNullable(closeReason.get());
+    }
+
+    public GremlinScriptEngine getScriptEngine(final Context context, final String language) {
+        return context.getGremlinExecutor().getScriptEngineManager().getEngineByName(language);
+    }
+
+    public void setSessionThread(final Thread runner) {
+        this.sessionThread = runner;
+    }
+
+    @Override
+    public void setSessionCancelFuture(final ScheduledFuture<?> f) {
+        if (!sessionCancelFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session cancellation future is already set");
+    }
+
+    @Override
+    public void setSessionFuture(final Future<?> f) {
+        if (!sessionFuture.compareAndSet(null, f))
+            throw new IllegalStateException("Session future is already set");
+    }
+
+    @Override
+    public synchronized void triggerTimeout(final long timeout, final boolean causedBySession) {
+        // triggering timeout triggers the stop of the Rexster Runnable which will end in close()
+        // for final cleanup
+        final Future<?> f = sessionFuture.get();
+        if (f != null && !f.isDone()) {
+            if (closeReason.compareAndSet(null, causedBySession ? CloseReason.SESSION_TIMEOUT : CloseReason.REQUEST_TIMEOUT)) {
+                actualTimeoutLengthWhenClosed = timeout;
+
+                // if caused by a session timeout for a session OR if it is a request timeout for a sessionless
+                // request then we can just straight cancel() the Rexster instance
+                if (causedBySession || !sessionIdOnRequest)
+                    cancel(true);
+                else
+                    sessionThread.interrupt();
+            }
+        }
+    }
+
+    protected void process(final Context gremlinContext) throws RexsterException {
+        final RequestMessage msg = gremlinContext.getRequestMessage();
+        final Map<String, Object> args = msg.getArgs();
+        final Object gremlinToExecute = args.get(Tokens.ARGS_GREMLIN);
+
+        // for strict transactions track the aliases used so that we can commit them and only them on close()
+        if (gremlinContext.getSettings().strictTransactionManagement)
+            msg.optionalArgs(Tokens.ARGS_ALIASES).ifPresent(m -> aliasesUsedByRexster.addAll(((Map<String,String>) m).values()));
+
+        try {
+            // itty is optional as Bytecode could be a "graph operation" rather than a Traversal. graph operations
+            // don't need to be iterated and handle their own lifecycle
+            final Optional<Iterator<?>> itty = gremlinToExecute instanceof Bytecode ?
+                    fromBytecode(gremlinContext, (Bytecode) gremlinToExecute) :
+                    Optional.of(fromScript(gremlinContext, (String) gremlinToExecute));
+
+            processAuditLog(gremlinContext.getSettings(), gremlinContext.getChannelHandlerContext(), gremlinToExecute);
+
+            if (itty.isPresent())
+                handleIterator(gremlinContext, itty.get());
+        } catch (Exception ex) {
+            handleException(gremlinContext, ex);
+        }
+    }
+
+    protected void handleException(final Context gremlinContext, final Throwable t) throws RexsterException {
+        if (t instanceof RexsterException) throw (RexsterException) t;
+
+        final Optional<Throwable> possibleTemporaryException = determineIfTemporaryException(t);
+        if (possibleTemporaryException.isPresent()) {
+            final Throwable temporaryException = possibleTemporaryException.get();
+            throw new RexsterException(temporaryException.getMessage(), t,
+                    ResponseMessage.build(gremlinContext.getRequestMessage())
+                            .code(ResponseStatusCode.SERVER_ERROR_TEMPORARY)
+                            .statusMessage(temporaryException.getMessage())
+                            .statusAttributeException(temporaryException).create());
+        }
+
+        final Throwable root = ExceptionUtils.getRootCause(t);
+
+        if (root instanceof TimedInterruptTimeoutException) {
+            // occurs when the TimedInterruptCustomizerProvider is in play
+            final String msg = String.format("A timeout occurred within the script during evaluation of [%s] - consider increasing the limit given to TimedInterruptCustomizerProvider",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage("Timeout during script evaluation triggered by TimedInterruptCustomizerProvider")
+                    .create());
+        }
+
+        if (root instanceof TimeoutException) {
+            final String errorMessage = String.format("Script evaluation exceeded the configured threshold for request [%s]",
+                    gremlinContext.getRequestMessage().getRequestId());
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_TIMEOUT)
+                    .statusMessage(t.getMessage())
+                    .create());
+        }
+
+        if (root instanceof InterruptedException ||
+                root instanceof TraversalInterruptedException ||
+                root instanceof InterruptedIOException) {
+            String msg = "Processing interrupted but the reason why was not known";
+            switch (closeReason.get()) {
+                case CHANNEL_CLOSED:
+                    msg = "Processing interrupted because the channel was closed";
+                    break;
+                case SESSION_TIMEOUT:
+                    msg = String.format("Session closed - %s - sessionLifetimeTimeout of %s ms exceeded", sessionId, actualTimeoutLengthWhenClosed);
+                    break;
+                case REQUEST_TIMEOUT:
+                    msg = String.format("Evaluation exceeded timeout threshold of %s ms", actualTimeoutLengthWhenClosed);
+                    break;
+            }
+            final ResponseStatusCode code = closeReason.get() == CloseReason.SESSION_TIMEOUT || closeReason.get() == CloseReason.REQUEST_TIMEOUT ?
+                    ResponseStatusCode.SERVER_ERROR_TIMEOUT : ResponseStatusCode.SERVER_ERROR;
+            throw new RexsterException(msg, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(code)
+                    .statusMessage(msg).create());
+        }
+
+        if (root instanceof MultipleCompilationErrorsException && root.getMessage().contains("Method too large") &&
+                ((MultipleCompilationErrorsException) root).getErrorCollector().getErrorCount() == 1) {
+            final String errorMessage = String.format("The Gremlin statement that was submitted exceeds the maximum compilation size allowed by the JVM, please split it into multiple smaller statements - %s", trimMessage(gremlinContext.getRequestMessage()));
+            logger.warn(errorMessage);
+            throw new RexsterException(errorMessage, root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(errorMessage)
+                    .statusAttributeException(root).create());
+        }
+
+        // GroovyRuntimeException will hit a pretty wide range of eval type errors, like MissingPropertyException,
+        // CompilationFailedException, MissingMethodException, etc. If more specific handling is required then
+        // try to catch it earlier above.
+        if (root instanceof GroovyRuntimeException ||
+                root instanceof VerificationException ||
+                root instanceof ScriptException) {
+            throw new RexsterException(root.getMessage(), root, ResponseMessage.build(gremlinContext.getRequestMessage())
+                    .code(ResponseStatusCode.SERVER_ERROR_EVALUATION)
+                    .statusMessage(root.getMessage())
+                    .statusAttributeException(root).create());
+        }
+
+        throw new RexsterException(root.getClass().getSimpleName() + ": " + root.getMessage(), root,
+                ResponseMessage.build(gremlinContext.getRequestMessage())
+                        .code(ResponseStatusCode.SERVER_ERROR)
+                        .statusAttributeException(root)
+                        .statusMessage(root.getMessage()).create());
+    }
+
+    /**
+     * Used to decrease the size of a Gremlin script that triggered a "method too large" exception so that it
+     * doesn't log a massive text string nor return a large error message.
+     */
+    private RequestMessage trimMessage(final RequestMessage msg) {
+        final RequestMessage trimmedMsg = RequestMessage.from(msg).create();
+        if (trimmedMsg.getArgs().containsKey(Tokens.ARGS_GREMLIN))
+            trimmedMsg.getArgs().put(Tokens.ARGS_GREMLIN, trimmedMsg.getArgs().get(Tokens.ARGS_GREMLIN).toString().substring(0, 1021) + "...");
+
+        return trimmedMsg;
+    }
+
+    /**
+     * Check if any exception in the chain is TemporaryException then we should respond with the right error code so
+     * that the client knows to retry.
+     */
+    protected Optional<Throwable> determineIfTemporaryException(final Throwable ex) {
+        return Stream.of(ExceptionUtils.getThrowables(ex)).
+                filter(i -> i instanceof TemporaryException).findFirst();
+    }
+
+    @Override
+    public synchronized void close() {
+        // already closing/closed
+        if (!sessions.containsKey(sessionId)) return;

Review comment:
       how do we handle this case:
   1. request1 for sessionA gets enqueued and starts processing
   2. request2 for sessionA gets enqueued and hasn't started processing yet
   3. client closes session
   4. request1 gets interrupted and rolledback. session ID is removed from the map
   5. request2 is dequeued, it tried to find if a sessionA exists and it finds that it doesn't so it creates another one and executes successfully.
   
   From a client perspective, they sent 2 requests for a session and then closed the session which should have rolledback both of them but now client observes that request2 is successful. We can say that request2 will not commit ever unless we get a commit from the client for sessionA but it still ends up using resources to execute.
   
   Instead of a Map, I think we should be using  DS where we can evict the closed sessions after some amount of time i.e. we still keep their IDs in the DS even when closed. If a request arrives for a closed session we throw exception. Since we don't want this DS to increase indefinitely, we should evict closed entries after some time. Ideally this DS should evict closed session after X number of requests have been dequeued where X is the number of requests in queue when the session was closed.
   




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