[GitHub] [kafka] divijvaidya commented on a diff in pull request #13795: KAFKA-14462; [17/N] Add CoordinatorRuntime

["divijvaidya (via GitHub)" <gi...@apache.org>]

divijvaidya commented on code in PR #13795:
URL: https://github.com/apache/kafka/pull/13795#discussion_r1223158227


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group-coordinator/src/main/java/org/apache/kafka/coordinator/group/runtime/CoordinatorRuntime.java:
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@@ -0,0 +1,1040 @@
+/*
+ * 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.kafka.coordinator.group.runtime;
+
+import org.apache.kafka.common.KafkaException;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.errors.CoordinatorLoadInProgressException;
+import org.apache.kafka.common.errors.NotCoordinatorException;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.deferred.DeferredEvent;
+import org.apache.kafka.deferred.DeferredEventQueue;
+import org.apache.kafka.timeline.SnapshotRegistry;
+import org.slf4j.Logger;
+
+import java.util.HashSet;
+import java.util.OptionalLong;
+import java.util.Set;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * The CoordinatorRuntime provides a framework to implement coordinators such as the group coordinator
+ * or the transaction coordinator.
+ *
+ * The runtime framework maps each underlying partitions (e.g. __consumer_offsets) that that broker is a
+ * leader of to a coordinator replicated state machine. A replicated state machine holds the hard and soft
+ * state of all the objects (e.g. groups or offsets) assigned to the partition. The hard state is stored in
+ * timeline datastructures backed by a SnapshotRegistry. The runtime supports two type of operations
+ * on state machines: (1) Writes and (2) Reads.
+ *
+ * (1) A write operation, aka a request, can read the full and potentially **uncommitted** state from state
+ * machine to handle the operation. A write operation typically generates a response and a list of
+ * records. The records are applies to the state machine and persisted to the partition. The response
+ * is parked until the records are committed and delivered when they are.
+ *
+ * (2) A read operation, aka a request, can only read the committed state from the state machine to handle
+ * the operation. A read operation typically generates a response that is immediately completed.
+ *
+ * The runtime framework exposes an asynchronous, future based, API to the world. All the operations
+ * are executed by an CoordinatorEventProcessor. The processor guarantees that operations for a
+ * single partition or state machine are not processed concurrently.
+ *
+ * @param <S> The type of the state machine.
+ * @param <U> The type of the record.
+ */
+public class CoordinatorRuntime<S extends Coordinator<U>, U> {
+
+    /**
+     * Builder to create a CoordinatorRuntime.
+     *
+     * @param <S> The type of the state machine.
+     * @param <U> The type of the record.
+     */
+    public static class Builder<S extends Coordinator<U>, U> {
+        private LogContext logContext;
+        private CoordinatorEventProcessor eventProcessor;
+        private PartitionWriter<U> partitionWriter;
+        private CoordinatorLoader<U> loader;
+        private CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier;
+
+        public Builder<S, U> withLogContext(LogContext logContext) {
+            this.logContext = logContext;
+            return this;
+        }
+
+        public Builder<S, U> withEventProcessor(CoordinatorEventProcessor eventProcessor) {
+            this.eventProcessor = eventProcessor;
+            return this;
+        }
+
+        public Builder<S, U> withPartitionWriter(PartitionWriter<U> partitionWriter) {
+            this.partitionWriter = partitionWriter;
+            return this;
+        }
+
+        public Builder<S, U> withLoader(CoordinatorLoader<U> loader) {
+            this.loader = loader;
+            return this;
+        }
+
+        public Builder<S, U> withCoordinatorBuilderSupplier(CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier) {
+            this.coordinatorBuilderSupplier = coordinatorBuilderSupplier;
+            return this;
+        }
+
+        public CoordinatorRuntime<S, U> build() {
+            if (logContext == null)
+                logContext = new LogContext();
+            if (eventProcessor == null)
+                throw new IllegalArgumentException("Event processor must be set.");
+            if (partitionWriter == null)
+                throw new IllegalArgumentException("Partition write must be set.");
+            if (loader == null)
+                throw new IllegalArgumentException("Loader must be set.");
+            if (coordinatorBuilderSupplier == null)
+                throw new IllegalArgumentException("State machine supplier must be set.");
+
+            return new CoordinatorRuntime<>(
+                logContext,
+                eventProcessor,
+                partitionWriter,
+                loader,
+                coordinatorBuilderSupplier
+            );
+        }
+    }
+
+    /**
+     * The various state that a coordinator for a partition can be in.
+     */
+    enum CoordinatorState {
+        /**
+         * Initial state when a coordinator is created.
+         */
+        INITIAL {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return false;
+            }
+        },
+
+        /**
+         * The coordinator is being loaded.
+         */
+        LOADING {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == INITIAL;
+            }
+        },
+
+        /**
+         * The coordinator is active and can service requests.
+         */
+        ACTIVE {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == ACTIVE || state == LOADING;
+            }
+        },
+
+        /**
+         * The coordinator is closed.
+         */
+        CLOSED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return true;
+            }
+        },
+
+        /**
+         * The coordinator loading has failed.
+         */
+        FAILED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == LOADING;
+            }
+        };
+
+        abstract boolean canTransitionFrom(CoordinatorState state);
+    }
+
+    /**
+     * CoordinatorContext holds all the metadata around a coordinator state machine.
+     */
+    class CoordinatorContext {
+        /**
+         * The topic partition backing the coordinator.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The snapshot registry backing the coordinator.
+         */
+        final SnapshotRegistry snapshotRegistry;
+
+        /**
+         * The deferred event queue used to park events waiting
+         * on records to be committed.
+         */
+        final DeferredEventQueue deferredEventQueue;
+
+        /**
+         * The current state.
+         */
+        volatile CoordinatorState state;
+
+        /**
+         * The actual state machine.
+         */
+        volatile S coordinator;
+
+        /**
+         * The current epoch of the coordinator. This represents
+         * the epoch of the partition leader.
+         */
+        volatile int epoch;
+
+        /**
+         * The last offset written to the partition.
+         */
+        volatile long lastWrittenOffset;
+
+        /**
+         * The last offset committed. This represents the high
+         * watermark of the partition.
+         */
+        volatile long lastCommittedOffset;
+
+        /**
+         * Constructor.
+         *
+         * @param tp The topic partition of the coordinator.
+         */
+        private CoordinatorContext(
+            TopicPartition tp
+        ) {
+            this.tp = tp;
+            this.snapshotRegistry = new SnapshotRegistry(logContext);
+            this.deferredEventQueue = new DeferredEventQueue(logContext);
+            this.state = CoordinatorState.INITIAL;
+            this.epoch = -1;
+            this.lastWrittenOffset = 0L;
+            this.lastCommittedOffset = 0L;
+        }
+
+        /**
+         * Updates the last written offset. This also create a new snapshot
+         * in the snapshot registry.
+         *
+         * @param offset The new last written offset.
+         */
+        private void updateLastWrittenOffset(
+            long offset
+        ) {
+            if (offset <= lastWrittenOffset) {
+                throw new IllegalStateException("New last written offset " + offset + " of " + tp +
+                    " must be larger than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Update last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.getOrCreateSnapshot(offset);
+        }
+
+        /**
+         * Reverts the last written offset. This also reverts the snapshot
+         * registry to this offset. All the changes applied after the offset
+         * are lost.
+         *
+         * @param offset The offset to revert to.
+         */
+        private void revertLastWrittenOffset(
+            long offset
+        ) {
+            if (offset > lastWrittenOffset) {
+                throw new IllegalStateException("New offset " + offset + " of " + tp +
+                    " must be smaller than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Revert last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.revertToSnapshot(offset);
+        }
+
+        /**
+         * Updates the last committed offset. This completes all the deferred
+         * events waiting on this offset. This also cleanups all the snapshots
+         * prior to this offset.
+         *
+         * @param offset The new last committed offset.
+         */
+        private void updateLastCommittedOffset(
+            long offset
+        ) {
+            if (offset <= lastCommittedOffset) {
+                throw new IllegalStateException("New committed offset " + offset + " of " + tp +
+                    " must be larger than " + lastCommittedOffset + ".");
+            }
+
+            log.debug("Update committed offset of {} to {}.", tp, offset);
+            lastCommittedOffset = offset;
+            deferredEventQueue.completeUpTo(offset);
+            snapshotRegistry.deleteSnapshotsUpTo(offset);
+        }
+
+        /**
+         * Transitions to the new state.
+         *
+         * @param newState The new state.
+         */
+        private void transitionTo(
+            CoordinatorState newState
+        ) {
+            if (!newState.canTransitionFrom(state)) {
+                throw new IllegalStateException("Cannot transition from " + state + " to " + newState);
+            }
+
+            log.debug("Transition from {} to {}.", state, newState);
+            switch (newState) {
+                case LOADING:
+                    state = CoordinatorState.LOADING;
+                    coordinator = coordinatorBuilderSupplier
+                        .get()
+                        .withSnapshotRegistry(snapshotRegistry)
+                        .build();
+                    break;
+
+                case ACTIVE:
+                    state = CoordinatorState.ACTIVE;
+                    snapshotRegistry.getOrCreateSnapshot(0);
+                    partitionWriter.registerListener(tp, highWatermarklistener);
+                    coordinator.onLoaded();
+                    break;
+
+                case FAILED:
+                    state = CoordinatorState.FAILED;
+                    unload();
+                    break;
+
+                case CLOSED:
+                    state = CoordinatorState.CLOSED;
+                    unload();
+                    break;
+
+                default:
+                    throw new IllegalArgumentException("Transitioning to " + newState + " is not supported.");
+            }
+        }
+
+        /**
+         * Unloads the coordinator.
+         */
+        private void unload() {
+            partitionWriter.deregisterListener(tp, highWatermarklistener);
+            deferredEventQueue.failAll(Errors.NOT_COORDINATOR.exception());
+            if (coordinator != null) {
+                coordinator.onUnloaded();
+                coordinator = null;
+            }
+        }
+    }
+
+    /**
+     * A coordinator write operation.
+     *
+     * @param <S> The type of the coordinator state machine.
+     * @param <T> The type of the response.
+     * @param <U> The type of the records.
+     */
+    public interface CoordinatorWriteOperation<S, T, U> {
+        /**
+         * Generates the records needed to implement this coordinator write operation. In general,
+         * this operation should not modify the hard state of the coordinator. That modifications
+         * will happen later on, when the records generated by this function are applied to the
+         * coordinator.
+         *
+         * @param coordinator The coordinator state machine.
+         * @return A result containing a list of records and the RPC result.
+         * @throws KafkaException
+         */
+        CoordinatorResult<T, U> generateRecordsAndResult(S coordinator) throws KafkaException;
+    }
+
+    /**
+     * A coordinator event that modifies the coordinator state.
+     *
+     * @param <T> The type of the response.
+     */
+    class CoordinatorWriteEvent<T> implements CoordinatorEvent, DeferredEvent {
+        /**
+         * The topic partition that this write event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The write operation to execute.
+         */
+        final CoordinatorWriteOperation<S, T, U> op;
+
+        /**
+         * The future that will be completed with the response
+         * generated by the write operation or an error.
+         */
+        final CompletableFuture<T> future;
+
+        /**
+         * The result of the write operation. It could be null
+         * if an exception is thrown before it is assigned.
+         */
+        CoordinatorResult<T, U> result;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The write operation.
+         */
+        CoordinatorWriteEvent(
+            String name,
+            TopicPartition tp,
+            CoordinatorWriteOperation<S, T, U> op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+            this.future = new CompletableFuture<>();
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                // Get the context of the coordinator or fail if the coordinator is not in active state.
+                CoordinatorContext context = activeContextOrThrow(tp);
+                long prevLastWrittenOffset = context.lastWrittenOffset;
+
+                // Execute the operation.
+                result = op.generateRecordsAndResult(context.coordinator);
+
+                if (result.records().isEmpty()) {
+                    // If the records are empty, it was a read operation after all. In this case,
+                    // the response can be returned directly iff there are no pending write operations;
+                    // otherwise, the read needs to wait on the last write operation to be completed.
+                    OptionalLong pendingOffset = context.deferredEventQueue.highestPendingOffset();
+                    if (pendingOffset.isPresent()) {
+                        context.deferredEventQueue.add(pendingOffset.getAsLong(), this);
+                    } else {
+                        complete(null);
+                    }
+                } else {
+                    // If the records are not empty, first, they are applied to the state machine,
+                    // second, then are written to the partition/log, and finally, the response
+                    // is put into the deferred event queue.
+                    try {
+                        // Apply the records to the state machine.
+                        result.records().forEach(context.coordinator::replay);
+
+                        // Write the records to the log and update the last written
+                        // offset.
+                        long offset = partitionWriter.append(tp, result.records());
+                        context.updateLastWrittenOffset(offset);
+
+                        // Add the response to the deferred queue.
+                        if (!future.isDone()) {
+                            context.deferredEventQueue.add(offset, this);
+                        } else {
+                            complete(null);
+                        }
+                    } catch (Throwable t) {
+                        context.revertLastWrittenOffset(prevLastWrittenOffset);
+                        complete(t);
+                    }
+                }
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Completes the future with either the result of the write operation
+         * or the provided exception.
+         *
+         * @param exception The exception to complete the future with.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception == null) {
+                future.complete(result.response());
+            } else {
+                future.completeExceptionally(exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "CoordinatorWriteEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * A coordinator read operation.
+     *
+     * @param <S> The type of the coordinator state machine.
+     * @param <T> The type of the response.
+     */
+    public interface CoordinatorReadOperation<S, T> {
+        /**
+         * Generates the response to implement this coordinator read operation. A read
+         * operation received the last committed offset. It must use it to ensure that
+         * it does not read uncommitted data from the timeline data structures.
+         *
+         * @param state     The coordinator state machine.
+         * @param offset    The last committed offset.
+         * @return A response.
+         * @throws KafkaException
+         */
+        T generateResponse(S state, long offset) throws KafkaException;
+    }
+
+    /**
+     * A coordinator that reads the committed coordinator state.
+     *
+     * @param <T> The type of the response.
+     */
+    class CoordinatorReadEvent<T> implements CoordinatorEvent {
+        /**
+         * The topic partition that this read event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The read operation to execute.
+         */
+        final CoordinatorReadOperation<S, T> op;
+
+        /**
+         * The future that will be completed with the response
+         * generated by the read operation or an error.
+         */
+        final CompletableFuture<T> future;
+
+        /**
+         * The result of the read operation. It could be null
+         * if an exception is thrown before it is assigned.
+         */
+        T response;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The read operation.
+         */
+        CoordinatorReadEvent(
+            String name,
+            TopicPartition tp,
+            CoordinatorReadOperation<S, T> op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+            this.future = new CompletableFuture<>();
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                // Get the context of the coordinator or fail if the coordinator is not in active state.
+                CoordinatorContext context = activeContextOrThrow(tp);
+
+                // Execute the read operation.
+                response = op.generateResponse(
+                    context.coordinator,
+                    context.lastCommittedOffset
+                );
+
+                // The response can be completed immediately.
+                complete(null);
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Completes the future with either the result of the read operation
+         * or the provided exception.
+         *
+         * @param exception The exception to complete the future with.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception == null) {
+                future.complete(response);
+            } else {
+                future.completeExceptionally(exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "CoordinatorReadEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * A coordinator internal event.
+     */
+    class CoordinatorInternalEvent implements CoordinatorEvent {
+        /**
+         * The topic partition that this internal event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The internal operation to execute.
+         */
+        final Runnable op;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The operation.
+         */
+        CoordinatorInternalEvent(
+            String name,
+            TopicPartition tp,
+            Runnable op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                op.run();
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Logs any exceptions thrown while the event is executed.
+         *
+         * @param exception The exception.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception != null) {
+                log.error("Execution of {} failed due to {}.", name, exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "InternalEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * Partition listener to be notified when the high watermark of the partitions
+     * backing the coordinator are updated.
+     */
+    class HighWatermarkListener implements PartitionWriter.Listener {
+        /**
+         * Updates the high watermark of the corresponding coordinator.
+         *
+         * @param tp        The topic partition.
+         * @param offset    The new high watermark.
+         */
+        @Override
+        public void onHighWatermarkUpdated(
+            TopicPartition tp,
+            long offset
+        ) {
+            log.debug("High watermark of {} incremented to {}.", tp, offset);
+            scheduleInternalOperation("HighWatermarkUpdated(tp=" + tp + ", offset=" + offset + ")", tp, () -> {
+                contextOrThrow(tp).updateLastCommittedOffset(offset);
+            });
+        }
+    }
+
+    /**
+     * The log context.
+     */
+    private final LogContext logContext;
+
+    /**
+     * The logger.
+     */
+    private final Logger log;
+
+    /**
+     * The coordinators keyed by topic partition.
+     */
+    private final ConcurrentHashMap<TopicPartition, CoordinatorContext> coordinators;
+
+    /**
+     * The event processor used by the runtime.
+     */
+    private final CoordinatorEventProcessor processor;
+
+    /**
+     * The partition writer used by the runtime to persist records.
+     */
+    private final PartitionWriter<U> partitionWriter;
+
+    /**
+     * The high watermark listener registered to all the partitions
+     * backing the coordinators.
+     */
+    private final HighWatermarkListener highWatermarklistener;
+
+    /**
+     * The coordinator loaded used by the runtime.
+     */
+    private final CoordinatorLoader<U> loader;
+
+    /**
+     * The coordinator state machine builder used by the runtime
+     * to instantiate a coordinator.
+     */
+    private final CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier;
+
+    /**
+     * Constructor.
+     *
+     * @param logContext                    The log context.
+     * @param processor                     The event processor.
+     * @param partitionWriter               The partition writer.
+     * @param loader                        The coordinator loader.
+     * @param coordinatorBuilderSupplier    The coordinator builder.
+     */
+    private CoordinatorRuntime(
+        LogContext logContext,
+        CoordinatorEventProcessor processor,
+        PartitionWriter<U> partitionWriter,
+        CoordinatorLoader<U> loader,
+        CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier
+    ) {
+        this.logContext = logContext;
+        this.log = logContext.logger(CoordinatorRuntime.class);
+        this.coordinators = new ConcurrentHashMap<>();
+        this.processor = processor;
+        this.partitionWriter = partitionWriter;
+        this.highWatermarklistener = new HighWatermarkListener();
+        this.loader = loader;
+        this.coordinatorBuilderSupplier = coordinatorBuilderSupplier;
+    }
+
+    /**
+     * @return The coordinator context if the coordinator is active or an exception otherwise.
+     * @throws NotCoordinatorException
+     * @throws CoordinatorLoadInProgressException
+     */
+    private CoordinatorContext activeContextOrThrow(TopicPartition tp) {
+        CoordinatorContext context = coordinators.get(tp);

Review Comment:
   I am not aware of the threading model here but since we are using a concurrent hash map to store coordinators, I am assuming that this function can be accessed by multiple threads.
   
   In multi thread access is true for coordinator when this function is executing, then there is a bug here because there might be a context switch after we have retrieved the value using `get` and by the time we execute the if/else below, the value of this context might have changed. We should use atomic primitives with ConcurrentHashMaps such as `computeIfAbsent`



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/runtime/CoordinatorRuntime.java:
##########
@@ -0,0 +1,1040 @@
+/*
+ * 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.kafka.coordinator.group.runtime;
+
+import org.apache.kafka.common.KafkaException;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.errors.CoordinatorLoadInProgressException;
+import org.apache.kafka.common.errors.NotCoordinatorException;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.deferred.DeferredEvent;
+import org.apache.kafka.deferred.DeferredEventQueue;
+import org.apache.kafka.timeline.SnapshotRegistry;
+import org.slf4j.Logger;
+
+import java.util.HashSet;
+import java.util.OptionalLong;
+import java.util.Set;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * The CoordinatorRuntime provides a framework to implement coordinators such as the group coordinator
+ * or the transaction coordinator.
+ *
+ * The runtime framework maps each underlying partitions (e.g. __consumer_offsets) that that broker is a
+ * leader of to a coordinator replicated state machine. A replicated state machine holds the hard and soft
+ * state of all the objects (e.g. groups or offsets) assigned to the partition. The hard state is stored in
+ * timeline datastructures backed by a SnapshotRegistry. The runtime supports two type of operations
+ * on state machines: (1) Writes and (2) Reads.
+ *
+ * (1) A write operation, aka a request, can read the full and potentially **uncommitted** state from state
+ * machine to handle the operation. A write operation typically generates a response and a list of
+ * records. The records are applies to the state machine and persisted to the partition. The response
+ * is parked until the records are committed and delivered when they are.
+ *
+ * (2) A read operation, aka a request, can only read the committed state from the state machine to handle
+ * the operation. A read operation typically generates a response that is immediately completed.
+ *
+ * The runtime framework exposes an asynchronous, future based, API to the world. All the operations
+ * are executed by an CoordinatorEventProcessor. The processor guarantees that operations for a
+ * single partition or state machine are not processed concurrently.
+ *
+ * @param <S> The type of the state machine.
+ * @param <U> The type of the record.
+ */
+public class CoordinatorRuntime<S extends Coordinator<U>, U> {
+
+    /**
+     * Builder to create a CoordinatorRuntime.
+     *
+     * @param <S> The type of the state machine.
+     * @param <U> The type of the record.
+     */
+    public static class Builder<S extends Coordinator<U>, U> {
+        private LogContext logContext;
+        private CoordinatorEventProcessor eventProcessor;
+        private PartitionWriter<U> partitionWriter;
+        private CoordinatorLoader<U> loader;
+        private CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier;
+
+        public Builder<S, U> withLogContext(LogContext logContext) {
+            this.logContext = logContext;
+            return this;
+        }
+
+        public Builder<S, U> withEventProcessor(CoordinatorEventProcessor eventProcessor) {
+            this.eventProcessor = eventProcessor;
+            return this;
+        }
+
+        public Builder<S, U> withPartitionWriter(PartitionWriter<U> partitionWriter) {
+            this.partitionWriter = partitionWriter;
+            return this;
+        }
+
+        public Builder<S, U> withLoader(CoordinatorLoader<U> loader) {
+            this.loader = loader;
+            return this;
+        }
+
+        public Builder<S, U> withCoordinatorBuilderSupplier(CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier) {
+            this.coordinatorBuilderSupplier = coordinatorBuilderSupplier;
+            return this;
+        }
+
+        public CoordinatorRuntime<S, U> build() {
+            if (logContext == null)
+                logContext = new LogContext();
+            if (eventProcessor == null)
+                throw new IllegalArgumentException("Event processor must be set.");
+            if (partitionWriter == null)
+                throw new IllegalArgumentException("Partition write must be set.");
+            if (loader == null)
+                throw new IllegalArgumentException("Loader must be set.");
+            if (coordinatorBuilderSupplier == null)
+                throw new IllegalArgumentException("State machine supplier must be set.");
+
+            return new CoordinatorRuntime<>(
+                logContext,
+                eventProcessor,
+                partitionWriter,
+                loader,
+                coordinatorBuilderSupplier
+            );
+        }
+    }
+
+    /**
+     * The various state that a coordinator for a partition can be in.
+     */
+    enum CoordinatorState {
+        /**
+         * Initial state when a coordinator is created.
+         */
+        INITIAL {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return false;
+            }
+        },
+
+        /**
+         * The coordinator is being loaded.
+         */
+        LOADING {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == INITIAL;
+            }
+        },
+
+        /**
+         * The coordinator is active and can service requests.
+         */
+        ACTIVE {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == ACTIVE || state == LOADING;
+            }
+        },
+
+        /**
+         * The coordinator is closed.
+         */
+        CLOSED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return true;
+            }
+        },
+
+        /**
+         * The coordinator loading has failed.
+         */
+        FAILED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == LOADING;
+            }
+        };
+
+        abstract boolean canTransitionFrom(CoordinatorState state);
+    }
+
+    /**
+     * CoordinatorContext holds all the metadata around a coordinator state machine.
+     */
+    class CoordinatorContext {
+        /**
+         * The topic partition backing the coordinator.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The snapshot registry backing the coordinator.
+         */
+        final SnapshotRegistry snapshotRegistry;
+
+        /**
+         * The deferred event queue used to park events waiting
+         * on records to be committed.
+         */
+        final DeferredEventQueue deferredEventQueue;
+
+        /**
+         * The current state.
+         */
+        volatile CoordinatorState state;
+
+        /**
+         * The actual state machine.
+         */
+        volatile S coordinator;
+
+        /**
+         * The current epoch of the coordinator. This represents
+         * the epoch of the partition leader.
+         */
+        volatile int epoch;
+
+        /**
+         * The last offset written to the partition.
+         */
+        volatile long lastWrittenOffset;
+
+        /**
+         * The last offset committed. This represents the high
+         * watermark of the partition.
+         */
+        volatile long lastCommittedOffset;
+
+        /**
+         * Constructor.
+         *
+         * @param tp The topic partition of the coordinator.
+         */
+        private CoordinatorContext(
+            TopicPartition tp
+        ) {
+            this.tp = tp;
+            this.snapshotRegistry = new SnapshotRegistry(logContext);
+            this.deferredEventQueue = new DeferredEventQueue(logContext);
+            this.state = CoordinatorState.INITIAL;
+            this.epoch = -1;
+            this.lastWrittenOffset = 0L;
+            this.lastCommittedOffset = 0L;
+        }
+
+        /**
+         * Updates the last written offset. This also create a new snapshot
+         * in the snapshot registry.
+         *
+         * @param offset The new last written offset.
+         */
+        private void updateLastWrittenOffset(
+            long offset
+        ) {
+            if (offset <= lastWrittenOffset) {
+                throw new IllegalStateException("New last written offset " + offset + " of " + tp +
+                    " must be larger than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Update last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.getOrCreateSnapshot(offset);
+        }
+
+        /**
+         * Reverts the last written offset. This also reverts the snapshot
+         * registry to this offset. All the changes applied after the offset
+         * are lost.
+         *
+         * @param offset The offset to revert to.
+         */
+        private void revertLastWrittenOffset(
+            long offset
+        ) {
+            if (offset > lastWrittenOffset) {
+                throw new IllegalStateException("New offset " + offset + " of " + tp +
+                    " must be smaller than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Revert last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.revertToSnapshot(offset);
+        }
+
+        /**
+         * Updates the last committed offset. This completes all the deferred
+         * events waiting on this offset. This also cleanups all the snapshots
+         * prior to this offset.
+         *
+         * @param offset The new last committed offset.
+         */
+        private void updateLastCommittedOffset(
+            long offset
+        ) {
+            if (offset <= lastCommittedOffset) {
+                throw new IllegalStateException("New committed offset " + offset + " of " + tp +
+                    " must be larger than " + lastCommittedOffset + ".");
+            }
+
+            log.debug("Update committed offset of {} to {}.", tp, offset);
+            lastCommittedOffset = offset;
+            deferredEventQueue.completeUpTo(offset);
+            snapshotRegistry.deleteSnapshotsUpTo(offset);

Review Comment:
   can this be done async by background thread pool?



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/runtime/CoordinatorRuntime.java:
##########
@@ -0,0 +1,1040 @@
+/*
+ * 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.kafka.coordinator.group.runtime;
+
+import org.apache.kafka.common.KafkaException;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.errors.CoordinatorLoadInProgressException;
+import org.apache.kafka.common.errors.NotCoordinatorException;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.deferred.DeferredEvent;
+import org.apache.kafka.deferred.DeferredEventQueue;
+import org.apache.kafka.timeline.SnapshotRegistry;
+import org.slf4j.Logger;
+
+import java.util.HashSet;
+import java.util.OptionalLong;
+import java.util.Set;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * The CoordinatorRuntime provides a framework to implement coordinators such as the group coordinator
+ * or the transaction coordinator.
+ *
+ * The runtime framework maps each underlying partitions (e.g. __consumer_offsets) that that broker is a
+ * leader of to a coordinator replicated state machine. A replicated state machine holds the hard and soft
+ * state of all the objects (e.g. groups or offsets) assigned to the partition. The hard state is stored in
+ * timeline datastructures backed by a SnapshotRegistry. The runtime supports two type of operations
+ * on state machines: (1) Writes and (2) Reads.
+ *
+ * (1) A write operation, aka a request, can read the full and potentially **uncommitted** state from state
+ * machine to handle the operation. A write operation typically generates a response and a list of
+ * records. The records are applies to the state machine and persisted to the partition. The response
+ * is parked until the records are committed and delivered when they are.
+ *
+ * (2) A read operation, aka a request, can only read the committed state from the state machine to handle
+ * the operation. A read operation typically generates a response that is immediately completed.
+ *
+ * The runtime framework exposes an asynchronous, future based, API to the world. All the operations
+ * are executed by an CoordinatorEventProcessor. The processor guarantees that operations for a
+ * single partition or state machine are not processed concurrently.
+ *
+ * @param <S> The type of the state machine.
+ * @param <U> The type of the record.
+ */
+public class CoordinatorRuntime<S extends Coordinator<U>, U> {
+
+    /**
+     * Builder to create a CoordinatorRuntime.
+     *
+     * @param <S> The type of the state machine.
+     * @param <U> The type of the record.
+     */
+    public static class Builder<S extends Coordinator<U>, U> {
+        private LogContext logContext;
+        private CoordinatorEventProcessor eventProcessor;
+        private PartitionWriter<U> partitionWriter;
+        private CoordinatorLoader<U> loader;
+        private CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier;
+
+        public Builder<S, U> withLogContext(LogContext logContext) {
+            this.logContext = logContext;
+            return this;
+        }
+
+        public Builder<S, U> withEventProcessor(CoordinatorEventProcessor eventProcessor) {
+            this.eventProcessor = eventProcessor;
+            return this;
+        }
+
+        public Builder<S, U> withPartitionWriter(PartitionWriter<U> partitionWriter) {
+            this.partitionWriter = partitionWriter;
+            return this;
+        }
+
+        public Builder<S, U> withLoader(CoordinatorLoader<U> loader) {
+            this.loader = loader;
+            return this;
+        }
+
+        public Builder<S, U> withCoordinatorBuilderSupplier(CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier) {
+            this.coordinatorBuilderSupplier = coordinatorBuilderSupplier;
+            return this;
+        }
+
+        public CoordinatorRuntime<S, U> build() {
+            if (logContext == null)
+                logContext = new LogContext();
+            if (eventProcessor == null)
+                throw new IllegalArgumentException("Event processor must be set.");
+            if (partitionWriter == null)
+                throw new IllegalArgumentException("Partition write must be set.");
+            if (loader == null)
+                throw new IllegalArgumentException("Loader must be set.");
+            if (coordinatorBuilderSupplier == null)
+                throw new IllegalArgumentException("State machine supplier must be set.");
+
+            return new CoordinatorRuntime<>(
+                logContext,
+                eventProcessor,
+                partitionWriter,
+                loader,
+                coordinatorBuilderSupplier
+            );
+        }
+    }
+
+    /**
+     * The various state that a coordinator for a partition can be in.
+     */
+    enum CoordinatorState {
+        /**
+         * Initial state when a coordinator is created.
+         */
+        INITIAL {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return false;
+            }
+        },
+
+        /**
+         * The coordinator is being loaded.
+         */
+        LOADING {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == INITIAL;
+            }
+        },
+
+        /**
+         * The coordinator is active and can service requests.
+         */
+        ACTIVE {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == ACTIVE || state == LOADING;
+            }
+        },
+
+        /**
+         * The coordinator is closed.
+         */
+        CLOSED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return true;
+            }
+        },
+
+        /**
+         * The coordinator loading has failed.
+         */
+        FAILED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == LOADING;
+            }
+        };
+
+        abstract boolean canTransitionFrom(CoordinatorState state);
+    }
+
+    /**
+     * CoordinatorContext holds all the metadata around a coordinator state machine.
+     */
+    class CoordinatorContext {
+        /**
+         * The topic partition backing the coordinator.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The snapshot registry backing the coordinator.
+         */
+        final SnapshotRegistry snapshotRegistry;
+
+        /**
+         * The deferred event queue used to park events waiting
+         * on records to be committed.
+         */
+        final DeferredEventQueue deferredEventQueue;
+
+        /**
+         * The current state.
+         */
+        volatile CoordinatorState state;
+
+        /**
+         * The actual state machine.
+         */
+        volatile S coordinator;
+
+        /**
+         * The current epoch of the coordinator. This represents
+         * the epoch of the partition leader.
+         */
+        volatile int epoch;
+
+        /**
+         * The last offset written to the partition.
+         */
+        volatile long lastWrittenOffset;
+
+        /**
+         * The last offset committed. This represents the high
+         * watermark of the partition.
+         */
+        volatile long lastCommittedOffset;
+
+        /**
+         * Constructor.
+         *
+         * @param tp The topic partition of the coordinator.
+         */
+        private CoordinatorContext(
+            TopicPartition tp
+        ) {
+            this.tp = tp;
+            this.snapshotRegistry = new SnapshotRegistry(logContext);
+            this.deferredEventQueue = new DeferredEventQueue(logContext);
+            this.state = CoordinatorState.INITIAL;
+            this.epoch = -1;
+            this.lastWrittenOffset = 0L;
+            this.lastCommittedOffset = 0L;
+        }
+
+        /**
+         * Updates the last written offset. This also create a new snapshot
+         * in the snapshot registry.
+         *
+         * @param offset The new last written offset.
+         */
+        private void updateLastWrittenOffset(
+            long offset
+        ) {
+            if (offset <= lastWrittenOffset) {
+                throw new IllegalStateException("New last written offset " + offset + " of " + tp +
+                    " must be larger than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Update last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.getOrCreateSnapshot(offset);
+        }
+
+        /**
+         * Reverts the last written offset. This also reverts the snapshot
+         * registry to this offset. All the changes applied after the offset
+         * are lost.
+         *
+         * @param offset The offset to revert to.
+         */
+        private void revertLastWrittenOffset(
+            long offset
+        ) {
+            if (offset > lastWrittenOffset) {
+                throw new IllegalStateException("New offset " + offset + " of " + tp +
+                    " must be smaller than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Revert last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.revertToSnapshot(offset);
+        }
+
+        /**
+         * Updates the last committed offset. This completes all the deferred
+         * events waiting on this offset. This also cleanups all the snapshots
+         * prior to this offset.
+         *
+         * @param offset The new last committed offset.
+         */
+        private void updateLastCommittedOffset(
+            long offset
+        ) {
+            if (offset <= lastCommittedOffset) {
+                throw new IllegalStateException("New committed offset " + offset + " of " + tp +
+                    " must be larger than " + lastCommittedOffset + ".");
+            }
+
+            log.debug("Update committed offset of {} to {}.", tp, offset);
+            lastCommittedOffset = offset;
+            deferredEventQueue.completeUpTo(offset);
+            snapshotRegistry.deleteSnapshotsUpTo(offset);
+        }
+
+        /**
+         * Transitions to the new state.
+         *
+         * @param newState The new state.
+         */
+        private void transitionTo(
+            CoordinatorState newState
+        ) {
+            if (!newState.canTransitionFrom(state)) {
+                throw new IllegalStateException("Cannot transition from " + state + " to " + newState);
+            }
+
+            log.debug("Transition from {} to {}.", state, newState);
+            switch (newState) {
+                case LOADING:
+                    state = CoordinatorState.LOADING;
+                    coordinator = coordinatorBuilderSupplier
+                        .get()
+                        .withSnapshotRegistry(snapshotRegistry)
+                        .build();
+                    break;
+
+                case ACTIVE:
+                    state = CoordinatorState.ACTIVE;
+                    snapshotRegistry.getOrCreateSnapshot(0);
+                    partitionWriter.registerListener(tp, highWatermarklistener);
+                    coordinator.onLoaded();
+                    break;
+
+                case FAILED:
+                    state = CoordinatorState.FAILED;
+                    unload();
+                    break;
+
+                case CLOSED:
+                    state = CoordinatorState.CLOSED;
+                    unload();
+                    break;
+
+                default:
+                    throw new IllegalArgumentException("Transitioning to " + newState + " is not supported.");
+            }
+        }
+
+        /**
+         * Unloads the coordinator.
+         */
+        private void unload() {
+            partitionWriter.deregisterListener(tp, highWatermarklistener);
+            deferredEventQueue.failAll(Errors.NOT_COORDINATOR.exception());
+            if (coordinator != null) {
+                coordinator.onUnloaded();
+                coordinator = null;
+            }
+        }
+    }
+
+    /**
+     * A coordinator write operation.
+     *
+     * @param <S> The type of the coordinator state machine.
+     * @param <T> The type of the response.
+     * @param <U> The type of the records.
+     */
+    public interface CoordinatorWriteOperation<S, T, U> {
+        /**
+         * Generates the records needed to implement this coordinator write operation. In general,
+         * this operation should not modify the hard state of the coordinator. That modifications
+         * will happen later on, when the records generated by this function are applied to the
+         * coordinator.
+         *
+         * @param coordinator The coordinator state machine.
+         * @return A result containing a list of records and the RPC result.
+         * @throws KafkaException
+         */
+        CoordinatorResult<T, U> generateRecordsAndResult(S coordinator) throws KafkaException;
+    }
+
+    /**
+     * A coordinator event that modifies the coordinator state.
+     *
+     * @param <T> The type of the response.
+     */
+    class CoordinatorWriteEvent<T> implements CoordinatorEvent, DeferredEvent {
+        /**
+         * The topic partition that this write event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The write operation to execute.
+         */
+        final CoordinatorWriteOperation<S, T, U> op;
+
+        /**
+         * The future that will be completed with the response
+         * generated by the write operation or an error.
+         */
+        final CompletableFuture<T> future;
+
+        /**
+         * The result of the write operation. It could be null
+         * if an exception is thrown before it is assigned.
+         */
+        CoordinatorResult<T, U> result;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The write operation.
+         */
+        CoordinatorWriteEvent(
+            String name,
+            TopicPartition tp,
+            CoordinatorWriteOperation<S, T, U> op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+            this.future = new CompletableFuture<>();
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                // Get the context of the coordinator or fail if the coordinator is not in active state.
+                CoordinatorContext context = activeContextOrThrow(tp);
+                long prevLastWrittenOffset = context.lastWrittenOffset;
+
+                // Execute the operation.
+                result = op.generateRecordsAndResult(context.coordinator);
+
+                if (result.records().isEmpty()) {
+                    // If the records are empty, it was a read operation after all. In this case,
+                    // the response can be returned directly iff there are no pending write operations;
+                    // otherwise, the read needs to wait on the last write operation to be completed.
+                    OptionalLong pendingOffset = context.deferredEventQueue.highestPendingOffset();
+                    if (pendingOffset.isPresent()) {
+                        context.deferredEventQueue.add(pendingOffset.getAsLong(), this);
+                    } else {
+                        complete(null);
+                    }
+                } else {
+                    // If the records are not empty, first, they are applied to the state machine,
+                    // second, then are written to the partition/log, and finally, the response
+                    // is put into the deferred event queue.
+                    try {
+                        // Apply the records to the state machine.
+                        result.records().forEach(context.coordinator::replay);
+
+                        // Write the records to the log and update the last written
+                        // offset.
+                        long offset = partitionWriter.append(tp, result.records());
+                        context.updateLastWrittenOffset(offset);
+
+                        // Add the response to the deferred queue.
+                        if (!future.isDone()) {
+                            context.deferredEventQueue.add(offset, this);
+                        } else {
+                            complete(null);
+                        }
+                    } catch (Throwable t) {
+                        context.revertLastWrittenOffset(prevLastWrittenOffset);
+                        complete(t);
+                    }
+                }
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Completes the future with either the result of the write operation
+         * or the provided exception.
+         *
+         * @param exception The exception to complete the future with.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception == null) {
+                future.complete(result.response());
+            } else {
+                future.completeExceptionally(exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "CoordinatorWriteEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * A coordinator read operation.
+     *
+     * @param <S> The type of the coordinator state machine.
+     * @param <T> The type of the response.
+     */
+    public interface CoordinatorReadOperation<S, T> {
+        /**
+         * Generates the response to implement this coordinator read operation. A read
+         * operation received the last committed offset. It must use it to ensure that
+         * it does not read uncommitted data from the timeline data structures.
+         *
+         * @param state     The coordinator state machine.
+         * @param offset    The last committed offset.
+         * @return A response.
+         * @throws KafkaException
+         */
+        T generateResponse(S state, long offset) throws KafkaException;
+    }
+
+    /**
+     * A coordinator that reads the committed coordinator state.
+     *
+     * @param <T> The type of the response.
+     */
+    class CoordinatorReadEvent<T> implements CoordinatorEvent {
+        /**
+         * The topic partition that this read event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The read operation to execute.
+         */
+        final CoordinatorReadOperation<S, T> op;
+
+        /**
+         * The future that will be completed with the response
+         * generated by the read operation or an error.
+         */
+        final CompletableFuture<T> future;
+
+        /**
+         * The result of the read operation. It could be null
+         * if an exception is thrown before it is assigned.
+         */
+        T response;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The read operation.
+         */
+        CoordinatorReadEvent(
+            String name,
+            TopicPartition tp,
+            CoordinatorReadOperation<S, T> op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+            this.future = new CompletableFuture<>();
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                // Get the context of the coordinator or fail if the coordinator is not in active state.
+                CoordinatorContext context = activeContextOrThrow(tp);
+
+                // Execute the read operation.
+                response = op.generateResponse(
+                    context.coordinator,
+                    context.lastCommittedOffset
+                );
+
+                // The response can be completed immediately.
+                complete(null);
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Completes the future with either the result of the read operation
+         * or the provided exception.
+         *
+         * @param exception The exception to complete the future with.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception == null) {
+                future.complete(response);
+            } else {
+                future.completeExceptionally(exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "CoordinatorReadEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * A coordinator internal event.
+     */
+    class CoordinatorInternalEvent implements CoordinatorEvent {
+        /**
+         * The topic partition that this internal event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The internal operation to execute.
+         */
+        final Runnable op;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The operation.
+         */
+        CoordinatorInternalEvent(
+            String name,
+            TopicPartition tp,
+            Runnable op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                op.run();
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Logs any exceptions thrown while the event is executed.
+         *
+         * @param exception The exception.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception != null) {
+                log.error("Execution of {} failed due to {}.", name, exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "InternalEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * Partition listener to be notified when the high watermark of the partitions
+     * backing the coordinator are updated.
+     */
+    class HighWatermarkListener implements PartitionWriter.Listener {
+        /**
+         * Updates the high watermark of the corresponding coordinator.
+         *
+         * @param tp        The topic partition.
+         * @param offset    The new high watermark.
+         */
+        @Override
+        public void onHighWatermarkUpdated(
+            TopicPartition tp,
+            long offset
+        ) {
+            log.debug("High watermark of {} incremented to {}.", tp, offset);
+            scheduleInternalOperation("HighWatermarkUpdated(tp=" + tp + ", offset=" + offset + ")", tp, () -> {
+                contextOrThrow(tp).updateLastCommittedOffset(offset);
+            });
+        }
+    }
+
+    /**
+     * The log context.
+     */
+    private final LogContext logContext;
+
+    /**
+     * The logger.
+     */
+    private final Logger log;
+
+    /**
+     * The coordinators keyed by topic partition.
+     */
+    private final ConcurrentHashMap<TopicPartition, CoordinatorContext> coordinators;
+
+    /**
+     * The event processor used by the runtime.
+     */
+    private final CoordinatorEventProcessor processor;
+
+    /**
+     * The partition writer used by the runtime to persist records.
+     */
+    private final PartitionWriter<U> partitionWriter;
+
+    /**
+     * The high watermark listener registered to all the partitions
+     * backing the coordinators.
+     */
+    private final HighWatermarkListener highWatermarklistener;
+
+    /**
+     * The coordinator loaded used by the runtime.
+     */
+    private final CoordinatorLoader<U> loader;
+
+    /**
+     * The coordinator state machine builder used by the runtime
+     * to instantiate a coordinator.
+     */
+    private final CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier;
+
+    /**
+     * Constructor.
+     *
+     * @param logContext                    The log context.
+     * @param processor                     The event processor.
+     * @param partitionWriter               The partition writer.
+     * @param loader                        The coordinator loader.
+     * @param coordinatorBuilderSupplier    The coordinator builder.
+     */
+    private CoordinatorRuntime(
+        LogContext logContext,
+        CoordinatorEventProcessor processor,
+        PartitionWriter<U> partitionWriter,
+        CoordinatorLoader<U> loader,
+        CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier
+    ) {
+        this.logContext = logContext;
+        this.log = logContext.logger(CoordinatorRuntime.class);
+        this.coordinators = new ConcurrentHashMap<>();
+        this.processor = processor;
+        this.partitionWriter = partitionWriter;
+        this.highWatermarklistener = new HighWatermarkListener();
+        this.loader = loader;
+        this.coordinatorBuilderSupplier = coordinatorBuilderSupplier;
+    }
+
+    /**
+     * @return The coordinator context if the coordinator is active or an exception otherwise.
+     * @throws NotCoordinatorException
+     * @throws CoordinatorLoadInProgressException
+     */
+    private CoordinatorContext activeContextOrThrow(TopicPartition tp) {
+        CoordinatorContext context = coordinators.get(tp);
+
+        if (context == null) {
+            throw Errors.NOT_COORDINATOR.exception();
+        } else {
+            switch (context.state) {
+                case INITIAL:
+                case FAILED:
+                case CLOSED:
+                    throw Errors.NOT_COORDINATOR.exception();
+
+                case LOADING:
+                    throw Errors.COORDINATOR_LOAD_IN_PROGRESS.exception();
+            }
+        }
+
+        return context;
+    }
+
+    /**
+     * @return The coordinator context. It is created if it does not exist.
+     */
+    private CoordinatorContext getOrCreateContext(TopicPartition tp) {
+        return coordinators.computeIfAbsent(tp, CoordinatorContext::new);
+    }
+
+    /**
+     * @return The coordinator context or thrown an exception if it does
+     * not exist.
+     * @throws NotCoordinatorException
+     * Package private for testing.
+     */
+    CoordinatorContext contextOrThrow(TopicPartition tp) {
+        CoordinatorContext context = coordinators.get(tp);
+
+        if (context == null) {
+            throw Errors.NOT_COORDINATOR.exception();
+        } else {
+            return context;
+        }
+    }
+
+    /**
+     * Schedules a write operation.
+     *
+     * @param name  The name of the write operation.
+     * @param tp    The address of the coordinator (aka its topic-partitions).
+     * @param op    The write operation.
+     *
+     * @return A future that will be completed with the result of the write operation
+     * when the operation is completed or an exception if the write operation failed.
+     *
+     * @param <T> The type of the result.
+     */
+    public <T> CompletableFuture<T> scheduleWriteOperation(
+        String name,
+        TopicPartition tp,
+        CoordinatorWriteOperation<S, T, U> op
+    ) {
+        log.debug("Scheduled execution of write operation {}.", name);
+        CoordinatorWriteEvent<T> event = new CoordinatorWriteEvent<>(name, tp, op);
+        processor.enqueue(event);
+        return event.future;
+    }
+
+    /**
+     * Schedules a read operation.
+     *
+     * @param name  The name of the write operation.
+     * @param tp    The address of the coordinator (aka its topic-partitions).
+     * @param op    The read operation.
+     *
+     * @return A future that will be completed with the result of the read operation
+     * when the operation is completed or an exception if the write operation failed.
+     *
+     * @param <T> The type of the result.
+     */
+    public <T> CompletableFuture<T> scheduleReadOperation(
+        String name,
+        TopicPartition tp,
+        CoordinatorReadOperation<S, T> op
+    ) {
+        log.debug("Scheduled execution of read operation {}.", name);
+        CoordinatorReadEvent<T> event = new CoordinatorReadEvent<>(name, tp, op);
+        processor.enqueue(event);
+        return event.future;
+    }
+
+    /**
+     * Schedules an internal event.
+     *
+     * @param name  The name of the write operation.
+     * @param tp    The address of the coordinator (aka its topic-partitions).
+     * @param op    The operation.
+     */
+    private void scheduleInternalOperation(
+        String name,
+        TopicPartition tp,
+        Runnable op
+    ) {
+        log.debug("Scheduled execution of internal operation {}.", name);
+        processor.enqueue(new CoordinatorInternalEvent(name, tp, op));
+    }
+
+    /**
+     * @return The topic partitions of the coordinators currently registered in the
+     * runtime.
+     */
+    public Set<TopicPartition> partitions() {
+        return new HashSet<>(coordinators.keySet());
+    }
+
+    /**
+     * Schedules the loading of a coordinator. This is called when the broker is elected as
+     * the leader for a partition.
+     *
+     * @param tp                The topic partition of the coordinator. Records from this
+     *                          partitions will be read and applied to the coordinator.
+     * @param partitionEpoch    The epoch of the partition.
+     */
+    public void scheduleLoadOperation(
+        TopicPartition tp,
+        int partitionEpoch
+    ) {
+        log.info("Scheduling loading of metadata from {} with epoch {}", tp, partitionEpoch);
+        // Touch the state to make the runtime immediately aware of the new coordinator.
+        getOrCreateContext(tp);
+
+        scheduleInternalOperation("Load(tp=" + tp + ", epoch=" + partitionEpoch + ")", tp, () -> {
+            CoordinatorContext context = getOrCreateContext(tp);
+
+            if (context.state == CoordinatorState.FAILED) {
+                // When the coordinator has failed, we create a new context instead of
+                // recycling the previous one because it is better to start from an
+                // empty state for timeline data structures.
+                coordinators.remove(tp);
+                context = getOrCreateContext(tp);
+            }
+
+            if (context.epoch < partitionEpoch) {
+                context.epoch = partitionEpoch;
+
+                switch (context.state) {
+                    case INITIAL:
+                        context.transitionTo(CoordinatorState.LOADING);
+                        loader.load(tp, context.coordinator).whenComplete((state, exception) -> {
+                            scheduleInternalOperation("CompleteLoad(tp=" + tp + ", epoch=" + partitionEpoch + ")", tp, () -> {
+                                CoordinatorContext ctx = contextOrThrow(tp);
+                                if (ctx.state != CoordinatorState.LOADING) {
+                                    log.info("Ignoring load completion from {} because context is in {} state.",
+                                        ctx.tp, ctx.state);
+                                    return;
+                                }
+                                try {
+                                    if (exception != null) throw exception;
+                                    ctx.transitionTo(CoordinatorState.ACTIVE);
+                                    log.info("Finished loading of metadata from {} with epoch {}.",
+                                        tp, partitionEpoch);
+                                } catch (Throwable ex) {
+                                    log.error("Failed to load metadata from {} with epoch {} due to {}.",
+                                        tp, partitionEpoch, ex.toString());
+                                    ctx.transitionTo(CoordinatorState.FAILED);
+                                }
+                            });
+                        });
+                        break;
+
+                    case LOADING:
+                        log.info("The coordinator {} is already loading metadata.", tp);
+                        break;
+
+                    case ACTIVE:
+                        log.info("The coordinator {} is already active.", tp);
+                        break;
+
+                    default:
+                        log.error("Cannot load coordinator {} in state {}.", tp, context.state);
+                }
+            } else {
+                log.info("Ignoring loading metadata from {} since current epoch {} is larger than or equals to {}.",
+                    context.tp, context.epoch, partitionEpoch);
+            }
+        });
+    }
+
+    /**
+     * Schedules the unloading of a coordinator. This is called when the broker is not the
+     * leader anymore.
+     *
+     * @param tp                The topic partition of the coordinator.
+     * @param partitionEpoch    The partition epoch.
+     */
+    public void scheduleUnloadOperation(
+        TopicPartition tp,
+        int partitionEpoch
+    ) {
+        log.info("Scheduling unloading of metadata for {} with epoch {}", tp, partitionEpoch);
+
+        scheduleInternalOperation("UnloadCoordinator(tp=" + tp + ", epoch=" + partitionEpoch + ")", tp, () -> {
+            CoordinatorContext context = contextOrThrow(tp);
+            if (context.epoch < partitionEpoch) {
+                log.info("Started unloading metadata for {} with epoch {}.", tp, partitionEpoch);
+                context.transitionTo(CoordinatorState.CLOSED);
+                coordinators.remove(tp);
+                log.info("Finished unloading metadata for {} with epoch {}.", tp, partitionEpoch);
+            } else {
+                log.info("Ignored unloading metadata for {} in epoch {} since current epoch is {}.",
+                    tp, partitionEpoch, context.epoch);
+            }
+        });
+    }
+
+    /**
+     * Closes the runtime. This closes all the coordinators currently registered
+     * in the runtime.
+     *
+     * @throws Exception
+     */
+    public void close() throws Exception {
+        log.info("Closing coordinator runtime.");
+        // This close the processor, drain all the pending events and
+        // reject any new events.
+        processor.close();

Review Comment:
   CloseQuietly?



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/runtime/CoordinatorRuntime.java:
##########
@@ -0,0 +1,1040 @@
+/*
+ * 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.kafka.coordinator.group.runtime;
+
+import org.apache.kafka.common.KafkaException;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.errors.CoordinatorLoadInProgressException;
+import org.apache.kafka.common.errors.NotCoordinatorException;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.deferred.DeferredEvent;
+import org.apache.kafka.deferred.DeferredEventQueue;
+import org.apache.kafka.timeline.SnapshotRegistry;
+import org.slf4j.Logger;
+
+import java.util.HashSet;
+import java.util.OptionalLong;
+import java.util.Set;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * The CoordinatorRuntime provides a framework to implement coordinators such as the group coordinator
+ * or the transaction coordinator.
+ *
+ * The runtime framework maps each underlying partitions (e.g. __consumer_offsets) that that broker is a
+ * leader of to a coordinator replicated state machine. A replicated state machine holds the hard and soft
+ * state of all the objects (e.g. groups or offsets) assigned to the partition. The hard state is stored in
+ * timeline datastructures backed by a SnapshotRegistry. The runtime supports two type of operations
+ * on state machines: (1) Writes and (2) Reads.
+ *
+ * (1) A write operation, aka a request, can read the full and potentially **uncommitted** state from state
+ * machine to handle the operation. A write operation typically generates a response and a list of
+ * records. The records are applies to the state machine and persisted to the partition. The response
+ * is parked until the records are committed and delivered when they are.
+ *
+ * (2) A read operation, aka a request, can only read the committed state from the state machine to handle
+ * the operation. A read operation typically generates a response that is immediately completed.
+ *
+ * The runtime framework exposes an asynchronous, future based, API to the world. All the operations
+ * are executed by an CoordinatorEventProcessor. The processor guarantees that operations for a
+ * single partition or state machine are not processed concurrently.
+ *
+ * @param <S> The type of the state machine.
+ * @param <U> The type of the record.
+ */
+public class CoordinatorRuntime<S extends Coordinator<U>, U> {
+
+    /**
+     * Builder to create a CoordinatorRuntime.
+     *
+     * @param <S> The type of the state machine.
+     * @param <U> The type of the record.
+     */
+    public static class Builder<S extends Coordinator<U>, U> {
+        private LogContext logContext;
+        private CoordinatorEventProcessor eventProcessor;
+        private PartitionWriter<U> partitionWriter;
+        private CoordinatorLoader<U> loader;
+        private CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier;
+
+        public Builder<S, U> withLogContext(LogContext logContext) {
+            this.logContext = logContext;
+            return this;
+        }
+
+        public Builder<S, U> withEventProcessor(CoordinatorEventProcessor eventProcessor) {
+            this.eventProcessor = eventProcessor;
+            return this;
+        }
+
+        public Builder<S, U> withPartitionWriter(PartitionWriter<U> partitionWriter) {
+            this.partitionWriter = partitionWriter;
+            return this;
+        }
+
+        public Builder<S, U> withLoader(CoordinatorLoader<U> loader) {
+            this.loader = loader;
+            return this;
+        }
+
+        public Builder<S, U> withCoordinatorBuilderSupplier(CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier) {
+            this.coordinatorBuilderSupplier = coordinatorBuilderSupplier;
+            return this;
+        }
+
+        public CoordinatorRuntime<S, U> build() {
+            if (logContext == null)
+                logContext = new LogContext();
+            if (eventProcessor == null)
+                throw new IllegalArgumentException("Event processor must be set.");
+            if (partitionWriter == null)
+                throw new IllegalArgumentException("Partition write must be set.");
+            if (loader == null)
+                throw new IllegalArgumentException("Loader must be set.");
+            if (coordinatorBuilderSupplier == null)
+                throw new IllegalArgumentException("State machine supplier must be set.");
+
+            return new CoordinatorRuntime<>(
+                logContext,
+                eventProcessor,
+                partitionWriter,
+                loader,
+                coordinatorBuilderSupplier
+            );
+        }
+    }
+
+    /**
+     * The various state that a coordinator for a partition can be in.
+     */
+    enum CoordinatorState {
+        /**
+         * Initial state when a coordinator is created.
+         */
+        INITIAL {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return false;
+            }
+        },
+
+        /**
+         * The coordinator is being loaded.
+         */
+        LOADING {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == INITIAL;
+            }
+        },
+
+        /**
+         * The coordinator is active and can service requests.
+         */
+        ACTIVE {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == ACTIVE || state == LOADING;
+            }
+        },
+
+        /**
+         * The coordinator is closed.
+         */
+        CLOSED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return true;
+            }
+        },
+
+        /**
+         * The coordinator loading has failed.
+         */
+        FAILED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == LOADING;
+            }
+        };
+
+        abstract boolean canTransitionFrom(CoordinatorState state);
+    }
+
+    /**
+     * CoordinatorContext holds all the metadata around a coordinator state machine.
+     */
+    class CoordinatorContext {
+        /**
+         * The topic partition backing the coordinator.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The snapshot registry backing the coordinator.
+         */
+        final SnapshotRegistry snapshotRegistry;
+
+        /**
+         * The deferred event queue used to park events waiting
+         * on records to be committed.
+         */
+        final DeferredEventQueue deferredEventQueue;
+
+        /**
+         * The current state.
+         */
+        volatile CoordinatorState state;
+
+        /**
+         * The actual state machine.
+         */
+        volatile S coordinator;
+
+        /**
+         * The current epoch of the coordinator. This represents
+         * the epoch of the partition leader.
+         */
+        volatile int epoch;
+
+        /**
+         * The last offset written to the partition.
+         */
+        volatile long lastWrittenOffset;
+
+        /**
+         * The last offset committed. This represents the high
+         * watermark of the partition.
+         */
+        volatile long lastCommittedOffset;
+
+        /**
+         * Constructor.
+         *
+         * @param tp The topic partition of the coordinator.
+         */
+        private CoordinatorContext(
+            TopicPartition tp
+        ) {
+            this.tp = tp;
+            this.snapshotRegistry = new SnapshotRegistry(logContext);
+            this.deferredEventQueue = new DeferredEventQueue(logContext);
+            this.state = CoordinatorState.INITIAL;
+            this.epoch = -1;
+            this.lastWrittenOffset = 0L;
+            this.lastCommittedOffset = 0L;
+        }
+
+        /**
+         * Updates the last written offset. This also create a new snapshot
+         * in the snapshot registry.
+         *
+         * @param offset The new last written offset.
+         */
+        private void updateLastWrittenOffset(
+            long offset
+        ) {
+            if (offset <= lastWrittenOffset) {
+                throw new IllegalStateException("New last written offset " + offset + " of " + tp +
+                    " must be larger than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Update last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.getOrCreateSnapshot(offset);
+        }
+
+        /**
+         * Reverts the last written offset. This also reverts the snapshot
+         * registry to this offset. All the changes applied after the offset
+         * are lost.
+         *
+         * @param offset The offset to revert to.
+         */
+        private void revertLastWrittenOffset(
+            long offset
+        ) {
+            if (offset > lastWrittenOffset) {
+                throw new IllegalStateException("New offset " + offset + " of " + tp +
+                    " must be smaller than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Revert last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.revertToSnapshot(offset);
+        }
+
+        /**
+         * Updates the last committed offset. This completes all the deferred
+         * events waiting on this offset. This also cleanups all the snapshots
+         * prior to this offset.
+         *
+         * @param offset The new last committed offset.
+         */
+        private void updateLastCommittedOffset(
+            long offset
+        ) {
+            if (offset <= lastCommittedOffset) {
+                throw new IllegalStateException("New committed offset " + offset + " of " + tp +
+                    " must be larger than " + lastCommittedOffset + ".");
+            }
+
+            log.debug("Update committed offset of {} to {}.", tp, offset);
+            lastCommittedOffset = offset;
+            deferredEventQueue.completeUpTo(offset);
+            snapshotRegistry.deleteSnapshotsUpTo(offset);
+        }
+
+        /**
+         * Transitions to the new state.
+         *
+         * @param newState The new state.
+         */
+        private void transitionTo(
+            CoordinatorState newState
+        ) {
+            if (!newState.canTransitionFrom(state)) {
+                throw new IllegalStateException("Cannot transition from " + state + " to " + newState);
+            }
+
+            log.debug("Transition from {} to {}.", state, newState);
+            switch (newState) {
+                case LOADING:
+                    state = CoordinatorState.LOADING;
+                    coordinator = coordinatorBuilderSupplier
+                        .get()
+                        .withSnapshotRegistry(snapshotRegistry)
+                        .build();
+                    break;
+
+                case ACTIVE:
+                    state = CoordinatorState.ACTIVE;
+                    snapshotRegistry.getOrCreateSnapshot(0);
+                    partitionWriter.registerListener(tp, highWatermarklistener);
+                    coordinator.onLoaded();
+                    break;
+
+                case FAILED:
+                    state = CoordinatorState.FAILED;
+                    unload();
+                    break;
+
+                case CLOSED:
+                    state = CoordinatorState.CLOSED;
+                    unload();
+                    break;
+
+                default:
+                    throw new IllegalArgumentException("Transitioning to " + newState + " is not supported.");
+            }
+        }
+
+        /**
+         * Unloads the coordinator.
+         */
+        private void unload() {
+            partitionWriter.deregisterListener(tp, highWatermarklistener);
+            deferredEventQueue.failAll(Errors.NOT_COORDINATOR.exception());
+            if (coordinator != null) {
+                coordinator.onUnloaded();
+                coordinator = null;
+            }
+        }
+    }
+
+    /**
+     * A coordinator write operation.
+     *
+     * @param <S> The type of the coordinator state machine.
+     * @param <T> The type of the response.
+     * @param <U> The type of the records.
+     */
+    public interface CoordinatorWriteOperation<S, T, U> {
+        /**
+         * Generates the records needed to implement this coordinator write operation. In general,
+         * this operation should not modify the hard state of the coordinator. That modifications
+         * will happen later on, when the records generated by this function are applied to the
+         * coordinator.
+         *
+         * @param coordinator The coordinator state machine.
+         * @return A result containing a list of records and the RPC result.
+         * @throws KafkaException
+         */
+        CoordinatorResult<T, U> generateRecordsAndResult(S coordinator) throws KafkaException;
+    }
+
+    /**
+     * A coordinator event that modifies the coordinator state.
+     *
+     * @param <T> The type of the response.
+     */
+    class CoordinatorWriteEvent<T> implements CoordinatorEvent, DeferredEvent {
+        /**
+         * The topic partition that this write event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The write operation to execute.
+         */
+        final CoordinatorWriteOperation<S, T, U> op;
+
+        /**
+         * The future that will be completed with the response
+         * generated by the write operation or an error.
+         */
+        final CompletableFuture<T> future;
+
+        /**
+         * The result of the write operation. It could be null
+         * if an exception is thrown before it is assigned.
+         */
+        CoordinatorResult<T, U> result;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The write operation.
+         */
+        CoordinatorWriteEvent(
+            String name,
+            TopicPartition tp,
+            CoordinatorWriteOperation<S, T, U> op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+            this.future = new CompletableFuture<>();
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                // Get the context of the coordinator or fail if the coordinator is not in active state.
+                CoordinatorContext context = activeContextOrThrow(tp);
+                long prevLastWrittenOffset = context.lastWrittenOffset;
+
+                // Execute the operation.
+                result = op.generateRecordsAndResult(context.coordinator);
+
+                if (result.records().isEmpty()) {
+                    // If the records are empty, it was a read operation after all. In this case,
+                    // the response can be returned directly iff there are no pending write operations;
+                    // otherwise, the read needs to wait on the last write operation to be completed.
+                    OptionalLong pendingOffset = context.deferredEventQueue.highestPendingOffset();
+                    if (pendingOffset.isPresent()) {
+                        context.deferredEventQueue.add(pendingOffset.getAsLong(), this);
+                    } else {
+                        complete(null);
+                    }
+                } else {
+                    // If the records are not empty, first, they are applied to the state machine,
+                    // second, then are written to the partition/log, and finally, the response
+                    // is put into the deferred event queue.
+                    try {
+                        // Apply the records to the state machine.
+                        result.records().forEach(context.coordinator::replay);
+
+                        // Write the records to the log and update the last written
+                        // offset.
+                        long offset = partitionWriter.append(tp, result.records());
+                        context.updateLastWrittenOffset(offset);
+
+                        // Add the response to the deferred queue.
+                        if (!future.isDone()) {
+                            context.deferredEventQueue.add(offset, this);
+                        } else {
+                            complete(null);
+                        }
+                    } catch (Throwable t) {
+                        context.revertLastWrittenOffset(prevLastWrittenOffset);
+                        complete(t);
+                    }
+                }
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Completes the future with either the result of the write operation
+         * or the provided exception.
+         *
+         * @param exception The exception to complete the future with.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception == null) {
+                future.complete(result.response());
+            } else {
+                future.completeExceptionally(exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "CoordinatorWriteEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * A coordinator read operation.
+     *
+     * @param <S> The type of the coordinator state machine.
+     * @param <T> The type of the response.
+     */
+    public interface CoordinatorReadOperation<S, T> {
+        /**
+         * Generates the response to implement this coordinator read operation. A read
+         * operation received the last committed offset. It must use it to ensure that
+         * it does not read uncommitted data from the timeline data structures.
+         *
+         * @param state     The coordinator state machine.
+         * @param offset    The last committed offset.
+         * @return A response.
+         * @throws KafkaException
+         */
+        T generateResponse(S state, long offset) throws KafkaException;
+    }
+
+    /**
+     * A coordinator that reads the committed coordinator state.
+     *
+     * @param <T> The type of the response.
+     */
+    class CoordinatorReadEvent<T> implements CoordinatorEvent {
+        /**
+         * The topic partition that this read event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The read operation to execute.
+         */
+        final CoordinatorReadOperation<S, T> op;
+
+        /**
+         * The future that will be completed with the response
+         * generated by the read operation or an error.
+         */
+        final CompletableFuture<T> future;
+
+        /**
+         * The result of the read operation. It could be null
+         * if an exception is thrown before it is assigned.
+         */
+        T response;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The read operation.
+         */
+        CoordinatorReadEvent(
+            String name,
+            TopicPartition tp,
+            CoordinatorReadOperation<S, T> op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+            this.future = new CompletableFuture<>();
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                // Get the context of the coordinator or fail if the coordinator is not in active state.
+                CoordinatorContext context = activeContextOrThrow(tp);
+
+                // Execute the read operation.
+                response = op.generateResponse(
+                    context.coordinator,
+                    context.lastCommittedOffset
+                );
+
+                // The response can be completed immediately.
+                complete(null);
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Completes the future with either the result of the read operation
+         * or the provided exception.
+         *
+         * @param exception The exception to complete the future with.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception == null) {
+                future.complete(response);
+            } else {
+                future.completeExceptionally(exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "CoordinatorReadEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * A coordinator internal event.
+     */
+    class CoordinatorInternalEvent implements CoordinatorEvent {
+        /**
+         * The topic partition that this internal event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The internal operation to execute.
+         */
+        final Runnable op;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The operation.
+         */
+        CoordinatorInternalEvent(
+            String name,
+            TopicPartition tp,
+            Runnable op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                op.run();
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Logs any exceptions thrown while the event is executed.
+         *
+         * @param exception The exception.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception != null) {
+                log.error("Execution of {} failed due to {}.", name, exception);

Review Comment:
   you can get rid of the second `{}` because last parameter can be added as a exception without having to parameterize it. see: https://www.slf4j.org/faq.html#paramException



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/runtime/CoordinatorRuntime.java:
##########
@@ -0,0 +1,1040 @@
+/*
+ * 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.kafka.coordinator.group.runtime;
+
+import org.apache.kafka.common.KafkaException;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.errors.CoordinatorLoadInProgressException;
+import org.apache.kafka.common.errors.NotCoordinatorException;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.deferred.DeferredEvent;
+import org.apache.kafka.deferred.DeferredEventQueue;
+import org.apache.kafka.timeline.SnapshotRegistry;
+import org.slf4j.Logger;
+
+import java.util.HashSet;
+import java.util.OptionalLong;
+import java.util.Set;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * The CoordinatorRuntime provides a framework to implement coordinators such as the group coordinator
+ * or the transaction coordinator.
+ *
+ * The runtime framework maps each underlying partitions (e.g. __consumer_offsets) that that broker is a
+ * leader of to a coordinator replicated state machine. A replicated state machine holds the hard and soft
+ * state of all the objects (e.g. groups or offsets) assigned to the partition. The hard state is stored in
+ * timeline datastructures backed by a SnapshotRegistry. The runtime supports two type of operations
+ * on state machines: (1) Writes and (2) Reads.
+ *
+ * (1) A write operation, aka a request, can read the full and potentially **uncommitted** state from state
+ * machine to handle the operation. A write operation typically generates a response and a list of
+ * records. The records are applies to the state machine and persisted to the partition. The response
+ * is parked until the records are committed and delivered when they are.
+ *
+ * (2) A read operation, aka a request, can only read the committed state from the state machine to handle
+ * the operation. A read operation typically generates a response that is immediately completed.
+ *
+ * The runtime framework exposes an asynchronous, future based, API to the world. All the operations
+ * are executed by an CoordinatorEventProcessor. The processor guarantees that operations for a
+ * single partition or state machine are not processed concurrently.
+ *
+ * @param <S> The type of the state machine.
+ * @param <U> The type of the record.
+ */
+public class CoordinatorRuntime<S extends Coordinator<U>, U> {
+
+    /**
+     * Builder to create a CoordinatorRuntime.
+     *
+     * @param <S> The type of the state machine.
+     * @param <U> The type of the record.
+     */
+    public static class Builder<S extends Coordinator<U>, U> {
+        private LogContext logContext;
+        private CoordinatorEventProcessor eventProcessor;
+        private PartitionWriter<U> partitionWriter;
+        private CoordinatorLoader<U> loader;
+        private CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier;
+
+        public Builder<S, U> withLogContext(LogContext logContext) {
+            this.logContext = logContext;
+            return this;
+        }
+
+        public Builder<S, U> withEventProcessor(CoordinatorEventProcessor eventProcessor) {
+            this.eventProcessor = eventProcessor;
+            return this;
+        }
+
+        public Builder<S, U> withPartitionWriter(PartitionWriter<U> partitionWriter) {
+            this.partitionWriter = partitionWriter;
+            return this;
+        }
+
+        public Builder<S, U> withLoader(CoordinatorLoader<U> loader) {
+            this.loader = loader;
+            return this;
+        }
+
+        public Builder<S, U> withCoordinatorBuilderSupplier(CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier) {
+            this.coordinatorBuilderSupplier = coordinatorBuilderSupplier;
+            return this;
+        }
+
+        public CoordinatorRuntime<S, U> build() {
+            if (logContext == null)
+                logContext = new LogContext();
+            if (eventProcessor == null)
+                throw new IllegalArgumentException("Event processor must be set.");
+            if (partitionWriter == null)
+                throw new IllegalArgumentException("Partition write must be set.");
+            if (loader == null)
+                throw new IllegalArgumentException("Loader must be set.");
+            if (coordinatorBuilderSupplier == null)
+                throw new IllegalArgumentException("State machine supplier must be set.");
+
+            return new CoordinatorRuntime<>(
+                logContext,
+                eventProcessor,
+                partitionWriter,
+                loader,
+                coordinatorBuilderSupplier
+            );
+        }
+    }
+
+    /**
+     * The various state that a coordinator for a partition can be in.
+     */
+    enum CoordinatorState {
+        /**
+         * Initial state when a coordinator is created.
+         */
+        INITIAL {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return false;
+            }
+        },
+
+        /**
+         * The coordinator is being loaded.
+         */
+        LOADING {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == INITIAL;
+            }
+        },
+
+        /**
+         * The coordinator is active and can service requests.
+         */
+        ACTIVE {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == ACTIVE || state == LOADING;
+            }
+        },
+
+        /**
+         * The coordinator is closed.
+         */
+        CLOSED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return true;
+            }
+        },
+
+        /**
+         * The coordinator loading has failed.
+         */
+        FAILED {
+            @Override
+            boolean canTransitionFrom(CoordinatorState state) {
+                return state == LOADING;
+            }
+        };
+
+        abstract boolean canTransitionFrom(CoordinatorState state);
+    }
+
+    /**
+     * CoordinatorContext holds all the metadata around a coordinator state machine.
+     */
+    class CoordinatorContext {
+        /**
+         * The topic partition backing the coordinator.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The snapshot registry backing the coordinator.
+         */
+        final SnapshotRegistry snapshotRegistry;
+
+        /**
+         * The deferred event queue used to park events waiting
+         * on records to be committed.
+         */
+        final DeferredEventQueue deferredEventQueue;
+
+        /**
+         * The current state.
+         */
+        volatile CoordinatorState state;
+
+        /**
+         * The actual state machine.
+         */
+        volatile S coordinator;
+
+        /**
+         * The current epoch of the coordinator. This represents
+         * the epoch of the partition leader.
+         */
+        volatile int epoch;
+
+        /**
+         * The last offset written to the partition.
+         */
+        volatile long lastWrittenOffset;
+
+        /**
+         * The last offset committed. This represents the high
+         * watermark of the partition.
+         */
+        volatile long lastCommittedOffset;
+
+        /**
+         * Constructor.
+         *
+         * @param tp The topic partition of the coordinator.
+         */
+        private CoordinatorContext(
+            TopicPartition tp
+        ) {
+            this.tp = tp;
+            this.snapshotRegistry = new SnapshotRegistry(logContext);
+            this.deferredEventQueue = new DeferredEventQueue(logContext);
+            this.state = CoordinatorState.INITIAL;
+            this.epoch = -1;
+            this.lastWrittenOffset = 0L;
+            this.lastCommittedOffset = 0L;
+        }
+
+        /**
+         * Updates the last written offset. This also create a new snapshot
+         * in the snapshot registry.
+         *
+         * @param offset The new last written offset.
+         */
+        private void updateLastWrittenOffset(
+            long offset
+        ) {
+            if (offset <= lastWrittenOffset) {
+                throw new IllegalStateException("New last written offset " + offset + " of " + tp +
+                    " must be larger than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Update last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.getOrCreateSnapshot(offset);
+        }
+
+        /**
+         * Reverts the last written offset. This also reverts the snapshot
+         * registry to this offset. All the changes applied after the offset
+         * are lost.
+         *
+         * @param offset The offset to revert to.
+         */
+        private void revertLastWrittenOffset(
+            long offset
+        ) {
+            if (offset > lastWrittenOffset) {
+                throw new IllegalStateException("New offset " + offset + " of " + tp +
+                    " must be smaller than " + lastWrittenOffset + ".");
+            }
+
+            log.debug("Revert last written offset of {} to {}.", tp, offset);
+            lastWrittenOffset = offset;
+            snapshotRegistry.revertToSnapshot(offset);
+        }
+
+        /**
+         * Updates the last committed offset. This completes all the deferred
+         * events waiting on this offset. This also cleanups all the snapshots
+         * prior to this offset.
+         *
+         * @param offset The new last committed offset.
+         */
+        private void updateLastCommittedOffset(
+            long offset
+        ) {
+            if (offset <= lastCommittedOffset) {
+                throw new IllegalStateException("New committed offset " + offset + " of " + tp +
+                    " must be larger than " + lastCommittedOffset + ".");
+            }
+
+            log.debug("Update committed offset of {} to {}.", tp, offset);
+            lastCommittedOffset = offset;
+            deferredEventQueue.completeUpTo(offset);
+            snapshotRegistry.deleteSnapshotsUpTo(offset);
+        }
+
+        /**
+         * Transitions to the new state.
+         *
+         * @param newState The new state.
+         */
+        private void transitionTo(
+            CoordinatorState newState
+        ) {
+            if (!newState.canTransitionFrom(state)) {
+                throw new IllegalStateException("Cannot transition from " + state + " to " + newState);
+            }
+
+            log.debug("Transition from {} to {}.", state, newState);
+            switch (newState) {
+                case LOADING:
+                    state = CoordinatorState.LOADING;
+                    coordinator = coordinatorBuilderSupplier
+                        .get()
+                        .withSnapshotRegistry(snapshotRegistry)
+                        .build();
+                    break;
+
+                case ACTIVE:
+                    state = CoordinatorState.ACTIVE;
+                    snapshotRegistry.getOrCreateSnapshot(0);
+                    partitionWriter.registerListener(tp, highWatermarklistener);
+                    coordinator.onLoaded();
+                    break;
+
+                case FAILED:
+                    state = CoordinatorState.FAILED;
+                    unload();
+                    break;
+
+                case CLOSED:
+                    state = CoordinatorState.CLOSED;
+                    unload();
+                    break;
+
+                default:
+                    throw new IllegalArgumentException("Transitioning to " + newState + " is not supported.");
+            }
+        }
+
+        /**
+         * Unloads the coordinator.
+         */
+        private void unload() {
+            partitionWriter.deregisterListener(tp, highWatermarklistener);
+            deferredEventQueue.failAll(Errors.NOT_COORDINATOR.exception());
+            if (coordinator != null) {
+                coordinator.onUnloaded();
+                coordinator = null;
+            }
+        }
+    }
+
+    /**
+     * A coordinator write operation.
+     *
+     * @param <S> The type of the coordinator state machine.
+     * @param <T> The type of the response.
+     * @param <U> The type of the records.
+     */
+    public interface CoordinatorWriteOperation<S, T, U> {
+        /**
+         * Generates the records needed to implement this coordinator write operation. In general,
+         * this operation should not modify the hard state of the coordinator. That modifications
+         * will happen later on, when the records generated by this function are applied to the
+         * coordinator.
+         *
+         * @param coordinator The coordinator state machine.
+         * @return A result containing a list of records and the RPC result.
+         * @throws KafkaException
+         */
+        CoordinatorResult<T, U> generateRecordsAndResult(S coordinator) throws KafkaException;
+    }
+
+    /**
+     * A coordinator event that modifies the coordinator state.
+     *
+     * @param <T> The type of the response.
+     */
+    class CoordinatorWriteEvent<T> implements CoordinatorEvent, DeferredEvent {
+        /**
+         * The topic partition that this write event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The write operation to execute.
+         */
+        final CoordinatorWriteOperation<S, T, U> op;
+
+        /**
+         * The future that will be completed with the response
+         * generated by the write operation or an error.
+         */
+        final CompletableFuture<T> future;
+
+        /**
+         * The result of the write operation. It could be null
+         * if an exception is thrown before it is assigned.
+         */
+        CoordinatorResult<T, U> result;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The write operation.
+         */
+        CoordinatorWriteEvent(
+            String name,
+            TopicPartition tp,
+            CoordinatorWriteOperation<S, T, U> op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+            this.future = new CompletableFuture<>();
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                // Get the context of the coordinator or fail if the coordinator is not in active state.
+                CoordinatorContext context = activeContextOrThrow(tp);
+                long prevLastWrittenOffset = context.lastWrittenOffset;
+
+                // Execute the operation.
+                result = op.generateRecordsAndResult(context.coordinator);
+
+                if (result.records().isEmpty()) {
+                    // If the records are empty, it was a read operation after all. In this case,
+                    // the response can be returned directly iff there are no pending write operations;
+                    // otherwise, the read needs to wait on the last write operation to be completed.
+                    OptionalLong pendingOffset = context.deferredEventQueue.highestPendingOffset();
+                    if (pendingOffset.isPresent()) {
+                        context.deferredEventQueue.add(pendingOffset.getAsLong(), this);
+                    } else {
+                        complete(null);
+                    }
+                } else {
+                    // If the records are not empty, first, they are applied to the state machine,
+                    // second, then are written to the partition/log, and finally, the response
+                    // is put into the deferred event queue.
+                    try {
+                        // Apply the records to the state machine.
+                        result.records().forEach(context.coordinator::replay);
+
+                        // Write the records to the log and update the last written
+                        // offset.
+                        long offset = partitionWriter.append(tp, result.records());
+                        context.updateLastWrittenOffset(offset);
+
+                        // Add the response to the deferred queue.
+                        if (!future.isDone()) {
+                            context.deferredEventQueue.add(offset, this);
+                        } else {
+                            complete(null);
+                        }
+                    } catch (Throwable t) {
+                        context.revertLastWrittenOffset(prevLastWrittenOffset);
+                        complete(t);
+                    }
+                }
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Completes the future with either the result of the write operation
+         * or the provided exception.
+         *
+         * @param exception The exception to complete the future with.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception == null) {
+                future.complete(result.response());
+            } else {
+                future.completeExceptionally(exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "CoordinatorWriteEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * A coordinator read operation.
+     *
+     * @param <S> The type of the coordinator state machine.
+     * @param <T> The type of the response.
+     */
+    public interface CoordinatorReadOperation<S, T> {
+        /**
+         * Generates the response to implement this coordinator read operation. A read
+         * operation received the last committed offset. It must use it to ensure that
+         * it does not read uncommitted data from the timeline data structures.
+         *
+         * @param state     The coordinator state machine.
+         * @param offset    The last committed offset.
+         * @return A response.
+         * @throws KafkaException
+         */
+        T generateResponse(S state, long offset) throws KafkaException;
+    }
+
+    /**
+     * A coordinator that reads the committed coordinator state.
+     *
+     * @param <T> The type of the response.
+     */
+    class CoordinatorReadEvent<T> implements CoordinatorEvent {
+        /**
+         * The topic partition that this read event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The read operation to execute.
+         */
+        final CoordinatorReadOperation<S, T> op;
+
+        /**
+         * The future that will be completed with the response
+         * generated by the read operation or an error.
+         */
+        final CompletableFuture<T> future;
+
+        /**
+         * The result of the read operation. It could be null
+         * if an exception is thrown before it is assigned.
+         */
+        T response;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The read operation.
+         */
+        CoordinatorReadEvent(
+            String name,
+            TopicPartition tp,
+            CoordinatorReadOperation<S, T> op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+            this.future = new CompletableFuture<>();
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                // Get the context of the coordinator or fail if the coordinator is not in active state.
+                CoordinatorContext context = activeContextOrThrow(tp);
+
+                // Execute the read operation.
+                response = op.generateResponse(
+                    context.coordinator,
+                    context.lastCommittedOffset
+                );
+
+                // The response can be completed immediately.
+                complete(null);
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Completes the future with either the result of the read operation
+         * or the provided exception.
+         *
+         * @param exception The exception to complete the future with.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception == null) {
+                future.complete(response);
+            } else {
+                future.completeExceptionally(exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "CoordinatorReadEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * A coordinator internal event.
+     */
+    class CoordinatorInternalEvent implements CoordinatorEvent {
+        /**
+         * The topic partition that this internal event is applied to.
+         */
+        final TopicPartition tp;
+
+        /**
+         * The operation name.
+         */
+        final String name;
+
+        /**
+         * The internal operation to execute.
+         */
+        final Runnable op;
+
+        /**
+         * Constructor.
+         *
+         * @param name  The operation name.
+         * @param tp    The topic partition that the operation is applied to.
+         * @param op    The operation.
+         */
+        CoordinatorInternalEvent(
+            String name,
+            TopicPartition tp,
+            Runnable op
+        ) {
+            this.tp = tp;
+            this.name = name;
+            this.op = op;
+        }
+
+        /**
+         * @return The key used by the CoordinatorEventProcessor to ensure
+         * that events with the same key are not processed concurrently.
+         */
+        @Override
+        public TopicPartition key() {
+            return tp;
+        }
+
+        /**
+         * Called by the CoordinatorEventProcessor when the event is executed.
+         */
+        @Override
+        public void run() {
+            try {
+                op.run();
+            } catch (Throwable t) {
+                complete(t);
+            }
+        }
+
+        /**
+         * Logs any exceptions thrown while the event is executed.
+         *
+         * @param exception The exception.
+         */
+        @Override
+        public void complete(Throwable exception) {
+            if (exception != null) {
+                log.error("Execution of {} failed due to {}.", name, exception);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "InternalEvent(name=" + name + ")";
+        }
+    }
+
+    /**
+     * Partition listener to be notified when the high watermark of the partitions
+     * backing the coordinator are updated.
+     */
+    class HighWatermarkListener implements PartitionWriter.Listener {
+        /**
+         * Updates the high watermark of the corresponding coordinator.
+         *
+         * @param tp        The topic partition.
+         * @param offset    The new high watermark.
+         */
+        @Override
+        public void onHighWatermarkUpdated(
+            TopicPartition tp,
+            long offset
+        ) {
+            log.debug("High watermark of {} incremented to {}.", tp, offset);
+            scheduleInternalOperation("HighWatermarkUpdated(tp=" + tp + ", offset=" + offset + ")", tp, () -> {
+                contextOrThrow(tp).updateLastCommittedOffset(offset);
+            });
+        }
+    }
+
+    /**
+     * The log context.
+     */
+    private final LogContext logContext;
+
+    /**
+     * The logger.
+     */
+    private final Logger log;
+
+    /**
+     * The coordinators keyed by topic partition.
+     */
+    private final ConcurrentHashMap<TopicPartition, CoordinatorContext> coordinators;
+
+    /**
+     * The event processor used by the runtime.
+     */
+    private final CoordinatorEventProcessor processor;
+
+    /**
+     * The partition writer used by the runtime to persist records.
+     */
+    private final PartitionWriter<U> partitionWriter;
+
+    /**
+     * The high watermark listener registered to all the partitions
+     * backing the coordinators.
+     */
+    private final HighWatermarkListener highWatermarklistener;
+
+    /**
+     * The coordinator loaded used by the runtime.
+     */
+    private final CoordinatorLoader<U> loader;
+
+    /**
+     * The coordinator state machine builder used by the runtime
+     * to instantiate a coordinator.
+     */
+    private final CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier;
+
+    /**
+     * Constructor.
+     *
+     * @param logContext                    The log context.
+     * @param processor                     The event processor.
+     * @param partitionWriter               The partition writer.
+     * @param loader                        The coordinator loader.
+     * @param coordinatorBuilderSupplier    The coordinator builder.
+     */
+    private CoordinatorRuntime(
+        LogContext logContext,
+        CoordinatorEventProcessor processor,
+        PartitionWriter<U> partitionWriter,
+        CoordinatorLoader<U> loader,
+        CoordinatorBuilderSupplier<S, U> coordinatorBuilderSupplier
+    ) {
+        this.logContext = logContext;
+        this.log = logContext.logger(CoordinatorRuntime.class);
+        this.coordinators = new ConcurrentHashMap<>();
+        this.processor = processor;
+        this.partitionWriter = partitionWriter;
+        this.highWatermarklistener = new HighWatermarkListener();
+        this.loader = loader;
+        this.coordinatorBuilderSupplier = coordinatorBuilderSupplier;
+    }
+
+    /**
+     * @return The coordinator context if the coordinator is active or an exception otherwise.
+     * @throws NotCoordinatorException
+     * @throws CoordinatorLoadInProgressException
+     */
+    private CoordinatorContext activeContextOrThrow(TopicPartition tp) {
+        CoordinatorContext context = coordinators.get(tp);
+
+        if (context == null) {
+            throw Errors.NOT_COORDINATOR.exception();
+        } else {
+            switch (context.state) {
+                case INITIAL:
+                case FAILED:
+                case CLOSED:
+                    throw Errors.NOT_COORDINATOR.exception();
+
+                case LOADING:
+                    throw Errors.COORDINATOR_LOAD_IN_PROGRESS.exception();
+            }
+        }
+
+        return context;
+    }
+
+    /**
+     * @return The coordinator context. It is created if it does not exist.
+     */
+    private CoordinatorContext getOrCreateContext(TopicPartition tp) {
+        return coordinators.computeIfAbsent(tp, CoordinatorContext::new);
+    }
+
+    /**
+     * @return The coordinator context or thrown an exception if it does
+     * not exist.
+     * @throws NotCoordinatorException
+     * Package private for testing.
+     */
+    CoordinatorContext contextOrThrow(TopicPartition tp) {
+        CoordinatorContext context = coordinators.get(tp);
+
+        if (context == null) {
+            throw Errors.NOT_COORDINATOR.exception();
+        } else {
+            return context;
+        }
+    }
+
+    /**
+     * Schedules a write operation.
+     *
+     * @param name  The name of the write operation.
+     * @param tp    The address of the coordinator (aka its topic-partitions).
+     * @param op    The write operation.
+     *
+     * @return A future that will be completed with the result of the write operation
+     * when the operation is completed or an exception if the write operation failed.
+     *
+     * @param <T> The type of the result.
+     */
+    public <T> CompletableFuture<T> scheduleWriteOperation(
+        String name,
+        TopicPartition tp,
+        CoordinatorWriteOperation<S, T, U> op
+    ) {
+        log.debug("Scheduled execution of write operation {}.", name);
+        CoordinatorWriteEvent<T> event = new CoordinatorWriteEvent<>(name, tp, op);
+        processor.enqueue(event);
+        return event.future;
+    }
+
+    /**
+     * Schedules a read operation.
+     *
+     * @param name  The name of the write operation.
+     * @param tp    The address of the coordinator (aka its topic-partitions).
+     * @param op    The read operation.
+     *
+     * @return A future that will be completed with the result of the read operation
+     * when the operation is completed or an exception if the write operation failed.
+     *
+     * @param <T> The type of the result.
+     */
+    public <T> CompletableFuture<T> scheduleReadOperation(
+        String name,
+        TopicPartition tp,
+        CoordinatorReadOperation<S, T> op
+    ) {
+        log.debug("Scheduled execution of read operation {}.", name);
+        CoordinatorReadEvent<T> event = new CoordinatorReadEvent<>(name, tp, op);
+        processor.enqueue(event);
+        return event.future;
+    }
+
+    /**
+     * Schedules an internal event.
+     *
+     * @param name  The name of the write operation.
+     * @param tp    The address of the coordinator (aka its topic-partitions).
+     * @param op    The operation.
+     */
+    private void scheduleInternalOperation(
+        String name,
+        TopicPartition tp,
+        Runnable op
+    ) {
+        log.debug("Scheduled execution of internal operation {}.", name);
+        processor.enqueue(new CoordinatorInternalEvent(name, tp, op));
+    }
+
+    /**
+     * @return The topic partitions of the coordinators currently registered in the
+     * runtime.
+     */
+    public Set<TopicPartition> partitions() {
+        return new HashSet<>(coordinators.keySet());
+    }
+
+    /**
+     * Schedules the loading of a coordinator. This is called when the broker is elected as
+     * the leader for a partition.
+     *
+     * @param tp                The topic partition of the coordinator. Records from this
+     *                          partitions will be read and applied to the coordinator.
+     * @param partitionEpoch    The epoch of the partition.
+     */
+    public void scheduleLoadOperation(
+        TopicPartition tp,
+        int partitionEpoch
+    ) {
+        log.info("Scheduling loading of metadata from {} with epoch {}", tp, partitionEpoch);
+        // Touch the state to make the runtime immediately aware of the new coordinator.
+        getOrCreateContext(tp);
+
+        scheduleInternalOperation("Load(tp=" + tp + ", epoch=" + partitionEpoch + ")", tp, () -> {
+            CoordinatorContext context = getOrCreateContext(tp);
+
+            if (context.state == CoordinatorState.FAILED) {
+                // When the coordinator has failed, we create a new context instead of
+                // recycling the previous one because it is better to start from an
+                // empty state for timeline data structures.
+                coordinators.remove(tp);
+                context = getOrCreateContext(tp);
+            }
+
+            if (context.epoch < partitionEpoch) {
+                context.epoch = partitionEpoch;
+
+                switch (context.state) {
+                    case INITIAL:
+                        context.transitionTo(CoordinatorState.LOADING);
+                        loader.load(tp, context.coordinator).whenComplete((state, exception) -> {
+                            scheduleInternalOperation("CompleteLoad(tp=" + tp + ", epoch=" + partitionEpoch + ")", tp, () -> {
+                                CoordinatorContext ctx = contextOrThrow(tp);
+                                if (ctx.state != CoordinatorState.LOADING) {
+                                    log.info("Ignoring load completion from {} because context is in {} state.",
+                                        ctx.tp, ctx.state);
+                                    return;
+                                }
+                                try {
+                                    if (exception != null) throw exception;
+                                    ctx.transitionTo(CoordinatorState.ACTIVE);
+                                    log.info("Finished loading of metadata from {} with epoch {}.",
+                                        tp, partitionEpoch);
+                                } catch (Throwable ex) {
+                                    log.error("Failed to load metadata from {} with epoch {} due to {}.",
+                                        tp, partitionEpoch, ex.toString());
+                                    ctx.transitionTo(CoordinatorState.FAILED);
+                                }
+                            });
+                        });
+                        break;
+
+                    case LOADING:
+                        log.info("The coordinator {} is already loading metadata.", tp);
+                        break;
+
+                    case ACTIVE:
+                        log.info("The coordinator {} is already active.", tp);
+                        break;
+
+                    default:
+                        log.error("Cannot load coordinator {} in state {}.", tp, context.state);
+                }
+            } else {
+                log.info("Ignoring loading metadata from {} since current epoch {} is larger than or equals to {}.",
+                    context.tp, context.epoch, partitionEpoch);
+            }
+        });
+    }
+
+    /**
+     * Schedules the unloading of a coordinator. This is called when the broker is not the
+     * leader anymore.
+     *
+     * @param tp                The topic partition of the coordinator.
+     * @param partitionEpoch    The partition epoch.
+     */
+    public void scheduleUnloadOperation(
+        TopicPartition tp,
+        int partitionEpoch
+    ) {
+        log.info("Scheduling unloading of metadata for {} with epoch {}", tp, partitionEpoch);
+
+        scheduleInternalOperation("UnloadCoordinator(tp=" + tp + ", epoch=" + partitionEpoch + ")", tp, () -> {
+            CoordinatorContext context = contextOrThrow(tp);
+            if (context.epoch < partitionEpoch) {
+                log.info("Started unloading metadata for {} with epoch {}.", tp, partitionEpoch);
+                context.transitionTo(CoordinatorState.CLOSED);
+                coordinators.remove(tp);
+                log.info("Finished unloading metadata for {} with epoch {}.", tp, partitionEpoch);
+            } else {
+                log.info("Ignored unloading metadata for {} in epoch {} since current epoch is {}.",
+                    tp, partitionEpoch, context.epoch);
+            }
+        });
+    }
+
+    /**
+     * Closes the runtime. This closes all the coordinators currently registered
+     * in the runtime.
+     *
+     * @throws Exception
+     */
+    public void close() throws Exception {
+        log.info("Closing coordinator runtime.");
+        // This close the processor, drain all the pending events and
+        // reject any new events.
+        processor.close();
+        // Unload all the coordinators.
+        coordinators.forEach((tp, context) -> {

Review Comment:
   note that this is not a thread safe operation since concurrentHashMap doesn't take an exclusive lock during forEach. Is that ok?



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