[GitHub] [accumulo] EdColeman commented on a change in pull request #2324: Add single node (per context) property cache and ZooKeeper storage

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

EdColeman commented on a change in pull request #2324:
URL: https://github.com/apache/accumulo/pull/2324#discussion_r741102164



##########
File path: server/base/src/main/java/org/apache/accumulo/server/conf/store/impl/ZooPropStore.java
##########
@@ -0,0 +1,397 @@
+/*
+ * 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.accumulo.server.conf.store.impl;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Objects;
+
+import org.apache.accumulo.core.conf.Property;
+import org.apache.accumulo.fate.zookeeper.ZooReaderWriter;
+import org.apache.accumulo.fate.zookeeper.ZooUtil;
+import org.apache.accumulo.server.ServerContext;
+import org.apache.accumulo.server.conf.codec.VersionedPropCodec;
+import org.apache.accumulo.server.conf.codec.VersionedProperties;
+import org.apache.accumulo.server.conf.store.PropCache;
+import org.apache.accumulo.server.conf.store.PropCacheId;
+import org.apache.accumulo.server.conf.store.PropChangeListener;
+import org.apache.accumulo.server.conf.store.PropStore;
+import org.apache.accumulo.server.conf.store.PropStoreException;
+import org.apache.zookeeper.KeeperException;
+import org.apache.zookeeper.data.Stat;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class ZooPropStore implements PropStore, PropChangeListener {
+
+  private final static Logger log = LoggerFactory.getLogger(ZooPropStore.class);
+
+  private final VersionedPropCodec propCodec;
+
+  private final ReadyMonitor zkReadyMon;
+
+  private final ServerContext context;
+  private final ZooReaderWriter zrw;
+  private final PropStoreWatcher propStoreWatcher;
+  private final PropCache cache;
+  private final PropStoreMetrics cacheMetrics = new PropStoreMetrics();
+
+  private Map<String,String> fixedProps = null;
+
+  /**
+   * Create instance using ZooPropStore.Builder
+   *
+   * @param context
+   *          the server context
+   * @param readyMonitor
+   *          coordination utility for ZooKeeper connection status.
+   * @param propStoreWatcher
+   *          an extended ZooKeeper watcher
+   */
+  private ZooPropStore(final ServerContext context, final ReadyMonitor readyMonitor,
+      final PropStoreWatcher propStoreWatcher) {
+
+    this.context = context;
+    this.zrw = context.getZooReaderWriter();
+    this.zkReadyMon = readyMonitor;
+    this.propStoreWatcher = propStoreWatcher;
+
+    this.propCodec = context.getVersionedPropertiesCodec();
+
+    ZooPropLoader propLoader = new ZooPropLoader(zrw, context.getVersionedPropertiesCodec(),
+        propStoreWatcher, cacheMetrics);
+
+    cache = new CaffeineCache.Builder(propLoader, cacheMetrics).build();
+
+    try {
+      var path = ZooUtil.getRoot(context.getInstanceID());
+      if (zrw.exists(path, propStoreWatcher)) {
+        log.debug("have a ZooKeeper connection");
+        zkReadyMon.setReady();
+      } else {
+        throw new IllegalStateException(
+            "Instance may not have been initialized, root node" + path + " does not exist");
+      }
+    } catch (InterruptedException | KeeperException ex) {
+      throw new IllegalStateException("Failed to read root node from ZooKeeper", ex);
+    }
+  }
+
+  /**
+   * Create initial system props for the instance. If the node already exists, no action is
+   * performed.
+   *
+   * @param context
+   *          the server context.
+   * @param initProps
+   *          map of k, v pairs of initial properties.
+   * @return true if props create, false if the node existed and initialization was skipped.
+   */
+  public synchronized static boolean initSysProps(final ServerContext context,
+      final Map<String,String> initProps) {
+    PropCacheId sysPropsId = PropCacheId.forSystem(context.getInstanceID());
+    return createInitialProps(context, sysPropsId, initProps);
+  }
+
+  /**
+   * Create initial properties if they do not exist. If the node exists, initialization will be
+   * skipped.
+   *
+   * @param context
+   *          the system context
+   * @param propCacheId
+   *          a prop id
+   * @param props
+   *          initial properties
+   * @return true if props create, false if the node existed and initialization was skipped.
+   */
+  public static boolean createInitialProps(final ServerContext context,
+      final PropCacheId propCacheId, Map<String,String> props) {
+
+    try {
+      ZooReaderWriter zrw = context.getZooReaderWriter();
+      if (zrw.exists(propCacheId.getPath())) {
+        return false;
+      }
+      VersionedProperties vProps = new VersionedProperties(props);
+      return zrw.putPersistentData(propCacheId.getPath(),
+          context.getVersionedPropertiesCodec().toBytes(vProps), ZooUtil.NodeExistsPolicy.FAIL);
+    } catch (InterruptedException ex) {
+      Thread.currentThread().interrupt();
+      throw new IllegalStateException("Interrupted creating node " + propCacheId, ex);
+    } catch (Exception ex) {
+      throw new IllegalStateException("Failed to create node " + propCacheId, ex);
+    }
+  }
+
+  public PropStoreMetrics getMetrics() {
+    return cacheMetrics;
+  }
+
+  // TODO - evaluate returning the props instead of boolean.
+  @Override
+  public boolean create(PropCacheId propCacheId, Map<String,String> props)
+      throws PropStoreException {
+
+    VersionedProperties vProps = new VersionedProperties(props);
+
+    try {
+
+      var path = propCacheId.getPath();
+      if (!zrw.putPersistentData(path, propCodec.toBytes(vProps), ZooUtil.NodeExistsPolicy.FAIL)) {
+        return false;
+      }
+
+      Stat stat = zrw.getStatus(path, propStoreWatcher);
+
+      if (stat.getVersion() != vProps.getNextVersion()) {
+        throw new PropStoreException(
+            "Invalid data version on create, received " + stat.getVersion(),
+            new IllegalStateException());
+      }
+
+      return true;
+
+    } catch (IOException | KeeperException | InterruptedException ex) {
+      throw new PropStoreException("Failed to serialize properties for " + propCacheId, ex);
+    }
+  }
+
+  @Override
+  public VersionedProperties get(final PropCacheId propCacheId) throws PropStoreException {
+    try {
+
+      checkZkConnection(); // if ZK not connected, block, do not just return a cached value.
+      propStoreWatcher.registerListener(propCacheId, this);
+      return cache.get(propCacheId);
+
+    } catch (Exception ex) {
+      throw new PropStoreException("read from prop store get() failed for: " + propCacheId, ex);
+    }
+  }
+
+  @Override
+  public boolean putAll(final PropCacheId propCacheId, final Map<String,String> props)
+      throws PropStoreException {
+
+    try {
+

Review comment:
       I had thought about allowing retries on a conflict, but I could not reason out how to handle a conflict.  If both requests changed different things, then maybe it would be okay.
   
   ```
   Thread-1 sets prop A to x
   Thread-2 sets prop B to y
   
   this seems "safe"
   ```
   
   But what if it was a change?
   ```
   prop A = z.
   Thread-1 sets prop A = x
   Thread-2 sets prop A = y
   who wins?  If both return success, then what is the the value of prop A after both return?  Would you need to set and then call read to see if you "won"?
   ````
   
   Rather than try to figure out logic that would be the least surprising, it seemed easiest to hard / quick fail and let the caller decide.  The calling function could make the decision to retry - it should have more context available to decide on what should happen in the case of a conflict.




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