Merge branch 'stable-3.2' * stable-3.2: Update plugin documentation Avoid multiple indexing operation for a single change Lock indexing events on forwarder site Change-Id: I81e61d5f128667c5e1c81ecd0b140922a7ded7f6

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/Configuration.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/Configuration.java
index 58a21b6..3bd58a3 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/Configuration.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/Configuration.java

@@ -45,6 +45,9 @@
 public class Configuration {
   private static final FluentLogger log = FluentLogger.forEnclosingClass();
 
+  public static final int DEFAULT_NUM_STRIPED_LOCKS = 10;
+  public static final int DEFAULT_TIMEOUT_MS = 5000;
+
   // common parameter to peerInfo section
   static final String PEER_INFO_SECTION = "peerInfo";
 
@@ -54,7 +57,6 @@
   static final int DEFAULT_INDEX_RETRY_INTERVAL = 30000;
   static final int DEFAULT_THREAD_POOL_SIZE = 4;
   static final String NUM_STRIPED_LOCKS = "numStripedLocks";
-  static final int DEFAULT_NUM_STRIPED_LOCKS = 10;
 
   private final Main main;
   private final AutoReindex autoReindex;
@@ -333,6 +335,9 @@
   }
 
   public static class Http {
+    public static final int DEFAULT_MAX_TRIES = 360;
+    public static final int DEFAULT_RETRY_INTERVAL = 10000;
+
     static final String HTTP_SECTION = "http";
     static final String USER_KEY = "user";
     static final String PASSWORD_KEY = "password";
@@ -341,10 +346,6 @@
     static final String MAX_TRIES_KEY = "maxTries";
     static final String RETRY_INTERVAL_KEY = "retryInterval";
 
-    static final int DEFAULT_TIMEOUT_MS = 5000;
-    static final int DEFAULT_MAX_TRIES = 360;
-    static final int DEFAULT_RETRY_INTERVAL = 10000;
-
     private final String user;
     private final String password;
     private final int connectionTimeout;
@@ -446,6 +447,7 @@
   public static class Index extends Forwarding {
     static final String INDEX_SECTION = "index";
     static final String MAX_TRIES_KEY = "maxTries";
+    static final String WAIT_TIMEOUT_KEY = "waitTimeout";
     static final String RETRY_INTERVAL_KEY = "retryInterval";
     static final String SYNCHRONIZE_FORCED_KEY = "synchronizeForced";
     static final boolean DEFAULT_SYNCHRONIZE_FORCED = true;
@@ -453,6 +455,7 @@
     private final int threadPoolSize;
     private final int retryInterval;
     private final int maxTries;
+    private final int waitTimeout;
     private final int numStripedLocks;
     private final boolean synchronizeForced;
 
@@ -462,6 +465,7 @@
       numStripedLocks = getInt(cfg, INDEX_SECTION, NUM_STRIPED_LOCKS, DEFAULT_NUM_STRIPED_LOCKS);
       retryInterval = getInt(cfg, INDEX_SECTION, RETRY_INTERVAL_KEY, DEFAULT_INDEX_RETRY_INTERVAL);
       maxTries = getInt(cfg, INDEX_SECTION, MAX_TRIES_KEY, DEFAULT_INDEX_MAX_TRIES);
+      waitTimeout = getInt(cfg, INDEX_SECTION, WAIT_TIMEOUT_KEY, DEFAULT_TIMEOUT_MS);
       synchronizeForced =
           cfg.getBoolean(INDEX_SECTION, SYNCHRONIZE_FORCED_KEY, DEFAULT_SYNCHRONIZE_FORCED);
     }
@@ -482,6 +486,10 @@
       return maxTries;
     }
 
+    public int waitTimeout() {
+      return waitTimeout;
+    }
+
     public boolean synchronizeForced() {
       return synchronizeForced;
     }

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/Setup.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/Setup.java
index 6635b80..8a267d6 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/Setup.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/Setup.java

@@ -24,6 +24,7 @@
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Cache.CACHE_SECTION;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Cache.PATTERN_KEY;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.DEFAULT_THREAD_POOL_SIZE;
+import static com.ericsson.gerrit.plugins.highavailability.Configuration.DEFAULT_TIMEOUT_MS;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Event.EVENT_SECTION;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Forwarding.DEFAULT_SYNCHRONIZE;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Forwarding.SYNCHRONIZE_KEY;
@@ -33,7 +34,6 @@
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.CONNECTION_TIMEOUT_KEY;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.DEFAULT_MAX_TRIES;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.DEFAULT_RETRY_INTERVAL;
-import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.DEFAULT_TIMEOUT_MS;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.HTTP_SECTION;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.MAX_TRIES_KEY;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.PASSWORD_KEY;

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexAccountHandler.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexAccountHandler.java
index a6dfc34..b1a595c 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexAccountHandler.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexAccountHandler.java

@@ -14,7 +14,6 @@
 
 package com.ericsson.gerrit.plugins.highavailability.forwarder;
 
-import com.ericsson.gerrit.plugins.highavailability.Configuration;
 import com.google.gerrit.entities.Account;
 import com.google.gerrit.server.index.account.AccountIndexer;
 import com.google.inject.Inject;
@@ -32,8 +31,7 @@
   private final AccountIndexer indexer;
 
   @Inject
-  ForwardedIndexAccountHandler(AccountIndexer indexer, Configuration config) {
-    super(config.index());
+  ForwardedIndexAccountHandler(AccountIndexer indexer) {
     this.indexer = indexer;
   }
 

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexChangeHandler.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexChangeHandler.java
index 667677d..064783c 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexChangeHandler.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexChangeHandler.java

@@ -55,7 +55,6 @@
       @ForwardedIndexExecutor ScheduledExecutorService indexExecutor,
       OneOffRequestContext oneOffCtx,
       ChangeCheckerImpl.Factory changeCheckerFactory) {
-    super(config.index());
     this.indexer = indexer;
     this.indexExecutor = indexExecutor;
     this.oneOffCtx = oneOffCtx;

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexGroupHandler.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexGroupHandler.java
index c0f9330..ab31659 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexGroupHandler.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexGroupHandler.java

@@ -14,7 +14,6 @@
 
 package com.ericsson.gerrit.plugins.highavailability.forwarder;
 
-import com.ericsson.gerrit.plugins.highavailability.Configuration;
 import com.google.gerrit.entities.AccountGroup;
 import com.google.gerrit.server.index.group.GroupIndexer;
 import com.google.inject.Inject;
@@ -32,8 +31,7 @@
   private final GroupIndexer indexer;
 
   @Inject
-  ForwardedIndexGroupHandler(GroupIndexer indexer, Configuration config) {
-    super(config.index());
+  ForwardedIndexGroupHandler(GroupIndexer indexer) {
     this.indexer = indexer;
   }
 

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexProjectHandler.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexProjectHandler.java
index e837770..20ffefd 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexProjectHandler.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexProjectHandler.java

@@ -14,7 +14,6 @@
 
 package com.ericsson.gerrit.plugins.highavailability.forwarder;
 
-import com.ericsson.gerrit.plugins.highavailability.Configuration;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.index.project.ProjectIndexer;
 import com.google.inject.Inject;
@@ -32,8 +31,7 @@
   private final ProjectIndexer indexer;
 
   @Inject
-  ForwardedIndexProjectHandler(ProjectIndexer indexer, Configuration config) {
-    super(config.index());
+  ForwardedIndexProjectHandler(ProjectIndexer indexer) {
     this.indexer = indexer;
   }
 

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexingHandler.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexingHandler.java
index 48dfba3..44e35a8 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexingHandler.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexingHandler.java

@@ -14,12 +14,12 @@
 
 package com.ericsson.gerrit.plugins.highavailability.forwarder;
 
-import com.ericsson.gerrit.plugins.highavailability.Configuration;
 import com.google.common.flogger.FluentLogger;
-import com.google.common.util.concurrent.Striped;
 import java.io.IOException;
+import java.util.Collections;
 import java.util.Optional;
-import java.util.concurrent.locks.Lock;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 /**
  * Base class to handle forwarded indexing. This class is meant to be extended by classes used on
@@ -29,6 +29,7 @@
  */
 public abstract class ForwardedIndexingHandler<T> {
   protected static final FluentLogger log = FluentLogger.forEnclosingClass();
+  private final Set<T> inFlightIndexing = Collections.newSetFromMap(new ConcurrentHashMap<>());
 
   public enum Operation {
     INDEX,
@@ -40,16 +41,10 @@
     }
   }
 
-  private final Striped<Lock> idLocks;
-
   protected abstract void doIndex(T id, Optional<IndexEvent> indexEvent) throws IOException;
 
   protected abstract void doDelete(T id, Optional<IndexEvent> indexEvent) throws IOException;
 
-  protected ForwardedIndexingHandler(Configuration.Index indexConfig) {
-    idLocks = Striped.lock(indexConfig.numStripedLocks());
-  }
-
   /**
    * Index an item in the local node, indexing will not be forwarded to the other node.
    *
@@ -60,11 +55,9 @@
    */
   public void index(T id, Operation operation, Optional<IndexEvent> indexEvent) throws IOException {
     log.atFine().log("%s %s %s", operation, id, indexEvent);
-    try {
-      Context.setForwardedEvent(true);
-      Lock idLock = idLocks.get(id);
-      idLock.lock();
+    if (inFlightIndexing.add(id)) {
       try {
+        Context.setForwardedEvent(true);
         switch (operation) {
           case INDEX:
             doIndex(id, indexEvent);
@@ -77,10 +70,12 @@
             break;
         }
       } finally {
-        idLock.unlock();
+        Context.unsetForwardedEvent();
+        inFlightIndexing.remove(id);
       }
-    } finally {
-      Context.unsetForwardedEvent();
+    } else {
+      throw new InFlightIndexedException(
+          String.format("Indexing for %s %s %s already in flight", operation, id, indexEvent));
     }
   }
 }

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/InFlightIndexedException.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/InFlightIndexedException.java
new file mode 100644
index 0000000..87d28b3
--- /dev/null
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/InFlightIndexedException.java

@@ -0,0 +1,26 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed 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 com.ericsson.gerrit.plugins.highavailability.forwarder;
+
+import java.io.IOException;
+
+public class InFlightIndexedException extends IOException {
+
+  private static final long serialVersionUID = 1L;
+
+  public InFlightIndexedException(String msg) {
+    super(msg);
+  }
+}

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventHandler.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventHandler.java
index 0697ad7..a352e38 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventHandler.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventHandler.java

@@ -14,6 +14,7 @@
 
 package com.ericsson.gerrit.plugins.highavailability.index;
 
+import com.ericsson.gerrit.plugins.highavailability.Configuration;
 import com.ericsson.gerrit.plugins.highavailability.forwarder.Context;
 import com.ericsson.gerrit.plugins.highavailability.forwarder.Forwarder;
 import com.ericsson.gerrit.plugins.highavailability.forwarder.IndexEvent;
@@ -28,7 +29,8 @@
 import java.util.Collections;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
-import java.util.concurrent.Executor;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
 
 class IndexEventHandler
     implements ChangeIndexedListener,
@@ -36,25 +38,34 @@
         GroupIndexedListener,
         ProjectIndexedListener {
   private static final FluentLogger log = FluentLogger.forEnclosingClass();
-  private final Executor executor;
+  private final ScheduledExecutorService executor;
   private final Forwarder forwarder;
   private final String pluginName;
   private final Set<IndexTask> queuedTasks = Collections.newSetFromMap(new ConcurrentHashMap<>());
   private final ChangeCheckerImpl.Factory changeChecker;
   private final CurrentRequestContext currCtx;
+  private final IndexEventLocks locks;
+
+  private final int retryInterval;
+  private final int maxTries;
 
   @Inject
   IndexEventHandler(
-      @IndexExecutor Executor executor,
+      @IndexExecutor ScheduledExecutorService executor,
       @PluginName String pluginName,
       Forwarder forwarder,
       ChangeCheckerImpl.Factory changeChecker,
-      CurrentRequestContext currCtx) {
+      CurrentRequestContext currCtx,
+      Configuration cfg,
+      IndexEventLocks locks) {
     this.forwarder = forwarder;
     this.executor = executor;
     this.pluginName = pluginName;
     this.changeChecker = changeChecker;
     this.currCtx = currCtx;
+    this.locks = locks;
+    this.retryInterval = cfg.http().retryInterval();
+    this.maxTries = cfg.http().maxTries();
   }
 
   @Override
@@ -128,6 +139,7 @@
 
   abstract class IndexTask implements Runnable {
     protected final IndexEvent indexEvent;
+    private int retryCount = 0;
 
     IndexTask() {
       indexEvent = new IndexEvent();
@@ -139,8 +151,22 @@
 
     @Override
     public void run() {
-      queuedTasks.remove(this);
-      execute();
+      locks.withLock(
+          this,
+          () -> {
+            queuedTasks.remove(this);
+            execute();
+          },
+          this::reschedule);
+    }
+
+    private void reschedule() {
+      if (++retryCount <= maxTries) {
+        log.atFine().log("Retrying %d times to %s", retryCount, this);
+        executor.schedule(this, retryInterval, TimeUnit.MILLISECONDS);
+      } else {
+        log.atSevere().log("Failed to %s after %d tries; giving up", this, maxTries);
+      }
     }
 
     abstract void execute();

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventLocks.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventLocks.java
new file mode 100644
index 0000000..4617e08
--- /dev/null
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventLocks.java

@@ -0,0 +1,67 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed 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 com.ericsson.gerrit.plugins.highavailability.index;
+
+import com.ericsson.gerrit.plugins.highavailability.Configuration;
+import com.ericsson.gerrit.plugins.highavailability.index.IndexEventHandler.IndexTask;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.flogger.FluentLogger;
+import com.google.common.util.concurrent.Striped;
+import com.google.inject.Inject;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.Lock;
+
+public class IndexEventLocks {
+  private static final FluentLogger log = FluentLogger.forEnclosingClass();
+
+  private static final int NUMBER_OF_INDEX_TASK_TYPES = 4;
+
+  private final Striped<Lock> locks;
+  private final long waitTimeout;
+
+  @Inject
+  public IndexEventLocks(Configuration cfg) {
+    this.locks = Striped.lock(NUMBER_OF_INDEX_TASK_TYPES * cfg.index().numStripedLocks());
+    this.waitTimeout = cfg.index().waitTimeout();
+  }
+
+  public void withLock(
+      IndexTask id, VoidFunction function, VoidFunction lockAcquireTimeoutCallback) {
+    Lock idLock = getLock(id);
+    try {
+      if (idLock.tryLock(waitTimeout, TimeUnit.MILLISECONDS)) {
+        try {
+          function.invoke();
+        } finally {
+          idLock.unlock();
+        }
+      } else {
+        lockAcquireTimeoutCallback.invoke();
+      }
+    } catch (InterruptedException e) {
+      log.atSevere().withCause(e).log("%s was interrupted; giving up", id);
+    }
+  }
+
+  @VisibleForTesting
+  protected Lock getLock(IndexTask id) {
+    return locks.get(id);
+  }
+
+  @FunctionalInterface
+  public interface VoidFunction {
+    void invoke();
+  }
+}

diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexModule.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexModule.java
index c17731f..9d0d03d 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexModule.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/IndexModule.java

@@ -20,18 +20,21 @@
 import com.google.gerrit.extensions.events.ProjectIndexedListener;
 import com.google.gerrit.extensions.registration.DynamicSet;
 import com.google.gerrit.lifecycle.LifecycleModule;
+import com.google.inject.Scopes;
 import com.google.inject.assistedinject.FactoryModuleBuilder;
-import java.util.concurrent.Executor;
 import java.util.concurrent.ScheduledExecutorService;
 
 public class IndexModule extends LifecycleModule {
 
   @Override
   protected void configure() {
-    bind(Executor.class).annotatedWith(IndexExecutor.class).toProvider(IndexExecutorProvider.class);
+    bind(ScheduledExecutorService.class)
+        .annotatedWith(IndexExecutor.class)
+        .toProvider(IndexExecutorProvider.class);
     bind(ScheduledExecutorService.class)
         .annotatedWith(ForwardedIndexExecutor.class)
         .toProvider(ForwardedIndexExecutorProvider.class);
+    bind(IndexEventLocks.class).in(Scopes.SINGLETON);
     listener().to(IndexExecutorProvider.class);
     DynamicSet.bind(binder(), ChangeIndexedListener.class).to(IndexEventHandler.class);
     DynamicSet.bind(binder(), AccountIndexedListener.class).to(IndexEventHandler.class);

diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
index 1d1a02b..fd0b56e 100644
--- a/src/main/resources/Documentation/about.md
+++ b/src/main/resources/Documentation/about.md

@@ -1,20 +1,19 @@
 
 This plugin allows making Gerrit highly available by having redundant Gerrit
-masters.
+active/passive instances.
 
-The masters must be:
+The Gerrit instances must be:
 
-* connecting to the same database
 * sharing the git repositories using a shared file system (e.g. NFS)
 * behind a load balancer (e.g. HAProxy)
 
-Currently, the mode supported is one primary (active) master and multiple backup
-(passive) masters but eventually the plan is to support `n` active masters. In
-the active/passive mode, the active master is handling all traffic while the
-passives are kept updated to be always ready to take over.
+Currently, the mode supported is one active instance and multiple backup
+(passive) instances but eventually the plan is to support `n` active instances.
+In the active/passive mode, the active instance is handling all traffic while
+the passives are kept updated to be always ready to take over.
 
-Even if database and git repositories are shared by the masters, there are a few
-areas of concern in order to be able to switch traffic between masters in a
+Even if git repositories are shared by the instances, there are a few areas
+of concern in order to be able to switch traffic between instances in a
 transparent manner from the user's perspective. The 4 areas of concern are
 things that Gerrit stores either in memory or locally in the review site:
 
@@ -23,69 +22,67 @@
 * stream-events
 * web sessions
 
-They need either to be shared or kept local to each master but synchronized.
-This plugin needs to be installed in all the masters and it will take care of sharing
-or synchronizing them.
+They need either to be shared or kept local to each instances but synchronized.
+This plugin needs to be installed in all the instances and it will take care of
+sharing or synchronizing them.
 
 #### Caches
-Every time a cache eviction occurs in one of the masters, the eviction will be
-forwarded the other masters so their caches do not contain stale entries.
+Every time a cache eviction occurs in one of the instances, the eviction will be
+forwarded the other nodes so their caches do not contain stale entries.
 
 #### Secondary indexes
-Every time the secondary index is modified in one of the masters, e.g., a change
-is added, updated or removed from the index, the others master's index are
-updated accordingly. This way, both indexes are kept synchronized.
+Every time the secondary index is modified in one of the instances, e.g., a change
+is added, updated or removed from the index, the others instances index are updated
+accordingly. This way, both indexes are kept synchronized.
 
 #### Stream events
-Every time a stream event occurs in one of the masters (see [more events info]
-(https://gerrit-review.googlesource.com/Documentation/cmd-stream-events.html#events)),
-the event is forwarded to the other masters which re-plays it. This way, the
-output of the stream-events command is the same, no matter which master a client
-is connected to.
+Every time a stream event occurs in one of the instances
+(see [more events info](https://gerrit-review.googlesource.com/Documentation/cmd-stream-events.html#events)),
+the event is forwarded to the other instances which re-plays it. This way, the output
+of the stream-events command is the same, no matter which  instance a client is
+connected to.
 
 #### Web session
 The built-in Gerrit H2 based web session cache is replaced with a file based
-implementation that is shared amongst the masters.
+implementation that is shared amongst the instances.
 
 ## Setup
 
 Prerequisites:
 
-* Unique database server must be accessible from all the masters
 * Git repositories must be located on a shared file system
 * A directory on a shared file system must be available for @PLUGIN@ to use
 
-For the masters:
+For the instances:
 
-* Configure the database section in gerrit.config to use the shared database
 * Configure gerrit.basePath in gerrit.config to the shared repositories location
 * Configure gerrit.serverId in gerrit.config based on [config](config.md)'s introduction
 * Install and configure this @PLUGIN@ plugin [further](config.md) or based on below.
 
 Here is an example of the minimal @PLUGIN@.config:
 
-Primary master
+Active instance
 
 ```
 [main]
-  sharedDirectory = /directory/accessible/from/both/masters
+  sharedDirectory = /directory/accessible/from/both/nodes
 
 [peerInfo "static"]
-  url = http://backupMasterHost1:8081/
+  url = http://backupNodeHost1:8081/
 
 [http]
   user = username
   password = password
 ```
 
-Backup master
+Backup instance
 
 ```
 [main]
-  sharedDirectory = /directory/accessible/from/both/masters
+  sharedDirectory = /directory/accessible/from/both/nodes
 
 [peerInfo "static"]
-  url = http://primaryMasterHost:8080/
+  url = http://primaryNodeHost:8080/
 
 [http]
   user = username
@@ -94,10 +91,10 @@
 
 ### HA replica site
 
-It is possible to create a copy of the master site and configure both sites to run
-in HA mode as peers. This is possible when the directory where the copy will be
-created is accessible from this machine. Such a replica site can be created by
-means of a gerrit [site init](../../../Documentation/pgm-init.html) step,
+It is possible to create a copy of the instance site and configure both
+sites to run in HA mode as peers. This is possible when the directory where
+the copy will be created is accessible from this machine. Such a replica site
+can be created by means of a gerrit [site init](../../../Documentation/pgm-init.html) step,
 contributed by the plugin under its init section.
 
 This init step is optional but defaults to creating the replica. If you want to

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index c0f2955..cdb7b47 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md

@@ -55,7 +55,7 @@
 ```
 
 ```main.sharedDirectory```
-:   Path to a directory accessible from both master instances.
+:   Path to a directory accessible from both instances.
     When given as a relative path, then it is resolved against the $SITE_PATH
     or Gerrit server. For example, if $SITE_PATH is "/gerrit/root" and
     sharedDirectory is given as "shared/dir" then the real path of the shared
@@ -160,8 +160,12 @@
 :   The interval of time in milliseconds between the subsequent auto-retries.
     When not specified, the default value is set to 10000ms.
 
-NOTE: the default settings for `http.timeout` and `http.maxTries` ensure that
-the plugin will keep retrying to forward a message for one hour.
+```index.waitTimeout```
+:   Maximum interval of time in milliseconds the plugin waits to acquire the lock for
+    an indexing call. When not specified, the default value is set to 5000ms.
+
+NOTE: the default settings for `http.timeout`, `http.maxTries` and `http.lockAcquireTimeout`
+ensure that the plugin will keep retrying to forward a message for one hour.
 
 ```cache.synchronize```
 :   Whether to synchronize cache evictions.
@@ -210,6 +214,11 @@
 :   The interval of time in milliseconds between the subsequent auto-retries.
     Defaults to 30000 (30 seconds).
 
+```index.waitTimeout```
+:   Maximum interval of time in milliseconds the plugin waits to acquire
+    the lock for an indexing call. When not specified, the default value
+    is set to 5000ms.
+
 ```websession.synchronize```
 :   Whether to synchronize web sessions.
     Defaults to true.

diff --git a/src/test/java/com/ericsson/gerrit/plugins/highavailability/ConfigurationTest.java b/src/test/java/com/ericsson/gerrit/plugins/highavailability/ConfigurationTest.java
index e28f8c9..a9537bd 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/highavailability/ConfigurationTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/ConfigurationTest.java

@@ -18,6 +18,7 @@
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Cache.PATTERN_KEY;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.DEFAULT_NUM_STRIPED_LOCKS;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.DEFAULT_THREAD_POOL_SIZE;
+import static com.ericsson.gerrit.plugins.highavailability.Configuration.DEFAULT_TIMEOUT_MS;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Event.EVENT_SECTION;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Forwarding.DEFAULT_SYNCHRONIZE;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Forwarding.SYNCHRONIZE_KEY;
@@ -27,7 +28,6 @@
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.CONNECTION_TIMEOUT_KEY;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.DEFAULT_MAX_TRIES;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.DEFAULT_RETRY_INTERVAL;
-import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.DEFAULT_TIMEOUT_MS;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.HTTP_SECTION;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.MAX_TRIES_KEY;
 import static com.ericsson.gerrit.plugins.highavailability.Configuration.Http.PASSWORD_KEY;

diff --git a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexAccountHandlerTest.java b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexAccountHandlerTest.java
index 33ca11c..0e9cee8 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexAccountHandlerTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexAccountHandlerTest.java

@@ -18,9 +18,7 @@
 import static com.google.gerrit.testing.GerritJUnit.assertThrows;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.when;
 
-import com.ericsson.gerrit.plugins.highavailability.Configuration;
 import com.ericsson.gerrit.plugins.highavailability.forwarder.ForwardedIndexingHandler.Operation;
 import com.google.gerrit.entities.Account;
 import com.google.gerrit.server.index.account.AccountIndexer;
@@ -37,16 +35,12 @@
 public class ForwardedIndexAccountHandlerTest {
 
   @Mock private AccountIndexer indexerMock;
-  @Mock private Configuration configMock;
-  @Mock private Configuration.Index indexMock;
   private ForwardedIndexAccountHandler handler;
   private Account.Id id;
 
   @Before
   public void setUp() throws Exception {
-    when(configMock.index()).thenReturn(indexMock);
-    when(indexMock.numStripedLocks()).thenReturn(10);
-    handler = new ForwardedIndexAccountHandler(indexerMock, configMock);
+    handler = new ForwardedIndexAccountHandler(indexerMock);
     id = Account.id(123);
   }
 

diff --git a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexChangeHandlerTest.java b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexChangeHandlerTest.java
index 9d852f2..7c1e6df 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexChangeHandlerTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexChangeHandlerTest.java

@@ -70,7 +70,6 @@
     Change change = new Change(null, id, null, null, TimeUtil.nowTs());
     when(changeNotes.getChange()).thenReturn(change);
     when(configMock.index()).thenReturn(indexMock);
-    when(indexMock.numStripedLocks()).thenReturn(10);
     when(changeCheckerFactoryMock.create(any())).thenReturn(changeCheckerAbsentMock);
     handler =
         new ForwardedIndexChangeHandler(

diff --git a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexGroupHandlerTest.java b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexGroupHandlerTest.java
index b4f887c..5fdb151 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexGroupHandlerTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexGroupHandlerTest.java

@@ -18,9 +18,7 @@
 import static com.google.gerrit.testing.GerritJUnit.assertThrows;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.when;
 
-import com.ericsson.gerrit.plugins.highavailability.Configuration;
 import com.ericsson.gerrit.plugins.highavailability.forwarder.ForwardedIndexingHandler.Operation;
 import com.google.gerrit.entities.AccountGroup;
 import com.google.gerrit.server.index.group.GroupIndexer;
@@ -37,16 +35,12 @@
 public class ForwardedIndexGroupHandlerTest {
 
   @Mock private GroupIndexer indexerMock;
-  @Mock private Configuration configMock;
-  @Mock private Configuration.Index indexMock;
   private ForwardedIndexGroupHandler handler;
   private AccountGroup.UUID uuid;
 
   @Before
   public void setUp() throws Exception {
-    when(configMock.index()).thenReturn(indexMock);
-    when(indexMock.numStripedLocks()).thenReturn(10);
-    handler = new ForwardedIndexGroupHandler(indexerMock, configMock);
+    handler = new ForwardedIndexGroupHandler(indexerMock);
     uuid = AccountGroup.uuid("123");
   }
 

diff --git a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexProjectHandlerTest.java b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexProjectHandlerTest.java
index 5e05fd6..0c896b3 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexProjectHandlerTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexProjectHandlerTest.java

@@ -18,9 +18,7 @@
 import static com.google.gerrit.testing.GerritJUnit.assertThrows;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.verify;
-import static org.mockito.Mockito.when;
 
-import com.ericsson.gerrit.plugins.highavailability.Configuration;
 import com.ericsson.gerrit.plugins.highavailability.forwarder.ForwardedIndexingHandler.Operation;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.index.project.ProjectIndexer;
@@ -37,16 +35,12 @@
 public class ForwardedIndexProjectHandlerTest {
 
   @Mock private ProjectIndexer indexerMock;
-  @Mock private Configuration configMock;
-  @Mock private Configuration.Index indexMock;
   private ForwardedIndexProjectHandler handler;
   private Project.NameKey nameKey;
 
   @Before
   public void setUp() {
-    when(configMock.index()).thenReturn(indexMock);
-    when(indexMock.numStripedLocks()).thenReturn(10);
-    handler = new ForwardedIndexProjectHandler(indexerMock, configMock);
+    handler = new ForwardedIndexProjectHandler(indexerMock);
     nameKey = Project.nameKey("project/name");
   }
 

diff --git a/src/test/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventHandlerTest.java b/src/test/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventHandlerTest.java
index eff8185..9d8f015 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventHandlerTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/index/IndexEventHandlerTest.java

@@ -32,20 +32,31 @@
 import com.ericsson.gerrit.plugins.highavailability.index.IndexEventHandler.IndexAccountTask;
 import com.ericsson.gerrit.plugins.highavailability.index.IndexEventHandler.IndexChangeTask;
 import com.ericsson.gerrit.plugins.highavailability.index.IndexEventHandler.IndexGroupTask;
-import com.google.common.util.concurrent.MoreExecutors;
+import com.ericsson.gerrit.plugins.highavailability.index.IndexEventHandler.IndexProjectTask;
 import com.google.gerrit.entities.Account;
 import com.google.gerrit.entities.AccountGroup;
 import com.google.gerrit.entities.Change;
 import com.google.gerrit.server.util.OneOffRequestContext;
 import com.google.gerrit.server.util.RequestContext;
 import com.google.gerrit.server.util.ThreadLocalRequestContext;
+import java.util.Collection;
+import java.util.List;
 import java.util.Optional;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Future;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.ScheduledThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.locks.Lock;
 import java.util.function.Consumer;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.mockito.Mock;
+import org.mockito.Mockito;
 import org.mockito.junit.MockitoJUnitRunner;
 
 @RunWith(MockitoJUnitRunner.class)
@@ -64,7 +75,10 @@
   private Change.Id changeId;
   private Account.Id accountId;
   private AccountGroup.UUID accountGroupUUID;
+  private ScheduledExecutorService executor = new CurrentThreadScheduledExecutorService();
   @Mock private RequestContext mockCtx;
+  @Mock private Configuration configuration;
+  private IndexEventLocks idLocks;
 
   private CurrentRequestContext currCtx =
       new CurrentRequestContext(null, null, null) {
@@ -82,17 +96,41 @@
     when(changeCheckerFactoryMock.create(any())).thenReturn(changeCheckerMock);
     when(changeCheckerMock.newIndexEvent()).thenReturn(Optional.of(new IndexEvent()));
 
+    Configuration.Index cfgIndex = mock(Configuration.Index.class);
+    when(configuration.index()).thenReturn(cfgIndex);
+    when(cfgIndex.numStripedLocks()).thenReturn(Configuration.DEFAULT_NUM_STRIPED_LOCKS);
+    when(cfgIndex.waitTimeout()).thenReturn(Configuration.DEFAULT_TIMEOUT_MS);
+
+    Configuration.Http http = mock(Configuration.Http.class);
+    when(configuration.http()).thenReturn(http);
+    when(http.maxTries()).thenReturn(Configuration.Http.DEFAULT_MAX_TRIES);
+    when(http.retryInterval()).thenReturn(Configuration.Http.DEFAULT_RETRY_INTERVAL);
+
+    idLocks = new IndexEventLocks(configuration);
     setUpIndexEventHandler(currCtx);
   }
 
   public void setUpIndexEventHandler(CurrentRequestContext currCtx) throws Exception {
+    setUpIndexEventHandler(currCtx, idLocks, configuration);
+  }
+
+  public void setUpIndexEventHandler(CurrentRequestContext currCtx, IndexEventLocks idLocks)
+      throws Exception {
+    setUpIndexEventHandler(currCtx, idLocks, configuration);
+  }
+
+  public void setUpIndexEventHandler(
+      CurrentRequestContext currCtx, IndexEventLocks idLocks, Configuration configuration)
+      throws Exception {
     indexEventHandler =
         new IndexEventHandler(
-            MoreExecutors.directExecutor(),
+            executor,
             PLUGIN_NAME,
             forwarder,
             changeCheckerFactoryMock,
-            currCtx);
+            currCtx,
+            configuration,
+            idLocks);
   }
 
   @Test
@@ -115,6 +153,153 @@
   }
 
   @Test
+  public void shouldNotIndexChangeWhenCannotAcquireLock() throws Exception {
+    IndexEventLocks locks = mock(IndexEventLocks.class);
+    Lock lock = mock(Lock.class);
+    when(locks.getLock(any(IndexChangeTask.class))).thenReturn(lock);
+    Mockito.doCallRealMethod().when(locks).withLock(any(), any(), any());
+    when(lock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS))).thenReturn(false);
+    setUpIndexEventHandler(currCtx, locks);
+
+    indexEventHandler.onChangeIndexed(PROJECT_NAME, changeId.get());
+
+    verify(forwarder, never()).indexChange(eq(PROJECT_NAME), eq(CHANGE_ID), any());
+  }
+
+  @Test
+  public void shouldNotIndexAccountWhenCannotAcquireLock() throws Exception {
+    IndexEventLocks locks = mock(IndexEventLocks.class);
+    Lock lock = mock(Lock.class);
+    when(locks.getLock(any(IndexAccountTask.class))).thenReturn(lock);
+    when(lock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS))).thenReturn(false);
+    Mockito.doCallRealMethod().when(locks).withLock(any(), any(), any());
+    setUpIndexEventHandler(currCtx, locks);
+
+    indexEventHandler.onAccountIndexed(accountId.get());
+
+    verify(forwarder, never()).indexAccount(eq(ACCOUNT_ID), any());
+  }
+
+  @Test
+  public void shouldNotDeleteChangeWhenCannotAcquireLock() throws Exception {
+    IndexEventLocks locks = mock(IndexEventLocks.class);
+    Lock lock = mock(Lock.class);
+    when(locks.getLock(any(DeleteChangeTask.class))).thenReturn(lock);
+    when(lock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS))).thenReturn(false);
+    Mockito.doCallRealMethod().when(locks).withLock(any(), any(), any());
+    setUpIndexEventHandler(currCtx, locks);
+
+    indexEventHandler.onChangeDeleted(changeId.get());
+
+    verify(forwarder, never()).deleteChangeFromIndex(eq(CHANGE_ID), any());
+  }
+
+  @Test
+  public void shouldNotIndexGroupWhenCannotAcquireLock() throws Exception {
+    IndexEventLocks locks = mock(IndexEventLocks.class);
+    Lock lock = mock(Lock.class);
+    when(locks.getLock(any(IndexGroupTask.class))).thenReturn(lock);
+    when(lock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS))).thenReturn(false);
+    Mockito.doCallRealMethod().when(locks).withLock(any(), any(), any());
+    setUpIndexEventHandler(currCtx, locks);
+
+    indexEventHandler.onGroupIndexed(accountGroupUUID.get());
+
+    verify(forwarder, never()).indexGroup(eq(UUID), any());
+  }
+
+  @Test
+  public void shouldNotIndexProjectWhenCannotAcquireLock() throws Exception {
+    IndexEventLocks locks = mock(IndexEventLocks.class);
+    Lock lock = mock(Lock.class);
+    when(locks.getLock(any(IndexProjectTask.class))).thenReturn(lock);
+    when(lock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS))).thenReturn(false);
+    Mockito.doCallRealMethod().when(locks).withLock(any(), any(), any());
+    setUpIndexEventHandler(currCtx, locks);
+
+    indexEventHandler.onProjectIndexed(PROJECT_NAME);
+
+    verify(forwarder, never()).indexProject(eq(PROJECT_NAME), any());
+  }
+
+  @Test
+  public void shouldRetryIndexChangeWhenCannotAcquireLock() throws Exception {
+    IndexEventLocks locks = mock(IndexEventLocks.class);
+    Lock lock = mock(Lock.class);
+    when(locks.getLock(any(IndexChangeTask.class))).thenReturn(lock);
+    Mockito.doCallRealMethod().when(locks).withLock(any(), any(), any());
+    when(lock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS)))
+        .thenReturn(false, true);
+    setUpIndexEventHandler(currCtx, locks);
+
+    indexEventHandler.onChangeIndexed(PROJECT_NAME, changeId.get());
+
+    verify(locks, times(2)).withLock(any(), any(), any());
+    verify(forwarder, times(1)).indexChange(eq(PROJECT_NAME), eq(CHANGE_ID), any());
+  }
+
+  @Test
+  public void shouldRetryUpToMaxTriesWhenCannotAcquireLock() throws Exception {
+    IndexEventLocks locks = mock(IndexEventLocks.class);
+    Lock lock = mock(Lock.class);
+    when(locks.getLock(any(IndexChangeTask.class))).thenReturn(lock);
+    Mockito.doCallRealMethod().when(locks).withLock(any(), any(), any());
+    when(lock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS))).thenReturn(false);
+
+    Configuration cfg = mock(Configuration.class);
+    Configuration.Http httpCfg = mock(Configuration.Http.class);
+    when(httpCfg.maxTries()).thenReturn(10);
+    when(cfg.http()).thenReturn(httpCfg);
+    setUpIndexEventHandler(currCtx, locks, cfg);
+
+    indexEventHandler.onChangeIndexed(PROJECT_NAME, changeId.get());
+
+    verify(locks, times(11)).withLock(any(), any(), any());
+    verify(forwarder, never()).indexChange(eq(PROJECT_NAME), eq(CHANGE_ID), any());
+  }
+
+  @Test
+  public void shouldNotRetryWhenMaxTriesLowerThanOne() throws Exception {
+    IndexEventLocks locks = mock(IndexEventLocks.class);
+    Lock lock = mock(Lock.class);
+    when(locks.getLock(any(IndexChangeTask.class))).thenReturn(lock);
+    Mockito.doCallRealMethod().when(locks).withLock(any(), any(), any());
+    when(lock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS))).thenReturn(false);
+
+    Configuration cfg = mock(Configuration.class);
+    Configuration.Http httpCfg = mock(Configuration.Http.class);
+    when(httpCfg.maxTries()).thenReturn(0);
+    when(cfg.http()).thenReturn(httpCfg);
+    setUpIndexEventHandler(currCtx, locks, cfg);
+
+    indexEventHandler.onChangeIndexed(PROJECT_NAME, changeId.get());
+
+    verify(locks, times(1)).withLock(any(), any(), any());
+    verify(forwarder, never()).indexChange(eq(PROJECT_NAME), eq(CHANGE_ID), any());
+  }
+
+  @Test
+  public void shouldLockPerIndexEventType() throws Exception {
+    IndexEventLocks locks = mock(IndexEventLocks.class);
+    Lock indexChangeLock = mock(Lock.class);
+    when(indexChangeLock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS)))
+        .thenReturn(false);
+    Lock accountChangeLock = mock(Lock.class);
+    when(accountChangeLock.tryLock(Mockito.anyLong(), Mockito.eq(TimeUnit.MILLISECONDS)))
+        .thenReturn(true);
+    when(locks.getLock(any(IndexChangeTask.class))).thenReturn(indexChangeLock);
+    when(locks.getLock(any(IndexAccountTask.class))).thenReturn(accountChangeLock);
+    Mockito.doCallRealMethod().when(locks).withLock(any(), any(), any());
+    setUpIndexEventHandler(currCtx, locks);
+
+    indexEventHandler.onChangeIndexed(PROJECT_NAME, changeId.get());
+    indexEventHandler.onAccountIndexed(accountId.get());
+
+    verify(forwarder, never()).indexChange(eq(PROJECT_NAME), eq(CHANGE_ID), any());
+    verify(forwarder).indexAccount(eq(ACCOUNT_ID), any());
+  }
+
+  @Test
   public void shouldReindexInRemoteWhenContextIsMissingButForcedIndexingEnabled() throws Exception {
     ThreadLocalRequestContext threadLocalCtxMock = mock(ThreadLocalRequestContext.class);
     OneOffRequestContext oneOffCtxMock = mock(OneOffRequestContext.class);
@@ -179,7 +364,14 @@
   public void duplicateChangeEventOfAQueuedEventShouldGetDiscarded() {
     ScheduledThreadPoolExecutor poolMock = mock(ScheduledThreadPoolExecutor.class);
     indexEventHandler =
-        new IndexEventHandler(poolMock, PLUGIN_NAME, forwarder, changeCheckerFactoryMock, currCtx);
+        new IndexEventHandler(
+            poolMock,
+            PLUGIN_NAME,
+            forwarder,
+            changeCheckerFactoryMock,
+            currCtx,
+            configuration,
+            idLocks);
     indexEventHandler.onChangeIndexed(PROJECT_NAME, changeId.get());
     indexEventHandler.onChangeIndexed(PROJECT_NAME, changeId.get());
     verify(poolMock, times(1))
@@ -190,7 +382,14 @@
   public void duplicateAccountEventOfAQueuedEventShouldGetDiscarded() {
     ScheduledThreadPoolExecutor poolMock = mock(ScheduledThreadPoolExecutor.class);
     indexEventHandler =
-        new IndexEventHandler(poolMock, PLUGIN_NAME, forwarder, changeCheckerFactoryMock, currCtx);
+        new IndexEventHandler(
+            poolMock,
+            PLUGIN_NAME,
+            forwarder,
+            changeCheckerFactoryMock,
+            currCtx,
+            configuration,
+            idLocks);
     indexEventHandler.onAccountIndexed(accountId.get());
     indexEventHandler.onAccountIndexed(accountId.get());
     verify(poolMock, times(1)).execute(indexEventHandler.new IndexAccountTask(ACCOUNT_ID));
@@ -200,7 +399,14 @@
   public void duplicateGroupEventOfAQueuedEventShouldGetDiscarded() {
     ScheduledThreadPoolExecutor poolMock = mock(ScheduledThreadPoolExecutor.class);
     indexEventHandler =
-        new IndexEventHandler(poolMock, PLUGIN_NAME, forwarder, changeCheckerFactoryMock, currCtx);
+        new IndexEventHandler(
+            poolMock,
+            PLUGIN_NAME,
+            forwarder,
+            changeCheckerFactoryMock,
+            currCtx,
+            configuration,
+            idLocks);
     indexEventHandler.onGroupIndexed(accountGroupUUID.get());
     indexEventHandler.onGroupIndexed(accountGroupUUID.get());
     verify(poolMock, times(1)).execute(indexEventHandler.new IndexGroupTask(UUID));
@@ -316,4 +522,98 @@
     assertThat(task.equals(differentGroupIdTask)).isFalse();
     assertThat(task.hashCode()).isNotEqualTo(differentGroupIdTask.hashCode());
   }
+
+  private class CurrentThreadScheduledExecutorService implements ScheduledExecutorService {
+
+    @Override
+    public void shutdown() {}
+
+    @Override
+    public List<Runnable> shutdownNow() {
+      return null;
+    }
+
+    @Override
+    public boolean isShutdown() {
+      return false;
+    }
+
+    @Override
+    public boolean isTerminated() {
+      return false;
+    }
+
+    @Override
+    public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
+      return false;
+    }
+
+    @Override
+    public <T> Future<T> submit(Callable<T> task) {
+      return null;
+    }
+
+    @Override
+    public <T> Future<T> submit(Runnable task, T result) {
+      return null;
+    }
+
+    @Override
+    public Future<?> submit(Runnable task) {
+      return null;
+    }
+
+    @Override
+    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
+        throws InterruptedException {
+      return null;
+    }
+
+    @Override
+    public <T> List<Future<T>> invokeAll(
+        Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
+        throws InterruptedException {
+      return null;
+    }
+
+    @Override
+    public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
+        throws InterruptedException, ExecutionException {
+      return null;
+    }
+
+    @Override
+    public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
+        throws InterruptedException, ExecutionException, TimeoutException {
+      return null;
+    }
+
+    @Override
+    public void execute(Runnable command) {
+      command.run();
+    }
+
+    @Override
+    public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {
+      command.run();
+      return null;
+    }
+
+    @Override
+    public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
+      return null;
+    }
+
+    @Override
+    public ScheduledFuture<?> scheduleAtFixedRate(
+        Runnable command, long initialDelay, long period, TimeUnit unit) {
+      return null;
+    }
+
+    @Override
+    public ScheduledFuture<?> scheduleWithFixedDelay(
+        Runnable command, long initialDelay, long delay, TimeUnit unit) {
+      return null;
+    }
+  }
 }