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 801bc9a..af1c4c3 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/Configuration.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/Configuration.java
@@ -54,6 +54,7 @@
 
   // common parameters to cache and index sections
   static final String THREAD_POOL_SIZE_KEY = "threadPoolSize";
+  static final String BATCH_THREAD_POOL_SIZE_KEY = "batchThreadPoolSize";
   static final int DEFAULT_INDEX_MAX_TRIES = 2;
   static final int DEFAULT_INDEX_RETRY_INTERVAL = 30000;
   static final int DEFAULT_THREAD_POOL_SIZE = 4;
@@ -455,6 +456,7 @@
     static final boolean DEFAULT_SYNCHRONIZE_FORCED = true;
 
     private final int threadPoolSize;
+    private final int batchThreadPoolSize;
     private final int retryInterval;
     private final int maxTries;
     private final int numStripedLocks;
@@ -463,6 +465,7 @@
     private Index(Config cfg) {
       super(cfg, INDEX_SECTION);
       threadPoolSize = getInt(cfg, INDEX_SECTION, THREAD_POOL_SIZE_KEY, DEFAULT_THREAD_POOL_SIZE);
+      batchThreadPoolSize = getInt(cfg, INDEX_SECTION, BATCH_THREAD_POOL_SIZE_KEY, threadPoolSize);
       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);
@@ -474,6 +477,10 @@
       return threadPoolSize;
     }
 
+    public int batchThreadPoolSize() {
+      return batchThreadPoolSize;
+    }
+
     public int numStripedLocks() {
       return numStripedLocks;
     }
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/autoreindex/IndexTs.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/autoreindex/IndexTs.java
index ff42a4e..62d9c10 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/autoreindex/IndexTs.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/autoreindex/IndexTs.java
@@ -48,7 +48,10 @@
 
   private final Path dataDir;
   private final ScheduledExecutorService exec;
-  private final FlusherRunner flusher;
+  private final FlusherRunner changeFlusher;
+  private final FlusherRunner accountFlusher;
+  private final FlusherRunner groupFlusher;
+  private final FlusherRunner projectFlusher;
   private final ChangeFinder changeFinder;
   private final CurrentRequestContext currCtx;
 
@@ -58,16 +61,11 @@
   private volatile LocalDateTime projectTs;
 
   class FlusherRunner implements Runnable {
+    private final AbstractIndexRestApiServlet.IndexName index;
 
     @Override
     public void run() {
-      store(AbstractIndexRestApiServlet.IndexName.CHANGE, changeTs);
-      store(AbstractIndexRestApiServlet.IndexName.ACCOUNT, accountTs);
-      store(AbstractIndexRestApiServlet.IndexName.GROUP, groupTs);
-      store(AbstractIndexRestApiServlet.IndexName.PROJECT, projectTs);
-    }
-
-    private void store(AbstractIndexRestApiServlet.IndexName index, LocalDateTime latestTs) {
+      LocalDateTime latestTs = getIndexTimeStamp();
       Optional<LocalDateTime> currTs = getUpdateTs(index);
       if (!currTs.isPresent() || latestTs.isAfter(currTs.get())) {
         Path indexTsFile = dataDir.resolve(index.name().toLowerCase());
@@ -78,6 +76,25 @@
         }
       }
     }
+
+    FlusherRunner(AbstractIndexRestApiServlet.IndexName index) {
+      this.index = index;
+    }
+
+    private LocalDateTime getIndexTimeStamp() {
+      switch (index) {
+        case CHANGE:
+          return changeTs;
+        case GROUP:
+          return groupTs;
+        case ACCOUNT:
+          return accountTs;
+        case PROJECT:
+          return projectTs;
+        default:
+          throw new IllegalArgumentException("Unsupported index " + index);
+      }
+    }
   }
 
   @Inject
@@ -88,7 +105,10 @@
       CurrentRequestContext currCtx) {
     this.dataDir = dataDir;
     this.exec = queue.getDefaultQueue();
-    this.flusher = new FlusherRunner();
+    this.changeFlusher = new FlusherRunner(AbstractIndexRestApiServlet.IndexName.CHANGE);
+    this.accountFlusher = new FlusherRunner(AbstractIndexRestApiServlet.IndexName.ACCOUNT);
+    this.groupFlusher = new FlusherRunner(AbstractIndexRestApiServlet.IndexName.GROUP);
+    this.projectFlusher = new FlusherRunner(AbstractIndexRestApiServlet.IndexName.PROJECT);
     this.changeFinder = changeFinder;
     this.currCtx = currCtx;
   }
@@ -147,19 +167,22 @@
     switch (index) {
       case CHANGE:
         changeTs = dateTime;
+        exec.execute(changeFlusher);
         break;
       case ACCOUNT:
         accountTs = dateTime;
+        exec.execute(accountFlusher);
         break;
       case GROUP:
         groupTs = dateTime;
+        exec.execute(groupFlusher);
         break;
       case PROJECT:
         projectTs = dateTime;
+        exec.execute(projectFlusher);
         break;
       default:
         throw new IllegalArgumentException("Unsupported index " + index);
     }
-    exec.execute(flusher);
   }
 }
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexBatchChangeHandler.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexBatchChangeHandler.java
new file mode 100644
index 0000000..dee8876
--- /dev/null
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/ForwardedIndexBatchChangeHandler.java
@@ -0,0 +1,38 @@
+// Copyright (C) 2021 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 com.ericsson.gerrit.plugins.highavailability.Configuration;
+import com.ericsson.gerrit.plugins.highavailability.index.ChangeCheckerImpl.Factory;
+import com.ericsson.gerrit.plugins.highavailability.index.ForwardedBatchIndexExecutor;
+import com.google.gerrit.server.index.change.ChangeIndexer;
+import com.google.gerrit.server.util.OneOffRequestContext;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import java.util.concurrent.ScheduledExecutorService;
+
+@Singleton
+public class ForwardedIndexBatchChangeHandler extends ForwardedIndexChangeHandler {
+
+  @Inject
+  ForwardedIndexBatchChangeHandler(
+      ChangeIndexer indexer,
+      Configuration config,
+      @ForwardedBatchIndexExecutor ScheduledExecutorService indexExecutor,
+      OneOffRequestContext oneOffCtx,
+      Factory changeCheckerFactory) {
+    super(indexer, config, indexExecutor, oneOffCtx, changeCheckerFactory);
+  }
+}
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/Forwarder.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/Forwarder.java
index bb47f11..b73b676 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/Forwarder.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/Forwarder.java
@@ -42,6 +42,18 @@
   CompletableFuture<Boolean> indexChange(String projectName, int changeId, IndexEvent indexEvent);
 
   /**
+   * Forward a change indexing event to the other master using batch index endpoint.
+   *
+   * @param projectName the project of the change to index.
+   * @param changeId the change to index.
+   * @param indexEvent the details of the index event.
+   * @return {@link CompletableFuture} of true if successful, otherwise {@link CompletableFuture} of
+   *     false.
+   */
+  CompletableFuture<Boolean> batchIndexChange(
+      String projectName, int changeId, IndexEvent indexEvent);
+
+  /**
    * Forward a delete change from index event to the other master.
    *
    * @param changeId the change to remove from the index.
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/IndexEvent.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/IndexEvent.java
index 037c1c6..71cf044 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/IndexEvent.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/IndexEvent.java
@@ -21,10 +21,14 @@
 public class IndexEvent {
   public long eventCreatedOn = System.currentTimeMillis() / 1000;
   public String targetSha;
+  public String metaSha;
 
   @Override
   public String toString() {
-    return "IndexEvent@" + format(eventCreatedOn) + ((targetSha != null) ? "/" + targetSha : "");
+    return "IndexEvent@"
+        + format(eventCreatedOn)
+        + ((targetSha != null) ? "/target:" + targetSha : "")
+        + ((metaSha != null) ? "/meta:" + metaSha : "");
   }
 
   public static String format(long eventTs) {
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/IndexBatchChangeRestApiServlet.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/IndexBatchChangeRestApiServlet.java
new file mode 100644
index 0000000..bebfae9
--- /dev/null
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/IndexBatchChangeRestApiServlet.java
@@ -0,0 +1,37 @@
+// Copyright (C) 2021 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.rest;
+
+import com.ericsson.gerrit.plugins.highavailability.forwarder.ForwardedIndexBatchChangeHandler;
+import com.google.gerrit.extensions.restapi.Url;
+import com.google.gerrit.server.events.EventGson;
+import com.google.gson.Gson;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+
+@Singleton
+class IndexBatchChangeRestApiServlet extends AbstractIndexRestApiServlet<String> {
+  private static final long serialVersionUID = -1L;
+
+  @Inject
+  IndexBatchChangeRestApiServlet(ForwardedIndexBatchChangeHandler handler, @EventGson Gson gson) {
+    super(handler, IndexName.CHANGE, true, gson);
+  }
+
+  @Override
+  String parse(String id) {
+    return Url.decode(id);
+  }
+}
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarder.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarder.java
index d404ab2..ea4ad10 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarder.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarder.java
@@ -83,6 +83,16 @@
         event);
   }
 
+  public CompletableFuture<Boolean> batchIndexChange(
+      String projectName, int changeId, IndexEvent event) {
+    return execute(
+        RequestMethod.POST,
+        "index change",
+        "index/change/batch",
+        buildIndexEndpoint(projectName, changeId),
+        event);
+  }
+
   @Override
   public CompletableFuture<Boolean> deleteChangeFromIndex(final int changeId, IndexEvent event) {
     return execute(
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarderServletModule.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarderServletModule.java
index 589bbef..4d3de37 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarderServletModule.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarderServletModule.java
@@ -20,6 +20,7 @@
   @Override
   protected void configureServlets() {
     serveRegex("/index/account/\\d+$").with(IndexAccountRestApiServlet.class);
+    serveRegex("/index/change/batch/.*$").with(IndexBatchChangeRestApiServlet.class);
     serveRegex("/index/change/.*$").with(IndexChangeRestApiServlet.class);
     serveRegex("/index/group/\\w+$").with(IndexGroupRestApiServlet.class);
     serveRegex("/index/project/.*$").with(IndexProjectRestApiServlet.class);
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/BatchIndexExecutor.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/BatchIndexExecutor.java
new file mode 100644
index 0000000..06da9f0
--- /dev/null
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/BatchIndexExecutor.java
@@ -0,0 +1,24 @@
+// Copyright (C) 2021 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 static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import com.google.inject.BindingAnnotation;
+import java.lang.annotation.Retention;
+
+@Retention(RUNTIME)
+@BindingAnnotation
+@interface BatchIndexExecutor {}
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/BatchIndexExecutorProvider.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/BatchIndexExecutorProvider.java
new file mode 100644
index 0000000..cfbd4fb
--- /dev/null
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/BatchIndexExecutorProvider.java
@@ -0,0 +1,30 @@
+// Copyright (C) 2021 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.ExecutorProvider;
+import com.google.gerrit.server.git.WorkQueue;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+
+@Singleton
+class BatchIndexExecutorProvider extends ExecutorProvider {
+
+  @Inject
+  BatchIndexExecutorProvider(WorkQueue workQueue, Configuration config) {
+    super(workQueue, config.index().batchThreadPoolSize(), "Forward-BatchIndex-Event");
+  }
+}
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ChangeCheckerImpl.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ChangeCheckerImpl.java
index fe04ee9..945cefa 100644
--- a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ChangeCheckerImpl.java
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ChangeCheckerImpl.java
@@ -18,6 +18,7 @@
 import com.google.common.flogger.FluentLogger;
 import com.google.gerrit.entities.Change;
 import com.google.gerrit.entities.Comment;
+import com.google.gerrit.entities.RefNames;
 import com.google.gerrit.server.CommentsUtil;
 import com.google.gerrit.server.change.ChangeFinder;
 import com.google.gerrit.server.git.GitRepositoryManager;
@@ -62,15 +63,25 @@
   }
 
   @Override
-  public Optional<IndexEvent> newIndexEvent() {
-    return getComputedChangeTs()
-        .map(
-            ts -> {
-              IndexEvent event = new IndexEvent();
-              event.eventCreatedOn = ts;
-              event.targetSha = getBranchTargetSha();
-              return event;
-            });
+  public Optional<IndexEvent> newIndexEvent() throws IOException {
+    Optional<Long> changeTs = getComputedChangeTs();
+    if (!changeTs.isPresent()) {
+      return Optional.empty();
+    }
+
+    long ts = changeTs.get();
+
+    IndexEvent event = new IndexEvent();
+    event.eventCreatedOn = ts;
+    try (Repository repo = gitRepoMgr.openRepository(changeNotes.get().getProjectName())) {
+      event.targetSha = getBranchTargetSha();
+      event.metaSha = getMetaSha(repo);
+      return Optional.of(event);
+    } catch (IOException e) {
+      log.atSevere().withCause(e).log(
+          "Unable to create index event for project %s", changeNotes.get().getProjectName());
+      throw e;
+    }
   }
 
   @Override
@@ -82,25 +93,31 @@
   }
 
   @Override
-  public boolean isChangeUpToDate(Optional<IndexEvent> indexEvent) {
+  public boolean isChangeUpToDate(Optional<IndexEvent> indexEventOption) throws IOException {
     getComputedChangeTs();
-    log.atFine().log("Checking change %s against index event %s", this, indexEvent);
+    log.atFine().log("Checking change %s against index event %s", this, indexEventOption);
     if (!computedChangeTs.isPresent()) {
       log.atWarning().log("Unable to compute last updated ts for change %s", changeId);
       return false;
     }
+    try {
+      if (indexEventOption.isPresent()) {
+        try (Repository repo = gitRepoMgr.openRepository(changeNotes.get().getProjectName())) {
+          IndexEvent indexEvent = indexEventOption.get();
+          return (computedChangeTs.get() > indexEvent.eventCreatedOn)
+              || (computedChangeTs.get() == indexEvent.eventCreatedOn)
+                  && (Objects.isNull(indexEvent.targetSha)
+                      || Objects.equals(getBranchTargetSha(), indexEvent.targetSha))
+                  && (Objects.isNull(indexEvent.metaSha)
+                      || Objects.equals(getMetaSha(repo), indexEvent.metaSha));
+        }
+      }
+      return true;
 
-    if (indexEvent.isPresent() && indexEvent.get().targetSha == null) {
-      return indexEvent.map(e -> (computedChangeTs.get() >= e.eventCreatedOn)).orElse(true);
+    } catch (IOException ex) {
+      log.atWarning().log("Unable to read meta sha for change %s", changeId);
+      return false;
     }
-
-    return indexEvent
-        .map(
-            e ->
-                (computedChangeTs.get() > e.eventCreatedOn)
-                    || (computedChangeTs.get() == e.eventCreatedOn)
-                        && (Objects.equals(getBranchTargetSha(), e.targetSha)))
-        .orElse(true);
   }
 
   @Override
@@ -113,12 +130,19 @@
 
   @Override
   public String toString() {
-    return "change-id="
-        + changeId
-        + "@"
-        + getComputedChangeTs().map(IndexEvent::format)
-        + "/"
-        + getBranchTargetSha();
+    try (Repository repo = gitRepoMgr.openRepository(changeNotes.get().getProjectName())) {
+      return "change-id="
+          + changeId
+          + "@"
+          + getComputedChangeTs().map(IndexEvent::format)
+          + "/target:"
+          + getBranchTargetSha()
+          + "/meta:"
+          + getMetaSha(repo);
+    } catch (IOException e) {
+      log.atSevere().withCause(e).log("Unable to render change %s", changeId);
+      return "change-id=" + changeId;
+    }
   }
 
   private String getBranchTargetSha() {
@@ -141,6 +165,16 @@
     return getChangeNotes().map(this::getTsFromChangeAndDraftComments);
   }
 
+  private String getMetaSha(Repository repo) throws IOException {
+    String refName = RefNames.changeMetaRef(changeNotes.get().getChange().getId());
+    Ref ref = repo.exactRef(refName);
+    if (ref == null) {
+      throw new IOException(
+          String.format("Unable to find meta ref %s for change %s", refName, changeId));
+    }
+    return ref.getTarget().getObjectId().getName();
+  }
+
   private long getTsFromChangeAndDraftComments(ChangeNotes notes) {
     Change change = notes.getChange();
     Timestamp changeTs = change.getLastUpdatedOn();
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ForwardedBatchIndexExecutor.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ForwardedBatchIndexExecutor.java
new file mode 100644
index 0000000..b5b6aab
--- /dev/null
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ForwardedBatchIndexExecutor.java
@@ -0,0 +1,24 @@
+// Copyright (C) 2021 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 static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import com.google.inject.BindingAnnotation;
+import java.lang.annotation.Retention;
+
+@Retention(RUNTIME)
+@BindingAnnotation
+public @interface ForwardedBatchIndexExecutor {}
diff --git a/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ForwardedBatchIndexExecutorProvider.java b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ForwardedBatchIndexExecutorProvider.java
new file mode 100644
index 0000000..f5bc85e
--- /dev/null
+++ b/src/main/java/com/ericsson/gerrit/plugins/highavailability/index/ForwardedBatchIndexExecutorProvider.java
@@ -0,0 +1,30 @@
+// Copyright (C) 2021 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.ExecutorProvider;
+import com.google.gerrit.server.git.WorkQueue;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+
+@Singleton
+class ForwardedBatchIndexExecutorProvider extends ExecutorProvider {
+
+  @Inject
+  ForwardedBatchIndexExecutorProvider(WorkQueue workQueue, Configuration config) {
+    super(workQueue, config.index().batchThreadPoolSize(), "Forwarded-BatchIndex-Event");
+  }
+}
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 368581c..73e842f 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
@@ -40,6 +40,7 @@
         ProjectIndexedListener {
   private static final FluentLogger log = FluentLogger.forEnclosingClass();
   private final ScheduledExecutorService executor;
+  private final ScheduledExecutorService batchExecutor;
   private final Forwarder forwarder;
   private final String pluginName;
   private final Set<IndexTask> queuedTasks = Collections.newSetFromMap(new ConcurrentHashMap<>());
@@ -53,6 +54,7 @@
   @Inject
   IndexEventHandler(
       @IndexExecutor ScheduledExecutorService executor,
+      @BatchIndexExecutor ScheduledExecutorService batchExecutor,
       @PluginName String pluginName,
       Forwarder forwarder,
       ChangeCheckerImpl.Factory changeChecker,
@@ -61,6 +63,7 @@
       IndexEventLocks locks) {
     this.forwarder = forwarder;
     this.executor = executor;
+    this.batchExecutor = batchExecutor;
     this.pluginName = pluginName;
     this.changeChecker = changeChecker;
     this.currCtx = currCtx;
@@ -95,11 +98,21 @@
         changeChecker
             .create(changeId)
             .newIndexEvent()
-            .map(event -> new IndexChangeTask(projectName, id, event))
+            .map(
+                event -> {
+                  if (Thread.currentThread().getName().contains("Batch")) {
+                    return new BatchIndexChangeTask(projectName, id, event);
+                  }
+                  return new IndexChangeTask(projectName, id, event);
+                })
             .ifPresent(
                 task -> {
                   if (queuedTasks.add(task)) {
-                    executor.execute(task);
+                    if (task instanceof BatchIndexChangeTask) {
+                      batchExecutor.execute(task);
+                    } else {
+                      executor.execute(task);
+                    }
                   }
                 });
       } catch (Exception e) {
@@ -215,6 +228,46 @@
     }
   }
 
+  class BatchIndexChangeTask extends IndexTask {
+    private final int changeId;
+    private final String projectName;
+
+    BatchIndexChangeTask(String projectName, int changeId, IndexEvent indexEvent) {
+      super(indexEvent);
+      this.projectName = projectName;
+      this.changeId = changeId;
+    }
+
+    @Override
+    public CompletableFuture<Boolean> execute() {
+      return forwarder.batchIndexChange(projectName, changeId, indexEvent);
+    }
+
+    @Override
+    String indexId() {
+      return "change/" + changeId;
+    }
+
+    @Override
+    public int hashCode() {
+      return Objects.hashCode(IndexEventHandler.BatchIndexChangeTask.class, changeId);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+      if (!(obj instanceof IndexEventHandler.BatchIndexChangeTask)) {
+        return false;
+      }
+      IndexEventHandler.BatchIndexChangeTask other = (IndexEventHandler.BatchIndexChangeTask) obj;
+      return changeId == other.changeId;
+    }
+
+    @Override
+    public String toString() {
+      return String.format("[%s] Index change %s in target instance", pluginName, changeId);
+    }
+  }
+
   class DeleteChangeTask extends IndexTask {
     private final int changeId;
 
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 4a25fc8..3bcc34f 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
@@ -35,6 +35,12 @@
         .annotatedWith(ForwardedIndexExecutor.class)
         .toProvider(ForwardedIndexExecutorProvider.class);
     bind(IndexEventLocks.class).in(Scopes.SINGLETON);
+    bind(ScheduledExecutorService.class)
+        .annotatedWith(BatchIndexExecutor.class)
+        .toProvider(BatchIndexExecutorProvider.class);
+    bind(ScheduledExecutorService.class)
+        .annotatedWith(ForwardedBatchIndexExecutor.class)
+        .toProvider(ForwardedBatchIndexExecutorProvider.class);
     listener().to(IndexExecutorProvider.class);
     DynamicSet.bind(binder(), ChangeIndexedListener.class)
         .to(IndexEventHandler.class)
diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
index fd0b56e..83d3862 100644
--- a/src/main/resources/Documentation/about.md
+++ b/src/main/resources/Documentation/about.md
@@ -103,3 +103,17 @@
 
 For further information and supported options, refer to [config](config.md)
 documentation.
+
+### Last index update timestamp storage
+
+The plugin keeps track of the timestamp when it lastly updated an index.
+When the [`autoReindex.enabled` option](config.md) is set to `true`, the timestamp
+is used to determine which changes to reindex when a node is temporarily out of
+sync with the primary, for example, after a node being offline for a long time.
+
+The HA plugin keeps the last update timestamp for each index in the following files:
+* `<gerrit_home>/data/high-availability/group`
+* `<gerrit_home>/data/high-availability/account`
+* `<gerrit_home>/data/high-availability/change`
+
+The timestamp is stored in this format `yyyy-mm-ddTHH:MM:SS.ss`, i.e.: `2020-12-18T12:17:53.25`.
diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 9ee2e92..3b1fee2 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -180,7 +180,11 @@
     Defaults to true.
 
 ```index.numStripedLocks```
-:   Number of striped locks to use during reindexing.
+:   Number of striped locks to use during reindexing. Should be of the same order
+    of magnitude of the open changes. I.e.: if one has 5000 changes, one might want
+    to have at least 1000 striped locks. The value has to be tuned empirically
+    by checking the number of failures in acquiring the locking. Checking the
+    `consider increasing the number of shards` warnings should help.
     Defaults to 10.
 
 ```index.synchronize```
@@ -196,6 +200,11 @@
 :   Maximum number of threads used to send index events to the target instance.
     Defaults to 4.
 
+```index.batchThreadPoolSize```
+:   Maximum number of threads used to send batch index events to the target instance
+    and not associated to an interactive action performed by a user.
+    Defaults equal index.threadPoolSize.
+
 ```index.maxTries```
 :   Maximum number of times the plugin should attempt to reindex changes.
     Setting this value to 0 will disable retries. After this number of failed tries,
diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index 002ef9c..5f455d4 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -1,4 +1,4 @@
-FROM gerritcodereview/gerrit:3.1.11
+FROM gerritcodereview/gerrit:3.1.12
 
 ENV GERRIT_BRANCH=stable-3.1
 
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 a9537bd..3444625 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/highavailability/ConfigurationTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/ConfigurationTest.java
@@ -14,6 +14,7 @@
 
 package com.ericsson.gerrit.plugins.highavailability;
 
+import static com.ericsson.gerrit.plugins.highavailability.Configuration.BATCH_THREAD_POOL_SIZE_KEY;
 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_NUM_STRIPED_LOCKS;
@@ -266,6 +267,19 @@
   }
 
   @Test
+  public void testGetBatchIndexThreadPoolSize() throws Exception {
+    assertThat(getConfiguration().index().batchThreadPoolSize())
+        .isEqualTo(DEFAULT_THREAD_POOL_SIZE);
+
+    globalPluginConfig.setInt(INDEX_SECTION, null, BATCH_THREAD_POOL_SIZE_KEY, THREAD_POOL_SIZE);
+    assertThat(getConfiguration().index().batchThreadPoolSize()).isEqualTo(THREAD_POOL_SIZE);
+
+    globalPluginConfig.setString(INDEX_SECTION, null, BATCH_THREAD_POOL_SIZE_KEY, INVALID_INT);
+    assertThat(getConfiguration().index().batchThreadPoolSize())
+        .isEqualTo(DEFAULT_THREAD_POOL_SIZE);
+  }
+
+  @Test
   public void testGetIndexSynchronize() throws Exception {
     assertThat(getConfiguration().index().synchronize()).isEqualTo(DEFAULT_SYNCHRONIZE);
 
diff --git a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarderTest.java b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarderTest.java
index 046d6ad..7908669 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarderTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/forwarder/rest/RestForwarderTest.java
@@ -68,6 +68,14 @@
               PLUGIN_NAME,
               "index/change",
               PROJECT_NAME_URL_END + "~" + CHANGE_NUMBER);
+  private static final String INDEX_BATCH_CHANGE_ENDPOINT =
+      Joiner.on("/")
+          .join(
+              URL,
+              PLUGINS,
+              PLUGIN_NAME,
+              "index/change/batch",
+              PROJECT_NAME_URL_END + "~" + CHANGE_NUMBER);
   private static final String DELETE_CHANGE_ENDPOINT =
       Joiner.on("/").join(URL, PLUGINS, PLUGIN_NAME, "index/change", "~" + CHANGE_NUMBER);
   private static final int ACCOUNT_NUMBER = 2;
@@ -198,6 +206,29 @@
   }
 
   @Test
+  public void testIndexBatchChangeOK() throws Exception {
+    when(httpSessionMock.post(eq(INDEX_BATCH_CHANGE_ENDPOINT), any()))
+        .thenReturn(new HttpResult(SUCCESSFUL, EMPTY_MSG));
+    assertThat(forwarder.batchIndexChange(PROJECT_NAME, CHANGE_NUMBER, new IndexEvent()).get())
+        .isTrue();
+  }
+
+  @Test
+  public void testIndexBatchChangeFailed() throws Exception {
+    when(httpSessionMock.post(eq(INDEX_BATCH_CHANGE_ENDPOINT), any()))
+        .thenReturn(new HttpResult(FAILED, EMPTY_MSG));
+    assertThat(forwarder.batchIndexChange(PROJECT_NAME, CHANGE_NUMBER, new IndexEvent()).get())
+        .isFalse();
+  }
+
+  @Test
+  public void testIndexBatchChangeThrowsException() throws Exception {
+    doThrow(new IOException()).when(httpSessionMock).post(eq(INDEX_BATCH_CHANGE_ENDPOINT), any());
+    assertThat(forwarder.batchIndexChange(PROJECT_NAME, CHANGE_NUMBER, new IndexEvent()).get())
+        .isFalse();
+  }
+
+  @Test
   public void testChangeDeletedFromIndexOK() throws Exception {
     when(httpSessionMock.delete(eq(DELETE_CHANGE_ENDPOINT)))
         .thenReturn(new HttpResult(SUCCESSFUL, EMPTY_MSG));
diff --git a/src/test/java/com/ericsson/gerrit/plugins/highavailability/index/ChangeCheckerIT.java b/src/test/java/com/ericsson/gerrit/plugins/highavailability/index/ChangeCheckerIT.java
new file mode 100644
index 0000000..febf8f0
--- /dev/null
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/index/ChangeCheckerIT.java
@@ -0,0 +1,127 @@
+// Copyright (C) 2021 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 static com.google.common.truth.Truth.assertThat;
+
+import com.ericsson.gerrit.plugins.highavailability.forwarder.IndexEvent;
+import com.google.gerrit.acceptance.LightweightPluginDaemonTest;
+import com.google.gerrit.acceptance.PushOneCommit.Result;
+import com.google.gerrit.acceptance.TestPlugin;
+import com.google.gerrit.entities.RefNames;
+import java.io.IOException;
+import java.util.Optional;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.Repository;
+import org.junit.Test;
+
+@TestPlugin(
+    name = "high-availability",
+    sysModule = "com.ericsson.gerrit.plugins.highavailability.Module",
+    httpModule = "com.ericsson.gerrit.plugins.highavailability.HttpModule")
+public class ChangeCheckerIT extends LightweightPluginDaemonTest {
+
+  ChangeCheckerImpl.Factory changeCheckerFactory;
+
+  @Override
+  public void setUpTestPlugin() throws Exception {
+    super.setUpTestPlugin();
+    changeCheckerFactory = plugin.getSysInjector().getInstance(ChangeCheckerImpl.Factory.class);
+  }
+
+  @Test
+  public void shouldPopulateMetaSha() throws Exception {
+    Result change = createChange();
+    ChangeChecker changeChecker = changeCheckerFactory.create(change.getChangeId());
+    Optional<IndexEvent> eventOption = changeChecker.newIndexEvent();
+
+    assertThat(eventOption.isPresent()).isTrue();
+    IndexEvent event = eventOption.get();
+    assertThat(event.metaSha).isNotNull();
+    assertThat(event.metaSha).isEqualTo(readMetaSha(change));
+  }
+
+  @Test
+  public void shouldReturnIsUpToDateTrueWhenEventContainsCorrectMetaAndTargetSha()
+      throws Exception {
+    Result change = createChange();
+    ChangeChecker changeChecker = changeCheckerFactory.create(change.getChangeId());
+    Optional<IndexEvent> event = changeChecker.newIndexEvent();
+
+    assertThat(changeChecker.isChangeUpToDate(event)).isTrue();
+  }
+
+  @Test
+  public void shouldReturnIsUpToDateTrueWhenTargetShaIsNull() throws Exception {
+    Result change = createChange();
+    ChangeChecker changeChecker = changeCheckerFactory.create(change.getChangeId());
+    Optional<IndexEvent> event =
+        changeChecker
+            .newIndexEvent()
+            .map(
+                e -> {
+                  e.targetSha = null;
+                  return e;
+                });
+
+    assertThat(changeChecker.isChangeUpToDate(event)).isTrue();
+  }
+
+  @Test
+  public void shouldReturnFalseWhenMetaShaIsNotUpToDate() throws Exception {
+    String testMetaRefSha = "6212efebe6e8b9f439a8ad013243e602afab7441";
+    Result change = createChange();
+    ChangeChecker changeChecker = changeCheckerFactory.create(change.getChangeId());
+    Optional<IndexEvent> event =
+        changeChecker
+            .newIndexEvent()
+            .map(
+                e -> {
+                  e.metaSha = testMetaRefSha;
+                  return e;
+                });
+
+    assertThat(changeChecker.isChangeUpToDate(event)).isFalse();
+  }
+
+  @Test
+  public void shouldReturnFalseWhenTargetShaIsNotUpToDate() throws Exception {
+    String testTargetRefSha = "abed47baf2818a86b68cf712073a748a6b5b293e";
+    Result change = createChange();
+    ChangeChecker changeChecker = changeCheckerFactory.create(change.getChangeId());
+    Optional<IndexEvent> event =
+        changeChecker
+            .newIndexEvent()
+            .map(
+                e -> {
+                  e.targetSha = testTargetRefSha;
+                  return e;
+                });
+
+    assertThat(changeChecker.isChangeUpToDate(event)).isFalse();
+  }
+
+  private String readMetaSha(Result change) throws IOException {
+    try (Repository repo = repoManager.openRepository(change.getChange().change().getProject())) {
+      String refName = RefNames.changeMetaRef(change.getChange().getId());
+      Ref ref = repo.exactRef(refName);
+      if (ref == null) {
+        return null;
+      }
+
+      return ref.getTarget().getObjectId().getName();
+    }
+  }
+}
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 875f203..578dc17 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
@@ -85,6 +85,7 @@
   private Account.Id accountId;
   private AccountGroup.UUID accountGroupUUID;
   private ScheduledExecutorService executor = new CurrentThreadScheduledExecutorService();
+  private ScheduledExecutorService batchExecutor = new CurrentThreadScheduledExecutorService();
   private ScheduledExecutorService testExecutor =
       Executors.newScheduledThreadPool(MAX_TEST_PARALLELISM);
   @Mock private RequestContext mockCtx;
@@ -142,6 +143,7 @@
     indexEventHandler =
         new IndexEventHandler(
             executor,
+            batchExecutor,
             PLUGIN_NAME,
             forwarder,
             changeCheckerFactoryMock,
@@ -387,9 +389,11 @@
   @Test
   public void duplicateChangeEventOfAQueuedEventShouldGetDiscarded() {
     ScheduledThreadPoolExecutor poolMock = mock(ScheduledThreadPoolExecutor.class);
+    ScheduledThreadPoolExecutor poolBatchMock = mock(ScheduledThreadPoolExecutor.class);
     indexEventHandler =
         new IndexEventHandler(
             poolMock,
+            poolBatchMock,
             PLUGIN_NAME,
             forwarder,
             changeCheckerFactoryMock,
@@ -405,9 +409,11 @@
   @Test
   public void duplicateAccountEventOfAQueuedEventShouldGetDiscarded() {
     ScheduledThreadPoolExecutor poolMock = mock(ScheduledThreadPoolExecutor.class);
+    ScheduledThreadPoolExecutor poolBatchMock = mock(ScheduledThreadPoolExecutor.class);
     indexEventHandler =
         new IndexEventHandler(
             poolMock,
+            poolBatchMock,
             PLUGIN_NAME,
             forwarder,
             changeCheckerFactoryMock,
@@ -422,9 +428,11 @@
   @Test
   public void duplicateGroupEventOfAQueuedEventShouldGetDiscarded() {
     ScheduledThreadPoolExecutor poolMock = mock(ScheduledThreadPoolExecutor.class);
+    ScheduledThreadPoolExecutor poolBatchMock = mock(ScheduledThreadPoolExecutor.class);
     indexEventHandler =
         new IndexEventHandler(
             poolMock,
+            poolBatchMock,
             PLUGIN_NAME,
             forwarder,
             changeCheckerFactoryMock,