diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexChangeHandler.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexChangeHandler.java
index 1e21e84..8d41500 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexChangeHandler.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexChangeHandler.java
@@ -23,15 +23,12 @@
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 import com.googlesource.gerrit.plugins.multisite.Configuration;
-import com.googlesource.gerrit.plugins.multisite.Configuration.Index;
 import com.googlesource.gerrit.plugins.multisite.forwarder.events.ChangeIndexEvent;
 import com.googlesource.gerrit.plugins.multisite.index.ChangeChecker;
 import com.googlesource.gerrit.plugins.multisite.index.ChangeCheckerImpl;
 import com.googlesource.gerrit.plugins.multisite.index.ForwardedIndexExecutor;
 import java.util.Optional;
-import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
-import java.util.concurrent.TimeUnit;
 
 /**
  * Index a change using {@link ChangeIndexer}. This class is meant to be used on the receiving side
@@ -41,12 +38,8 @@
  */
 @Singleton
 public class ForwardedIndexChangeHandler
-    extends ForwardedIndexingHandler<String, ChangeIndexEvent> {
+    extends ForwardedIndexingHandlerWithRetries<String, ChangeIndexEvent> {
   private final ChangeIndexer indexer;
-  private final ScheduledExecutorService indexExecutor;
-  private final OneOffRequestContext oneOffCtx;
-  private final int retryInterval;
-  private final int maxTries;
   private final ChangeCheckerImpl.Factory changeCheckerFactory;
 
   @Inject
@@ -56,43 +49,22 @@
       @ForwardedIndexExecutor ScheduledExecutorService indexExecutor,
       OneOffRequestContext oneOffCtx,
       ChangeCheckerImpl.Factory changeCheckerFactory) {
-    super(configuration.index().numStripedLocks());
+    super(indexExecutor, configuration, oneOffCtx);
     this.indexer = indexer;
-    this.indexExecutor = indexExecutor;
-    this.oneOffCtx = oneOffCtx;
     this.changeCheckerFactory = changeCheckerFactory;
-
-    Index indexConfig = configuration.index();
-    this.retryInterval = indexConfig != null ? indexConfig.retryInterval() : 0;
-    this.maxTries = indexConfig != null ? indexConfig.maxTries() : 0;
   }
 
   @Override
   protected void doIndex(String id, Optional<ChangeIndexEvent> indexEvent) {
-    doIndex(id, indexEvent, 0);
+    attemptToIndex(id, indexEvent, 0);
   }
 
-  private void doIndex(String id, Optional<ChangeIndexEvent> indexEvent, int retryCount) {
+  @Override
+  protected void attemptToIndex(String id, Optional<ChangeIndexEvent> indexEvent, int retryCount) {
     ChangeChecker checker = changeCheckerFactory.create(id);
     Optional<ChangeNotes> changeNotes = checker.getChangeNotes();
     if (changeNotes.isPresent()) {
-      ChangeNotes notes = changeNotes.get();
-      reindex(notes);
-
-      if (checker.isChangeUpToDate(indexEvent)) {
-        if (retryCount > 0) {
-          log.warn("Change {} has been eventually indexed after {} attempt(s)", id, retryCount);
-        } else {
-          log.debug("Change {} successfully indexed", id);
-        }
-      } else {
-        log.warn(
-            "Change {} seems too old compared to the event timestamp (event={} >> change-Ts={})",
-            id,
-            indexEvent,
-            checker);
-        rescheduleIndex(id, indexEvent, retryCount + 1);
-      }
+      reindexAndCheckIsUpToDate(id, indexEvent, checker, retryCount);
     } else {
       log.warn(
           "Change {} not present yet in local Git repository (event={}) after {} attempt(s)",
@@ -106,42 +78,20 @@
     }
   }
 
-  private void reindex(ChangeNotes notes) {
+  @Override
+  protected void reindex(String id) {
     try (ManualRequestContext ctx = oneOffCtx.open()) {
+      ChangeChecker checker = changeCheckerFactory.create(id);
+      Optional<ChangeNotes> changeNotes = checker.getChangeNotes();
+      ChangeNotes notes = changeNotes.get();
       notes.reload();
       indexer.index(notes.getChange());
     }
   }
 
-  private boolean rescheduleIndex(
-      String id, Optional<ChangeIndexEvent> indexEvent, int retryCount) {
-    if (retryCount > maxTries) {
-      log.error(
-          "Change {} could not be indexed after {} retries. Change index could be stale.",
-          id,
-          retryCount);
-      return false;
-    }
-
-    log.warn(
-        "Retrying for the #{} time to index Change {} after {} msecs",
-        retryCount,
-        id,
-        retryInterval);
-    @SuppressWarnings("unused")
-    Future<?> possiblyIgnoredError =
-        indexExecutor.schedule(
-            () -> {
-              try (ManualRequestContext ctx = oneOffCtx.open()) {
-                Context.setForwardedEvent(true);
-                doIndex(id, indexEvent, retryCount);
-              } catch (Exception e) {
-                log.warn("Change {} could not be indexed", id, e);
-              }
-            },
-            retryInterval,
-            TimeUnit.MILLISECONDS);
-    return true;
+  @Override
+  protected String indexName() {
+    return "change";
   }
 
   @Override
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexGroupHandler.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexGroupHandler.java
index 4e154ef..c4906a9 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexGroupHandler.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexGroupHandler.java
@@ -16,11 +16,15 @@
 
 import com.google.gerrit.entities.AccountGroup;
 import com.google.gerrit.server.index.group.GroupIndexer;
+import com.google.gerrit.server.util.OneOffRequestContext;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 import com.googlesource.gerrit.plugins.multisite.Configuration;
 import com.googlesource.gerrit.plugins.multisite.forwarder.events.GroupIndexEvent;
+import com.googlesource.gerrit.plugins.multisite.index.ForwardedIndexExecutor;
+import com.googlesource.gerrit.plugins.multisite.index.GroupChecker;
 import java.util.Optional;
+import java.util.concurrent.ScheduledExecutorService;
 
 /**
  * Index a group using {@link GroupIndexer}. This class is meant to be used on the receiving side of
@@ -29,19 +33,42 @@
  * done for the same group uuid
  */
 @Singleton
-public class ForwardedIndexGroupHandler extends ForwardedIndexingHandler<String, GroupIndexEvent> {
+public class ForwardedIndexGroupHandler
+    extends ForwardedIndexingHandlerWithRetries<String, GroupIndexEvent> {
   private final GroupIndexer indexer;
+  private final GroupChecker groupChecker;
 
   @Inject
-  ForwardedIndexGroupHandler(GroupIndexer indexer, Configuration config) {
-    super(config.index().numStripedLocks());
+  ForwardedIndexGroupHandler(
+      GroupIndexer indexer,
+      Configuration config,
+      GroupChecker groupChecker,
+      OneOffRequestContext oneOffRequestContext,
+      @ForwardedIndexExecutor ScheduledExecutorService indexExecutor) {
+    super(indexExecutor, config, oneOffRequestContext);
     this.indexer = indexer;
+    this.groupChecker = groupChecker;
   }
 
   @Override
   protected void doIndex(String uuid, Optional<GroupIndexEvent> event) {
-    indexer.index(AccountGroup.uuid(uuid));
-    log.debug("Group {} successfully indexed", uuid);
+    attemptToIndex(uuid, event, 0);
+  }
+
+  @Override
+  protected void reindex(String id) {
+    indexer.index(AccountGroup.uuid(id));
+  }
+
+  @Override
+  protected String indexName() {
+    return "group";
+  }
+
+  @Override
+  protected void attemptToIndex(
+      String uuid, Optional<GroupIndexEvent> groupIndexEvent, int retryCount) {
+    reindexAndCheckIsUpToDate(uuid, groupIndexEvent, groupChecker, retryCount);
   }
 
   @Override
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexProjectHandler.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexProjectHandler.java
index ce6804e..3787a80 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexProjectHandler.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexProjectHandler.java
@@ -16,6 +16,7 @@
 
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.index.project.ProjectIndexer;
+import com.google.gerrit.server.util.OneOffRequestContext;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 import com.googlesource.gerrit.plugins.multisite.Configuration;
@@ -24,7 +25,6 @@
 import com.googlesource.gerrit.plugins.multisite.index.ProjectChecker;
 import java.util.Optional;
 import java.util.concurrent.ScheduledExecutorService;
-import java.util.concurrent.TimeUnit;
 
 /**
  * Index a project using {@link ProjectIndexer}. This class is meant to be used on the receiving
@@ -34,78 +34,40 @@
  */
 @Singleton
 public class ForwardedIndexProjectHandler
-    extends ForwardedIndexingHandler<String, ProjectIndexEvent> {
+    extends ForwardedIndexingHandlerWithRetries<String, ProjectIndexEvent> {
   private final ProjectIndexer indexer;
-  private final int retryInterval;
-  private final int maxTries;
   private final ProjectChecker projectChecker;
-  private final ScheduledExecutorService indexExecutor;
 
   @Inject
   ForwardedIndexProjectHandler(
       ProjectIndexer indexer,
       ProjectChecker projectChecker,
+      OneOffRequestContext oneOffRequestContext,
       @ForwardedIndexExecutor ScheduledExecutorService indexExecutor,
       Configuration config) {
-    super(config.index().numStripedLocks());
+    super(indexExecutor, config, oneOffRequestContext);
     this.indexer = indexer;
-    Configuration.Index indexConfig = config.index();
-    this.retryInterval = indexConfig != null ? indexConfig.retryInterval() : 0;
-    this.maxTries = indexConfig != null ? indexConfig.maxTries() : 0;
-    this.indexExecutor = indexExecutor;
     this.projectChecker = projectChecker;
   }
 
   @Override
   protected void doIndex(String projectName, Optional<ProjectIndexEvent> event) {
-    if (!attemptIndex(projectName, event)) {
-      log.warn("First Attempt failed, scheduling again after {} msecs", retryInterval);
-      rescheduleIndex(projectName, event, 1);
-    }
+    attemptToIndex(projectName, event, 0);
   }
 
-  public boolean attemptIndex(String projectName, Optional<ProjectIndexEvent> event) {
-    log.debug("Attempt to index project {}, event: [{}]", projectName, event);
-    final Project.NameKey projectNameKey = Project.nameKey(projectName);
-    if (projectChecker.isProjectUpToDate(projectNameKey)) {
-      indexer.index(projectNameKey);
-      log.debug("Project {} successfully indexed", projectName);
-      return true;
-    }
-    return false;
+  @Override
+  protected void reindex(String id) {
+    indexer.index(Project.nameKey(id));
   }
 
-  public void rescheduleIndex(
-      String projectName, Optional<ProjectIndexEvent> event, int retryCount) {
-    if (retryCount > maxTries) {
-      log.error(
-          "Project {} could not be indexed after {} retries. index could be stale.",
-          projectName,
-          retryCount);
+  @Override
+  protected String indexName() {
+    return "project";
+  }
 
-      return;
-    }
-
-    log.warn(
-        "Retrying for the #{} time to index project {} after {} msecs",
-        retryCount,
-        projectName,
-        retryInterval);
-
-    indexExecutor.schedule(
-        () -> {
-          Context.setForwardedEvent(true);
-          if (!attemptIndex(projectName, event)) {
-            log.warn(
-                "Attempt {} to index project {} failed, scheduling again after {} msecs",
-                retryCount,
-                projectName,
-                retryInterval);
-            rescheduleIndex(projectName, event, retryCount + 1);
-          }
-        },
-        retryInterval,
-        TimeUnit.MILLISECONDS);
+  @Override
+  protected void attemptToIndex(String id, Optional<ProjectIndexEvent> indexEvent, int retryCount) {
+    reindexAndCheckIsUpToDate(id, indexEvent, projectChecker, retryCount);
   }
 
   @Override
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexingHandlerWithRetries.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexingHandlerWithRetries.java
new file mode 100644
index 0000000..5c64431
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexingHandlerWithRetries.java
@@ -0,0 +1,109 @@
+// 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.googlesource.gerrit.plugins.multisite.forwarder;
+
+import com.google.gerrit.server.util.ManualRequestContext;
+import com.google.gerrit.server.util.OneOffRequestContext;
+import com.googlesource.gerrit.plugins.multisite.Configuration;
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.IndexEvent;
+import com.googlesource.gerrit.plugins.multisite.index.UpToDateChecker;
+import java.util.Optional;
+import java.util.concurrent.Future;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Base class to handle forwarded indexing. This class is meant to be extended by classes used on
+ * the receiving side of the {@link IndexEvent} since it will prevent indexing to be forwarded again
+ * causing an infinite forwarding loop between the 2 nodes. It will also make sure no concurrent
+ * indexing is done for the same id.
+ */
+public abstract class ForwardedIndexingHandlerWithRetries<T, E extends IndexEvent>
+    extends ForwardedIndexingHandler<T, E> {
+
+  private final int retryInterval;
+  private final int maxTries;
+  private final ScheduledExecutorService indexExecutor;
+  protected final OneOffRequestContext oneOffCtx;
+
+  ForwardedIndexingHandlerWithRetries(
+      ScheduledExecutorService indexExecutor,
+      Configuration configuration,
+      OneOffRequestContext oneOffCtx) {
+    super(configuration.index().numStripedLocks());
+
+    Configuration.Index indexConfig = configuration.index();
+    this.oneOffCtx = oneOffCtx;
+    this.indexExecutor = indexExecutor;
+    this.retryInterval = indexConfig != null ? indexConfig.retryInterval() : 0;
+    this.maxTries = indexConfig != null ? indexConfig.maxTries() : 0;
+  }
+
+  protected abstract void reindex(T id);
+
+  protected abstract String indexName();
+
+  protected abstract void attemptToIndex(T id, Optional<E> indexEvent, int retryCount);
+
+  protected boolean rescheduleIndex(T id, Optional<E> indexEvent, int retryCount) {
+    if (retryCount > maxTries) {
+      log.error(
+          "{} {} could not be indexed after {} retries. {} index could be stale.",
+          indexName(),
+          id,
+          retryCount,
+          indexName());
+      return false;
+    }
+
+    log.warn(
+        "Retrying for the #{} time to index {} {} after {} msecs",
+        retryCount,
+        indexName(),
+        id,
+        retryInterval);
+    @SuppressWarnings("unused")
+    Future<?> possiblyIgnoredError =
+        indexExecutor.schedule(
+            () -> {
+              try (ManualRequestContext ctx = oneOffCtx.open()) {
+                Context.setForwardedEvent(true);
+                attemptToIndex(id, indexEvent, retryCount);
+              } catch (Exception e) {
+                log.warn("{} {} could not be indexed", indexName(), id, e);
+              }
+            },
+            retryInterval,
+            TimeUnit.MILLISECONDS);
+    return true;
+  }
+
+  public final void reindexAndCheckIsUpToDate(
+      T id, Optional<E> indexEvent, UpToDateChecker<E> upToDateChecker, int retryCount) {
+    reindex(id);
+
+    if (!upToDateChecker.isUpToDate(indexEvent)) {
+      log.warn("{} {} is not up-to-date. Rescheduling", indexName(), id);
+      rescheduleIndex(id, indexEvent, retryCount + 1);
+      return;
+    }
+    if (retryCount > 0) {
+      log.warn(
+          "{} {} has been eventually indexed after {} attempt(s)", indexName(), id, retryCount);
+    } else {
+      log.debug("{} {} successfully indexed", indexName(), id);
+    }
+  }
+}
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/events/GroupIndexEvent.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/events/GroupIndexEvent.java
index e452e27..4981b2f 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/events/GroupIndexEvent.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/forwarder/events/GroupIndexEvent.java
@@ -15,27 +15,30 @@
 package com.googlesource.gerrit.plugins.multisite.forwarder.events;
 
 import com.google.common.base.Objects;
+import com.google.gerrit.common.Nullable;
+import org.eclipse.jgit.lib.ObjectId;
 
 public class GroupIndexEvent extends IndexEvent {
   static final String TYPE = "group-index";
 
-  public String groupUUID;
+  public final String groupUUID;
+  public final ObjectId sha1;
 
-  public GroupIndexEvent(String groupUUID) {
+  public GroupIndexEvent(String groupUUID, @Nullable ObjectId sha1) {
     super(TYPE);
     this.groupUUID = groupUUID;
+    this.sha1 = sha1;
   }
 
-  @Override
   public boolean equals(Object o) {
     if (this == o) return true;
     if (o == null || getClass() != o.getClass()) return false;
     GroupIndexEvent that = (GroupIndexEvent) o;
-    return Objects.equal(groupUUID, that.groupUUID);
+    return Objects.equal(groupUUID, that.groupUUID) && Objects.equal(sha1, that.sha1);
   }
 
   @Override
   public int hashCode() {
-    return Objects.hashCode(groupUUID);
+    return Objects.hashCode(groupUUID, sha1);
   }
 }
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ChangeChecker.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ChangeChecker.java
index 3646b3a..9ee59eb 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ChangeChecker.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ChangeChecker.java
@@ -20,7 +20,7 @@
 import java.util.Optional;
 
 /** Encapsulates the logic of verifying the up-to-date status of a change. */
-public interface ChangeChecker {
+public interface ChangeChecker extends UpToDateChecker<ChangeIndexEvent> {
 
   /**
    * Return the Change nodes read from ReviewDb or NoteDb.
@@ -48,7 +48,7 @@
    * @param indexEvent indexing event
    * @return true if the local Change is up-to-date, false otherwise.
    */
-  public boolean isChangeUpToDate(Optional<ChangeIndexEvent> indexEvent);
+  public boolean isUpToDate(Optional<ChangeIndexEvent> indexEvent);
 
   /**
    * Return the last computed up-to-date Change time-stamp.
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ChangeCheckerImpl.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ChangeCheckerImpl.java
index 15b1e09..02972b0 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ChangeCheckerImpl.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ChangeCheckerImpl.java
@@ -85,7 +85,7 @@
   }
 
   @Override
-  public boolean isChangeUpToDate(Optional<ChangeIndexEvent> indexEvent) {
+  public boolean isUpToDate(Optional<ChangeIndexEvent> indexEvent) {
     getComputedChangeTs();
     if (!computedChangeTs.isPresent()) {
       log.warn("Unable to compute last updated ts for change {}", changeId);
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/GroupChecker.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/GroupChecker.java
new file mode 100644
index 0000000..c19d04a
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/GroupChecker.java
@@ -0,0 +1,27 @@
+// 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.googlesource.gerrit.plugins.multisite.index;
+
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.GroupIndexEvent;
+import java.util.Optional;
+import org.eclipse.jgit.lib.ObjectId;
+
+public interface GroupChecker extends UpToDateChecker<GroupIndexEvent> {
+
+  @Override
+  boolean isUpToDate(Optional<GroupIndexEvent> groupIndexEvent);
+
+  ObjectId getGroupHead(String groupUUID);
+}
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImpl.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImpl.java
new file mode 100644
index 0000000..e9e40f3
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImpl.java
@@ -0,0 +1,101 @@
+// 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.googlesource.gerrit.plugins.multisite.index;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.flogger.FluentLogger;
+import com.google.gerrit.entities.AccountGroup;
+import com.google.gerrit.entities.RefNames;
+import com.google.gerrit.server.config.AllUsersName;
+import com.google.gerrit.server.git.GitRepositoryManager;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.GroupIndexEvent;
+import java.io.IOException;
+import java.util.Optional;
+import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.revwalk.RevWalk;
+
+@Singleton
+class GroupCheckerImpl implements GroupChecker {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+  private final GitRepositoryManager repoManager;
+  private final AllUsersName allUsers;
+
+  @Inject
+  GroupCheckerImpl(GitRepositoryManager repoManager, AllUsersName allUsers) {
+    this.repoManager = repoManager;
+    this.allUsers = allUsers;
+  }
+
+  @Override
+  public boolean isUpToDate(Optional<GroupIndexEvent> groupIndexEvent) {
+    if (!groupIndexEvent.isPresent()) {
+      logger.atWarning().log("Group Index empty, considering this group up-to-date");
+      return true;
+    }
+    GroupIndexEvent event = groupIndexEvent.get();
+    AccountGroup.UUID groupUUID = AccountGroup.uuid(event.groupUUID);
+
+    if (event.sha1 == null) {
+      logger.atWarning().log(
+          "Event for group '%s' does not contain sha1, consider group up-to-date for compatibility.",
+          groupUUID);
+      return true;
+    }
+
+    try (Repository repo = repoManager.openRepository(allUsers)) {
+      if (commitExistsInRepo(repo, event.sha1)) {
+        logger.atInfo().log(
+            "Group '%s' up-to-date: sha1 '%s' exists in All-Users", groupUUID, event.sha1);
+        return true;
+      } else {
+        logger.atWarning().log(
+            "Group '%s' NOT up-to-date: sha1 '%s' still missing in All-Users",
+            groupUUID, event.sha1);
+      }
+    } catch (Exception e) {
+      logger.atSevere().withCause(e).log(
+          "Could not check whether Group '%s' is up-to-date", groupUUID);
+    }
+    return false;
+  }
+
+  @Override
+  public ObjectId getGroupHead(String groupUUID) {
+    try (Repository repo = repoManager.openRepository(allUsers)) {
+      return Optional.ofNullable(repo.exactRef(RefNames.refsGroups(AccountGroup.uuid(groupUUID))))
+          .map(Ref::getObjectId)
+          .orElse(ObjectId.zeroId());
+    } catch (Exception e) {
+      logger.atSevere().withCause(e).log("Fatal: could not get head of group %s.", groupUUID);
+      return ObjectId.zeroId();
+    }
+  }
+
+  @VisibleForTesting
+  boolean commitExistsInRepo(Repository repo, ObjectId sha1) throws IOException {
+    try (RevWalk revWalk = new RevWalk(repo)) {
+      revWalk.parseCommit(sha1);
+      return true;
+    } catch (MissingObjectException e) {
+      logger.atWarning().log("Commit %s does not exist in All-Users", sha1);
+    }
+    return false;
+  }
+}
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/IndexEventHandler.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/IndexEventHandler.java
index fe6bdac..5effd0e 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/IndexEventHandler.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/IndexEventHandler.java
@@ -47,17 +47,20 @@
   private final Set<IndexTask> queuedTasks = Collections.newSetFromMap(new ConcurrentHashMap<>());
   private final ChangeCheckerImpl.Factory changeChecker;
   private final ProjectsFilter projectsFilter;
+  private final GroupChecker groupChecker;
 
   @Inject
   IndexEventHandler(
       @IndexExecutor Executor executor,
       DynamicSet<IndexEventForwarder> forwarders,
       ChangeCheckerImpl.Factory changeChecker,
-      ProjectsFilter projectsFilter) {
+      ProjectsFilter projectsFilter,
+      GroupChecker groupChecker) {
     this.forwarders = forwarders;
     this.executor = executor;
     this.changeChecker = changeChecker;
     this.projectsFilter = projectsFilter;
+    this.groupChecker = groupChecker;
   }
 
   @Override
@@ -83,7 +86,8 @@
   @Override
   public void onGroupIndexed(String groupUUID) {
     if (!Context.isForwardedEvent()) {
-      IndexGroupTask task = new IndexGroupTask(new GroupIndexEvent(groupUUID));
+      IndexGroupTask task =
+          new IndexGroupTask(new GroupIndexEvent(groupUUID, groupChecker.getGroupHead(groupUUID)));
       if (queuedTasks.add(task)) {
         executor.execute(task);
       }
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/IndexModule.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/IndexModule.java
index 1f54385..075388f 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/IndexModule.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/IndexModule.java
@@ -39,6 +39,7 @@
     DynamicSet.bind(binder(), ProjectIndexedListener.class).to(IndexEventHandler.class);
 
     bind(ProjectChecker.class).to(ProjectCheckerImpl.class);
+    bind(GroupChecker.class).to(GroupCheckerImpl.class);
 
     install(
         new FactoryModuleBuilder()
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ProjectChecker.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ProjectChecker.java
index ece2443..e040fce 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ProjectChecker.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ProjectChecker.java
@@ -14,9 +14,7 @@
 
 package com.googlesource.gerrit.plugins.multisite.index;
 
-import com.google.gerrit.entities.Project;
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.ProjectIndexEvent;
 
 /** Encapsulates the logic of verifying the up-to-date status of a project. */
-public interface ProjectChecker {
-  boolean isProjectUpToDate(Project.NameKey projectName);
-}
+public interface ProjectChecker extends UpToDateChecker<ProjectIndexEvent> {}
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ProjectCheckerImpl.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ProjectCheckerImpl.java
index d9851f3..8655824 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ProjectCheckerImpl.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/ProjectCheckerImpl.java
@@ -17,6 +17,8 @@
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.server.project.ProjectCache;
 import com.google.inject.Inject;
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.ProjectIndexEvent;
+import java.util.Optional;
 
 public class ProjectCheckerImpl implements ProjectChecker {
   private final ProjectCache projectCache;
@@ -27,7 +29,9 @@
   }
 
   @Override
-  public boolean isProjectUpToDate(Project.NameKey projectName) {
-    return projectCache.get(projectName).isPresent();
+  public boolean isUpToDate(Optional<ProjectIndexEvent> indexEvent) {
+    return indexEvent
+        .flatMap(event -> projectCache.get(Project.nameKey(event.projectName)))
+        .isPresent();
   }
 }
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/index/UpToDateChecker.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/UpToDateChecker.java
new file mode 100644
index 0000000..8a7a231
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/index/UpToDateChecker.java
@@ -0,0 +1,28 @@
+// 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.googlesource.gerrit.plugins.multisite.index;
+
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.IndexEvent;
+import java.util.Optional;
+
+public interface UpToDateChecker<E extends IndexEvent> {
+  /**
+   * Check if the local Change is aligned with the indexEvent received.
+   *
+   * @param indexEvent indexing event
+   * @return true if the local Change is up-to-date, false otherwise.
+   */
+  boolean isUpToDate(Optional<E> indexEvent);
+}
diff --git a/src/test/java/com/googlesource/gerrit/plugins/multisite/event/IndexEventRouterTest.java b/src/test/java/com/googlesource/gerrit/plugins/multisite/event/IndexEventRouterTest.java
index 31ca13d..1dc07bd 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/multisite/event/IndexEventRouterTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/multisite/event/IndexEventRouterTest.java
@@ -35,6 +35,7 @@
 import com.googlesource.gerrit.plugins.multisite.forwarder.router.StreamEventRouter;
 import com.googlesource.gerrit.plugins.replication.RefReplicationDoneEvent;
 import java.util.Optional;
+import org.eclipse.jgit.lib.ObjectId;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -95,7 +96,7 @@
   @Test
   public void routerShouldSendEventsToTheAppropriateHandler_GroupIndex() throws Exception {
     final String groupId = "12";
-    final GroupIndexEvent event = new GroupIndexEvent(groupId);
+    final GroupIndexEvent event = new GroupIndexEvent(groupId, ObjectId.zeroId());
     router.route(event);
 
     verify(indexGroupHandler)
diff --git a/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexChangeHandlerTest.java b/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexChangeHandlerTest.java
index c52dfff..96470b6 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexChangeHandlerTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexChangeHandlerTest.java
@@ -189,6 +189,6 @@
       }
     }
 
-    when(changeCheckerPresentMock.isChangeUpToDate(any())).thenReturn(changeIsUpToDate);
+    when(changeCheckerPresentMock.isUpToDate(any())).thenReturn(changeIsUpToDate);
   }
 }
diff --git a/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexGroupHandlerTest.java b/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexGroupHandlerTest.java
index 2fe6e2e..982ac52 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexGroupHandlerTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexGroupHandlerTest.java
@@ -16,16 +16,23 @@
 
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.gerrit.testing.GerritJUnit.assertThrows;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 import com.google.gerrit.entities.AccountGroup;
 import com.google.gerrit.server.index.group.GroupIndexer;
+import com.google.gerrit.server.util.OneOffRequestContext;
 import com.googlesource.gerrit.plugins.multisite.Configuration;
 import com.googlesource.gerrit.plugins.multisite.forwarder.ForwardedIndexingHandler.Operation;
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.GroupIndexEvent;
+import com.googlesource.gerrit.plugins.multisite.index.TestGroupChecker;
 import java.io.IOException;
 import java.util.Optional;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -40,16 +47,22 @@
 
   @Rule public ExpectedException exception = ExpectedException.none();
   @Mock private GroupIndexer indexerMock;
+  @Mock private OneOffRequestContext ctxMock;
+  @Mock private ScheduledExecutorService indexExecutorMock;
   @Mock private Configuration config;
   @Mock private Configuration.Index index;
   private ForwardedIndexGroupHandler handler;
   private String uuid;
+  private static final int RETRY_INTERVAL = 1000;
+  private static final int MAX_TRIES = 2;
 
   @Before
   public void setUp() throws Exception {
     when(config.index()).thenReturn(index);
     when(index.numStripedLocks()).thenReturn(10);
-    handler = new ForwardedIndexGroupHandler(indexerMock, config);
+    when(index.retryInterval()).thenReturn(RETRY_INTERVAL);
+    when(index.maxTries()).thenReturn(MAX_TRIES);
+    handler = groupHandler(true);
     uuid = "123";
   }
 
@@ -106,4 +119,28 @@
 
     verify(indexerMock).index(AccountGroup.uuid(uuid));
   }
+
+  @Test
+  public void shouldChangeIndexEventWheNotUpToDate() throws IOException {
+    ForwardedIndexGroupHandler groupHandlerWithOutdatedEvent = groupHandler(false);
+    groupHandlerWithOutdatedEvent.index(uuid, Operation.INDEX, groupIndexEvent(uuid));
+    verify(indexerMock).index(AccountGroup.uuid(uuid));
+  }
+
+  @Test
+  public void shouldRescheduleGroupIndexingWhenItIsNotUpToDate() throws IOException {
+    ForwardedIndexGroupHandler groupHandlerWithOutdatedEvent = groupHandler(false);
+    groupHandlerWithOutdatedEvent.index(uuid, Operation.INDEX, groupIndexEvent(uuid));
+    verify(indexExecutorMock)
+        .schedule(any(Runnable.class), eq(new Long(RETRY_INTERVAL)), eq(TimeUnit.MILLISECONDS));
+  }
+
+  private ForwardedIndexGroupHandler groupHandler(boolean checkIsUpToDate) {
+    return new ForwardedIndexGroupHandler(
+        indexerMock, config, new TestGroupChecker(checkIsUpToDate), ctxMock, indexExecutorMock);
+  }
+
+  private Optional<GroupIndexEvent> groupIndexEvent(String uuid) {
+    return Optional.of(new GroupIndexEvent(uuid, null));
+  }
 }
diff --git a/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexProjectHandlerTest.java b/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexProjectHandlerTest.java
index 1102cd2..3ce5e14 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexProjectHandlerTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/multisite/forwarder/ForwardedIndexProjectHandlerTest.java
@@ -23,6 +23,7 @@
 
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.index.project.ProjectIndexer;
+import com.google.gerrit.server.util.OneOffRequestContext;
 import com.googlesource.gerrit.plugins.multisite.Configuration;
 import com.googlesource.gerrit.plugins.multisite.forwarder.ForwardedIndexingHandler.Operation;
 import com.googlesource.gerrit.plugins.multisite.index.ProjectChecker;
@@ -44,6 +45,7 @@
   @Rule public ExpectedException exception = ExpectedException.none();
   @Mock private ProjectIndexer indexerMock;
   @Mock private Configuration configMock;
+  @Mock private OneOffRequestContext ctxMock;
   @Mock private ProjectChecker projectCheckerMock;
   @Mock private Configuration.Index indexMock;
   @Mock private ScheduledExecutorService indexExecutorMock;
@@ -56,10 +58,10 @@
     when(indexMock.numStripedLocks()).thenReturn(10);
     when(indexMock.retryInterval()).thenReturn(0);
     when(indexMock.maxTries()).thenReturn(2);
-    when(projectCheckerMock.isProjectUpToDate(any())).thenReturn(true);
+    when(projectCheckerMock.isUpToDate(any())).thenReturn(true);
     handler =
         new ForwardedIndexProjectHandler(
-            indexerMock, projectCheckerMock, indexExecutorMock, configMock);
+            indexerMock, projectCheckerMock, ctxMock, indexExecutorMock, configMock);
     nameKey = "project/name";
   }
 
@@ -116,13 +118,4 @@
 
     verify(indexerMock).index(Project.nameKey(nameKey));
   }
-
-  @Test
-  public void indexAttemptShouldFailWhenCheckerFails() throws Exception {
-    handler =
-        new ForwardedIndexProjectHandler(
-            indexerMock, (projectName) -> false, indexExecutorMock, configMock);
-
-    assertThat(handler.attemptIndex(nameKey, Optional.empty())).isFalse();
-  }
 }
diff --git a/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImplTest.java b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImplTest.java
new file mode 100644
index 0000000..6403cce
--- /dev/null
+++ b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImplTest.java
@@ -0,0 +1,123 @@
+// 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.googlesource.gerrit.plugins.multisite.index;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.mockito.Mockito.doReturn;
+
+import com.google.gerrit.common.Nullable;
+import com.google.gerrit.entities.AccountGroup;
+import com.google.gerrit.entities.RefNames;
+import com.google.gerrit.server.config.AllUsersName;
+import com.google.gerrit.server.config.AllUsersNameProvider;
+import com.google.gerrit.server.git.GitRepositoryManager;
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.GroupIndexEvent;
+import java.io.IOException;
+import java.util.Optional;
+import java.util.UUID;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectIdRef;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.RefDatabase;
+import org.eclipse.jgit.lib.Repository;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+
+@RunWith(MockitoJUnitRunner.class)
+public class GroupCheckerImplTest {
+  ObjectId AN_OBJECT_ID = ObjectId.fromString("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
+  AllUsersName allUsers = new AllUsersName(AllUsersNameProvider.DEFAULT);
+
+  GroupCheckerImpl objectUnderTest;
+  @Mock private GitRepositoryManager repoManagerMock;
+  @Mock private RefDatabase refDatabaseMock;
+  @Mock private Repository repoMock;
+
+  @Before
+  public void setUp() throws Exception {
+    doReturn(repoMock).when(repoManagerMock).openRepository(allUsers);
+    doReturn(refDatabaseMock).when(repoMock).getRefDatabase();
+    objectUnderTest = new GroupCheckerImpl(repoManagerMock, allUsers);
+  }
+
+  @Test
+  public void isGroupUpToDate_shouldReturnTrueWhenEventIsEmpty() {
+    assertThat(objectUnderTest.isUpToDate(Optional.empty())).isTrue();
+  }
+
+  @Test
+  public void isGroupUpToDate_shouldReturnFalseWhenSha1DoesNotExistInAllUsers() {
+    setCommitExistsInRepo(false);
+    assertThat(
+            objectUnderTest.isUpToDate(groupIndexEvent(UUID.randomUUID().toString(), AN_OBJECT_ID)))
+        .isFalse();
+  }
+
+  @Test
+  public void isGroupUpToDate_shouldReturnFalseWhenSha1ExistsInAllUsers() {
+    setCommitExistsInRepo(true);
+    assertThat(
+            objectUnderTest.isUpToDate(groupIndexEvent(UUID.randomUUID().toString(), AN_OBJECT_ID)))
+        .isTrue();
+  }
+
+  @Test
+  public void isGroupUpToDate_shouldReturnTrueWhenSha1IsNotDefined() {
+    UUID groupUUID = UUID.randomUUID();
+    setCommitExistsInRepo(true);
+
+    assertThat(objectUnderTest.isUpToDate(groupIndexEvent(groupUUID.toString(), null))).isTrue();
+  }
+
+  @Test
+  public void getGroupHead_shouldReturnTheExactReValueWhenDefined() throws IOException {
+    UUID groupUUID = UUID.randomUUID();
+    setupExactRefInGroup(groupUUID, AN_OBJECT_ID);
+
+    assertThat(objectUnderTest.getGroupHead(groupUUID.toString())).isEqualTo(AN_OBJECT_ID);
+  }
+
+  @Test
+  public void getGroupHead_shouldReturnObjectIdZeroWhenExactRefIsNull() throws IOException {
+    UUID groupUUID = UUID.randomUUID();
+    setupExactRefInGroup(groupUUID, null);
+
+    assertThat(objectUnderTest.getGroupHead(groupUUID.toString())).isEqualTo(ObjectId.zeroId());
+  }
+
+  private Optional<GroupIndexEvent> groupIndexEvent(String uuid, @Nullable ObjectId sha1) {
+    return Optional.of(new GroupIndexEvent(uuid, sha1));
+  }
+
+  private void setCommitExistsInRepo(boolean commitExists) {
+    objectUnderTest =
+        new GroupCheckerImpl(repoManagerMock, allUsers) {
+          @Override
+          boolean commitExistsInRepo(Repository repo, ObjectId sha1) {
+            return commitExists;
+          }
+        };
+  }
+
+  private void setupExactRefInGroup(UUID groupUUID, @Nullable ObjectId objectId)
+      throws IOException {
+    String groupRefName = RefNames.refsGroups(AccountGroup.uuid(groupUUID.toString()));
+    ObjectIdRef.Unpeeled aRef = new ObjectIdRef.Unpeeled(Ref.Storage.LOOSE, groupRefName, objectId);
+    doReturn(objectId == null ? null : aRef).when(refDatabaseMock).exactRef(groupRefName);
+  }
+}
diff --git a/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupEventIndexTest.java b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupEventIndexTest.java
new file mode 100644
index 0000000..93cec05
--- /dev/null
+++ b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupEventIndexTest.java
@@ -0,0 +1,45 @@
+// 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.googlesource.gerrit.plugins.multisite.index;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.gerritforge.gerrit.eventbroker.EventGsonProvider;
+import com.google.gson.Gson;
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.GroupIndexEvent;
+import java.util.UUID;
+import org.eclipse.jgit.lib.ObjectId;
+import org.junit.Test;
+
+public class GroupEventIndexTest {
+  private static final Gson gson = new EventGsonProvider().get();
+
+  @Test
+  public void groupEventIndexRoundTripWithSha1() {
+    String aGroupUUID = UUID.randomUUID().toString();
+    ObjectId anObjectId = ObjectId.fromString("deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
+    GroupIndexEvent original = new GroupIndexEvent(aGroupUUID, anObjectId);
+
+    assertThat(gson.fromJson(gson.toJson(original), GroupIndexEvent.class)).isEqualTo(original);
+  }
+
+  @Test
+  public void groupEventIndexRoundTripWithoutSha1() {
+    String aGroupUUID = UUID.randomUUID().toString();
+    GroupIndexEvent original = new GroupIndexEvent(aGroupUUID, null);
+
+    assertThat(gson.fromJson(gson.toJson(original), GroupIndexEvent.class)).isEqualTo(original);
+  }
+}
diff --git a/src/test/java/com/googlesource/gerrit/plugins/multisite/index/IndexEventHandlerTest.java b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/IndexEventHandlerTest.java
index 0a57851..022dec5 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/multisite/index/IndexEventHandlerTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/IndexEventHandlerTest.java
@@ -46,7 +46,11 @@
   public void setUp() {
     eventHandler =
         new IndexEventHandler(
-            MoreExecutors.directExecutor(), asDynamicSet(forwarder), changeChecker, projectsFilter);
+            MoreExecutors.directExecutor(),
+            asDynamicSet(forwarder),
+            changeChecker,
+            projectsFilter,
+            new TestGroupChecker(true));
   }
 
   private DynamicSet<IndexEventForwarder> asDynamicSet(IndexEventForwarder forwarder) {
diff --git a/src/test/java/com/googlesource/gerrit/plugins/multisite/index/TestGroupChecker.java b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/TestGroupChecker.java
new file mode 100644
index 0000000..0af7e9e
--- /dev/null
+++ b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/TestGroupChecker.java
@@ -0,0 +1,42 @@
+// Copyright (C) 2018 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.googlesource.gerrit.plugins.multisite.index;
+
+import com.googlesource.gerrit.plugins.multisite.forwarder.events.GroupIndexEvent;
+import java.util.Optional;
+import org.eclipse.jgit.lib.ObjectId;
+import org.junit.Ignore;
+
+@Ignore
+public class TestGroupChecker implements GroupChecker {
+
+  private final boolean isUpToDate;
+
+  public TestGroupChecker(boolean isUpToDate) {
+    this.isUpToDate = isUpToDate;
+  }
+
+  private static final String someObjectId = "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef";
+
+  @Override
+  public boolean isUpToDate(Optional<GroupIndexEvent> groupIndexEvent) {
+    return isUpToDate;
+  }
+
+  @Override
+  public ObjectId getGroupHead(String groupUUID) {
+    return ObjectId.fromString(someObjectId);
+  }
+}
diff --git a/src/test/scala/com/googlesource/gerrit/plugins/multisite/scenarios/CreateChangeUsingMultiGerrit.scala b/src/test/scala/com/googlesource/gerrit/plugins/multisite/scenarios/CreateChangeUsingMultiGerrit.scala
index eade1cf..fdacd81 100644
--- a/src/test/scala/com/googlesource/gerrit/plugins/multisite/scenarios/CreateChangeUsingMultiGerrit.scala
+++ b/src/test/scala/com/googlesource/gerrit/plugins/multisite/scenarios/CreateChangeUsingMultiGerrit.scala
@@ -25,7 +25,6 @@
 class CreateChangeUsingMultiGerrit extends GerritSimulation {
   private val data: FeederBuilder = jsonFile(resource).convert(keys).queue
   private val projectName = className
-  private val numberKey = "_number"
 
   override def relativeRuntimeWeight = 10
 
@@ -33,7 +32,7 @@
     .feed(data)
     .exec(httpRequest
       .body(ElFileBody(body)).asJson
-      .check(regex("\"" + numberKey + "\":(\\d+),").saveAs(numberKey)))
+      .check(regex("\"_" + numberKey + "\":(\\d+),").saveAs(numberKey)))
     .exec(session => {
       deleteChange.number = Some(session(numberKey).as[Int])
       session
diff --git a/src/test/scala/com/googlesource/gerrit/plugins/multisite/scenarios/DeleteChangeUsingMultiGerrit1.scala b/src/test/scala/com/googlesource/gerrit/plugins/multisite/scenarios/DeleteChangeUsingMultiGerrit1.scala
index 457de0d..b509a19 100644
--- a/src/test/scala/com/googlesource/gerrit/plugins/multisite/scenarios/DeleteChangeUsingMultiGerrit1.scala
+++ b/src/test/scala/com/googlesource/gerrit/plugins/multisite/scenarios/DeleteChangeUsingMultiGerrit1.scala
@@ -40,12 +40,12 @@
     .feed(data)
     .exec(session => {
       if (number.nonEmpty) {
-        session.set("number", number.get)
+        session.set(numberKey, number.get)
       } else {
         session
       }
     })
-    .exec(http(uniqueName).delete("${url}${number}"))
+    .exec(http(uniqueName).delete("${url}${"+ numberKey +"}"))
 
   setUp(
     test.inject(