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How Hazelcast Split Brain Merge Happens

Here is, step by step, how Hazelcast split brain merge happens:

  1. The oldest member of the cluster checks if there is another cluster with the same group-name and group-password in the network.
  2. If the oldest member finds such a cluster, then it figures out which cluster should merge to the other.

  3. Each member of the merging cluster will do the following.

  4. Pause.

  5. Take locally owned map entries.
  6. Close all of its network connections (detach from its cluster).
  7. Join to the new cluster.
  8. Send merge request for each of its locally owned map entry.
  9. Resume.

Each member of the merging cluster rejoins the new cluster and sends a merge request for each of its locally owned map entries. Two important points:

  • The smaller cluster will merge into the bigger one. If they have equal number of members then a hashing algorithm determines the merging cluster.
  • Each cluster may have different versions of the same key in the same map. The destination cluster will decide how to handle merging entry based on the MergePolicy set for that map. There are built-in merge policies such as PassThroughMergePolicy, PutIfAbsentMapMergePolicy, HigherHitsMapMergePolicy and LatestUpdateMapMergePolicy. You can develop your own merge policy by implementing com.hazelcast.map.merge.MapMergePolicy. You should set the full class name of your implementation to the merge-policy configuration.
public interface MergePolicy {
  /**
  * Returns the value of the entry after the merge
  * of entries with the same key. Returning value can be
  * You should consider the case where existingEntry is null.
  *
  * @param mapName       name of the map
  * @param mergingEntry  entry merging into the destination cluster
  * @param existingEntry existing entry in the destination cluster
  * @return final value of the entry. If returns null then entry will be removed.
  */
  Object merge( String mapName, EntryView mergingEntry, EntryView existingEntry );
}