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Safety Checking Cluster Members

To prevent data loss when shutting down a cluster member, Hazelcast provides a graceful shutdown feature. You perform this shutdown by calling the method HazelcastInstance.shutdown(). Once this method is called, it checks the following conditions to ensure the member is safe to shutdown.

  • There is no active migration.
  • At least one backup of partitions are synced with primary ones.

Even if the above conditions are not met, HazelcastInstance.shutdown() will force them to be completed. When this method eventually returns, the member has been brought to a safe state and it can be shut down without any data loss.

Ensuring Safe State with PartitionService

What if you want to be sure that your cluster is in a safe state, as in safe to shutdown without any data loss? For example, you may have some use cases like rolling upgrades, development/testing, or other logic that requires a cluster/member to be safe.

To provide this safety, Hazelcast offers the PartitionService interface with the methods isClusterSafe, isMemberSafe, isLocalMemberSafe and forceLocalMemberToBeSafe. These methods can be deemed as decoupled pieces from the method Hazelcast.shutdown. 

public interface PartitionService {
   ...
   ...
    boolean isClusterSafe();
    boolean isMemberSafe(Member member);
    boolean isLocalMemberSafe();
    boolean forceLocalMemberToBeSafe(long timeout, TimeUnit unit);
}

The method isClusterSafe checks whether the cluster is in a safe state. It returns true if there are no active partition migrations and there are sufficient backups for each partition. Once it returns true, the cluster is safe and a node can be shut down without data loss.

The method isMemberSafe checks whether a specific member is in a safe state. This check controls if the first backups of partitions of the given member are synced with the primary ones. Once it returns true, the given member is safe and it can be shut down without data loss.

Similarly, the method isLocalMemberSafe does the same check for the local member. The method forceLocalMemberToBeSafe forces the owned and backup partitions to be synchronized, making the local member safe.

image NOTE: These methods are available starting with Hazelcast 3.3.

Example PartitionService Code

PartitionService partitionService = hazelcastInstance.getPartitionService();
if (partitionService.isClusterSafe()) {
  hazelcastInstance.shutdown(); // or terminate
}

OR 

PartitionService partitionService = hazelcastInstance.getPartitionService();
if (partitionService.isLocalMemberSafe()) {
  hazelcastInstance.shutdown(); // or terminate
}



RELATED INFORMATION

For more code samples please refer to PartitionService Code Samples.