Go to hazelcast.org/documentation

Cluster-Member Safety Check

To prevent data loss when shutting down a node, Hazelcast provides a graceful shutdown feature. You perform this by calling the method HazelcastInstance.shutdown(). Once this method is called, it checks the following conditions to ensure the node 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. Eventually, when this method returns, it means the node has been brought to a safe state and it can be shut down without any data loss.

What if you want to be sure that your cluster is in a safe state? What does it mean that cluster is safe to shutdown without any data loss?

There may be some use cases like rolling upgrades, development/testing or any logic that require a cluster/member to be safe. To provide this, 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 node is in a safe state. This check controls if the first backups of partitions of the given node are synced with the primary ones. Once it returns true, the given node 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 from Hazelcast 3.3.

Sample Codes

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

OR 

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



RELATED INFORMATION

For more code samples please refer to PartitionService Code Samples.