Hazelcast IdGenerator is used to generate cluster-wide unique identifiers. Generated identifiers are long type primitive values between 0 and Long.MAX_VALUE.

ID generation occurs almost at the speed of AtomicLong.incrementAndGet(). A group of 1 million identifiers is allocated for each cluster member. In the background, this allocation takes place with an IAtomicLong incremented by 1 million. Once cluster member claims to generate IDs (allocation is done), IdGenerator is able to increment a local counter. If a cluster member uses all IDs in the group, it will have another 1 million IDs. By this way, only one time of network traffic is needed, meaning 999.999 identifiers are generated in memory. And this is fast.

Let's write a sample identifier generator.

public class IdGeneratorExample {
  public static void main( String[] args ) throws Exception {
    HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
    IdGenerator idGen = hazelcastInstance.getIdGenerator( "newId" );
    while (true) {
      Long id = idGen.newId();
      System.err.println( "Id: " + id );
      Thread.sleep( 1000 );

And let's run the above code two times. Output will be similar to the below.

Members [1] {
  Member []:5701 this
Id: 1
Id: 2
Id: 3
Members [2] {
  Member []:5701
  Member []:5702 this
Id: 1000001
Id: 1000002
Id: 1000003

You can see that the generated IDs are unique and counting upwards. If you see duplicated identifiers, it means your instances could not form a cluster.

ATTENTION: Generated IDs are unique during the life cycle of the cluster. If the entire cluster is restarted, IDs start from 0 again or you can initialize to a value using the init() method of IdGenerator.

ATTENTION: IdGenerator has 1 synchronous backup and no asynchronous backups. Its backup count is not configurable.