Distributed Data Structures

As mentioned in the Overview section, Hazelcast offers distributed implementations of Java interfaces. Below is the Java interface list with links to each section in this manual.

  • Standard utility collections:

    • Map: The distributed implementation of java.util.Map lets you read from and write to a Hazelcast map with methods like get and put.
    • Queue: The distributed queue is an implementation of java.util.concurrent.BlockingQueue. You can add an item in one machine and remove it from another one.
    • Set: The distributed and concurrent implementation of java.util.Set. It does not allow duplicate elements and does not preserve their order.
    • List: Very similar to Hazelcast List, except that it allows duplicate elements and preserves their order.
    • MultiMap: This is a specialized Hazelcast map. It is distributed, where multiple values under a single key can be stored.
    • ReplicatedMap: This does not partition data, i.e. it does not spread data to different cluster members. Instead, it replicates the data to all nodes.
  • Topic: Distributed mechanism for publishing messages that are delivered to multiple subscribers; this is also known as a publish/subscribe (pub/sub) messaging model. Please see the Topic section for more information.
  • Concurrency utilities:
    • Lock: Distributed implementation of java.util.concurrent.locks.Lock. When you lock using Hazelcast Lock, the critical section that it guards is guaranteed to be executed by only one thread in the entire cluster.
    • Semaphore: Distributed implementation of java.util.concurrent.Semaphore. When performing concurrent activities, semaphores offer permits to control the thread counts.
    • AtomicLong: Distributed implementation of java.util.concurrent.atomic.AtomicLong. Most of AtomicLong's operations are available. However, these operations involve remote calls and hence their performances differ from AtomicLong, due to being distributed.
    • AtomicReference: When you need to deal with a reference in a distributed environment, you can use Hazelcast AtomicReference. This is the distributed version of java.util.concurrent.atomic.AtomicReference.
    • IdGenerator: You use Hazelcast IdGenerator to generate cluster-wide unique identifiers. ID generation occurs almost at the speed of AtomicLong.incrementAndGet().
    • CountdownLatch: Distributed implementation of java.util.concurrent.CountDownLatch. Hazelcast CountDownLatch is a gate keeper for concurrent activities, enabling the threads to wait for other threads to complete their operations.

Common Features of all Hazelcast Data Structures:

  • If a member goes down, its backup replica (which holds the same data) will dynamically redistribute the data, including the ownership and locks on them, to the remaining live nodes. As a result, no data will be lost.
  • There is no single cluster master that can cause single point of failure. Every node in the cluster has equal rights and responsibilities. No single node is superior. There is no dependency on an external 'server' or 'master'.

Here is an example of how you can retrieve existing data structure instances (map, queue, set, lock, topic, etc.) and how you can listen for instance events, such as an instance being created or destroyed.

import java.util.Collection;
import com.hazelcast.config.Config;
import com.hazelcast.core.*;

public class Sample implements DistributedObjectListener {
  public static void main(String[] args) {
    Sample sample = new Sample();

    Config config = new Config();
    HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance(config);

    Collection<DistributedObject> distributedObjects = hazelcastInstance.getDistributedObjects();
    for (DistributedObject distributedObject : distributedObjects) {
      System.out.println(distributedObject.getName() + "," + distributedObject.getId());

  public void distributedObjectCreated(DistributedObjectEvent event) {
    DistributedObject instance = event.getDistributedObject();
    System.out.println("Created " + instance.getName() + "," + instance.getId());

  public void distributedObjectDestroyed(DistributedObjectEvent event) {
    DistributedObject instance = event.getDistributedObject();
    System.out.println("Destroyed " + instance.getName() + "," + instance.getId());