Distributed Data Structures

Common Features of all Hazelcast Data Structures:

• Data in the cluster is almost evenly distributed (partitioned) across all nodes. So each node carries ~ (1/n * total-data) + backups , n being the number of nodes in the cluster.

• If a member goes down, its backup replica that also holds the same data, will dynamically redistribute the data including the ownership and locks on them to remaining live nodes. As a result, no data will get lost.

• When a new node joins the cluster, new node takes ownership(responsibility) and load of -some- of the entire data in the cluster. Eventually the new node will carry almost (1/n * total-data) + backups and becomes the new partition reducing the load on others.

• There is no single cluster master or something that can cause single point of failure. Every node in the cluster has equal rights and responsibilities. No-one is superior. And no dependency on external 'server' or 'master' kind of concept.

Here is how you can retrieve existing data structure instances (map, queue, set, lock, topic, etc.) and how you can listen for instance events to get notified when an instance is 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 cfg = new Config();
        HazelcastInstance hz = Hazelcast.newHazelcastInstance(cfg);
        hz.addDistributedObjectListener(sample);

        Collection<DistributedObject> distributedObjects = hz.getDistributedObjects();

        for (DistributedObject distributedObject : distributedObjects) {
            System.out.println(distributedObject.getName() + "," + distributedObject.getId());
        }
    }

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

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

    }
}