Go to hazelcast.org/documentation

Partition Migration

In the previous section, we created a real distributed counter. Now, we need to make sure that the content of the partition containers is migrated to different cluster members when a member joins or leaves the cluster. To make this happen, first we need to add three new methods (applyMigrationData, toMigrationData and clear) to the Container, as shown below.

import java.util.HashMap;
import java.util.Map;

class Container {
    private final Map<String, Integer> values = new HashMap();

    int inc(String id, int amount) {
        Integer counter = values.get(id);
        if (counter == null) {
            counter = 0;
        }
        counter += amount;
        values.put(id, counter);
        return counter;
    }

    void clear() {
        values.clear();
    }

    void applyMigrationData(Map<String, Integer> migrationData) {
        values.putAll(migrationData);
    }

    Map<String, Integer> toMigrationData() {
        return new HashMap(values);
    }

    public void init(String objectName) {
        values.put(objectName,0);
    }

    public void destroy(String objectName) {
        values.remove(objectName);
    }
}
  • toMigrationData: This method is called when Hazelcast wants to start the partition migration from the member owning the partition. The result of the toMigrationData method is the partition data in a form that can be serialized to another member.
  • applyMigrationData: This method is called when migrationData (created by the method toMigrationData) will be applied to the member that will be the new partition owner.
  • clear: This method is called when the partition migration is successfully completed and the old partition owner gets rid of all data in the partition. This method is also called when the partition migration operation fails and the to-be-the-new partition owner needs to roll back its changes.

Transferring migrationData

After you add these three methods to the Container, you need to create a CounterMigrationOperation class that transfers migrationData from one member to another and calls the method applyMigrationData on the correct partition of the new partition owner. A sample is shown below.

import com.hazelcast.nio.ObjectDataInput;
import com.hazelcast.nio.ObjectDataOutput;
import com.hazelcast.spi.AbstractOperation;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class CounterMigrationOperation extends AbstractOperation {

    Map<String, Integer> migrationData;

    public CounterMigrationOperation() {
    }

    public CounterMigrationOperation(Map<String, Integer> migrationData) {
        this.migrationData = migrationData;
    }

    @Override
    public void run() throws Exception {
        CounterService service = getService();
        Container container = service.containers[getPartitionId()];
        container.applyMigrationData(migrationData);
    }

    @Override
    protected void writeInternal(ObjectDataOutput out) throws IOException {
        out.writeInt(migrationData.size());
        for (Map.Entry<String, Integer> entry : migrationData.entrySet()) {
            out.writeUTF(entry.getKey());
            out.writeInt(entry.getValue());
        }
    }

    @Override
    protected void readInternal(ObjectDataInput in) throws IOException {
        int size = in.readInt();
        migrationData = new HashMap<String, Integer>();
        for (int i = 0; i < size; i++)
            migrationData.put(in.readUTF(), in.readInt());
    }
}



image NOTE: During a partition migration, no other operations are executed on the related partition.

Letting Hazelcast Know CounterService Can Do Partition Migrations

We need to make our CounterService class implement the MigrationAwareService interface. This will let Hazelcast know that the CounterService can perform partition migration. See the below code.

import com.hazelcast.core.DistributedObject;
import com.hazelcast.partition.MigrationEndpoint;
import com.hazelcast.spi.*;

import java.util.Map;
import java.util.Properties;

public class CounterService implements ManagedService, RemoteService, MigrationAwareService {
    public final static String NAME = "CounterService";
    Container[] containers;
    private NodeEngine nodeEngine;

    @Override
    public void init(NodeEngine nodeEngine, Properties properties) {
        this.nodeEngine = nodeEngine;
        containers = new Container[nodeEngine.getPartitionService().getPartitionCount()];
        for (int k = 0; k < containers.length; k++)
            containers[k] = new Container();
    }

    @Override
    public void shutdown(boolean terminate) {
    }

    @Override
    public DistributedObject createDistributedObject(String objectName) {
        int partitionId = nodeEngine.getPartitionService().getPartitionId(objectName);
        Container container = containers[partitionId];
        container.init(objectName);
        return new CounterProxy(objectName, nodeEngine,this);
    }

    @Override
    public void destroyDistributedObject(String objectName) {
        int partitionId = nodeEngine.getPartitionService().getPartitionId(objectName);
        Container container = containers[partitionId];
        container.destroy(objectName);
    }

    @Override
    public void beforeMigration(PartitionMigrationEvent e) {
        //no-op
    }

    @Override
    public void clearPartitionReplica(int partitionId) {
        Container container = containers[partitionId];
        container.clear();
    }

    @Override
    public Operation prepareReplicationOperation(PartitionReplicationEvent e) {
        if (e.getReplicaIndex() > 1) {
            return null;
        }
        Container container = containers[e.getPartitionId()];
        Map<String, Integer> data = container.toMigrationData();
        return data.isEmpty() ? null : new CounterMigrationOperation(data);
    }

    @Override
    public void commitMigration(PartitionMigrationEvent e) {
        if (e.getMigrationEndpoint() == MigrationEndpoint.SOURCE) {
            Container c = containers[e.getPartitionId()];
            c.clear();
        }

        //todo
    }

    @Override
    public void rollbackMigration(PartitionMigrationEvent e) {
        if (e.getMigrationEndpoint() == MigrationEndpoint.DESTINATION) {
            Container c = containers[e.getPartitionId()];
            c.clear();
        }
    }

    @Override
    public void reset() {
    }
}

With the MigrationAwareService interface, some additional methods are exposed. For example, the method prepareMigrationOperation returns all the data of the partition that is going to be moved.

The method commitMigration commits the data, meaning in this case, it clears the partition container of the old owner.

Running the Sample Code

We can run the following code.

import com.hazelcast.core.Hazelcast;
import com.hazelcast.core.HazelcastInstance;

public class Member {
    public static void main(String[] args) throws Exception {
        HazelcastInstance[] instances = new HazelcastInstance[3];
        for (int k = 0; k < instances.length; k++)
            instances[k] = Hazelcast.newHazelcastInstance();

        Counter[] counters = new Counter[4];
        for (int k = 0; k < counters.length; k++)
            counters[k] = instances[0].getDistributedObject(CounterService.NAME, k + "counter");

        for (Counter counter : counters)
            System.out.println(counter.inc(1));

        Thread.sleep(10000);

        System.out.println("Creating new members");

        for (int k = 0; k < 3; k++) {
            Hazelcast.newHazelcastInstance();
        }

        Thread.sleep(10000);

        for (Counter counter : counters)
            System.out.println(counter.inc(1));

        System.out.println("Finished");
        System.exit(0);
    }
}

And we get the following output.

Executing 0counter.inc() on: Address[192.168.1.103]:5702
Executing backup 0counter.inc() on: Address[192.168.1.103]:5703
1
Executing 1counter.inc() on: Address[192.168.1.103]:5703
Executing backup 1counter.inc() on: Address[192.168.1.103]:5701
1
Executing 2counter.inc() on: Address[192.168.1.103]:5701
Executing backup 2counter.inc() on: Address[192.168.1.103]:5703
1
Executing 3counter.inc() on: Address[192.168.1.103]:5701
Executing backup 3counter.inc() on: Address[192.168.1.103]:5703
1
Creating new members
Executing 0counter.inc() on: Address[192.168.1.103]:5705
Executing backup 0counter.inc() on: Address[192.168.1.103]:5703
2
Executing 1counter.inc() on: Address[192.168.1.103]:5703
Executing backup 1counter.inc() on: Address[192.168.1.103]:5704
2
Executing 2counter.inc() on: Address[192.168.1.103]:5705
Executing backup 2counter.inc() on: Address[192.168.1.103]:5704
2
Executing 3counter.inc() on: Address[192.168.1.103]:5704
Executing backup 3counter.inc() on: Address[192.168.1.103]:5705
2
Finished

You can see that the counters have moved. 0counter moved from 192.168.1.103:5702 to 192.168.1.103:5705 and it is incremented correctly. Our counters can now move around in the cluster. You will see the the counters will be redistributed once you add or remove a cluster member.