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CP Subsystem Listeners

The CP Subsystem provides the following listeners:

CP membership listeners
CP group availability listeners

The following sections explain each listener and how to register them.

CP Membership Listener

CPMembershipListener is notified when a CP member is added to or removed from the CP Subsystem. Its signature is very similar to Hazelcast’s usual MembershipListener.

The listener interface has methods that are invoked for the following events:

memberAdded: A new CP member is added to the CP subsystem.
memberRemoved: An existing CP member is removed from the CP subsystem.

To get notified for CP membership events, you implement the CPMembershipListener interface.

The following is an example CPMembershipListener class:

public class CPMembershipListenerImpl implements CPMembershipListener {

    /**
     * Called when a new CP member is added to the CP Subsystem.
     */
    public void memberAdded(CPMembershipEvent event) {
        System.out.println("Added: " + event);
    }

    /**
     * Called when a CP member is removed from the CP Subsystem.
     */
    public void memberRemoved(CPMembershipEvent event) {
        System.out.println("Removed: " + event);
    }
}

Registering CP Membership Listeners

CPMembershipListener can be defined in the configuration or can be registered in runtime via the CPSubsystem API.

Below is an example registering the listener in runtime using the CPSubsystem.addMembershipListener method:

// Either server or client
HazelcastInstance hazelcastInstance = ...;
hazelcastInstance.getCPSubsystem().addMembershipListener(new CPMembershipListenerImpl());

The following is an example programmatic configuration:

Server
Client

Config config = new Config();
config.addListenerConfig(new ListenerConfig("com.yourpackage.CPMembershipListenerImpl"));

ClientConfig config = new ClientConfig();
config.addListenerConfig(new ListenerConfig("com.yourpackage.CPMembershipListenerImpl"));

The followings are examples of the equivalent declarative configurations:

Server XML
Server YAML
Client XML
Client YAML

<hazelcast>
    ...
    <listeners>
        <listener>
            com.yourpackage.CPMembershipListenerImpl
        </listener>
    </listeners>
    ...
</hazelcast>

hazelcast:
  ...
  listeners:
    - com.yourpackage.CPMembershipListenerImpl

<hazelcast-client>
    ...
    <listeners>
        <listener>
            com.yourpackage.CPMembershipListenerImpl
        </listener>
    </listeners>
    ...
</hazelcast-client>

hazelcast-client:
  ...
  listeners:
    - com.yourpackage.CPMembershipListenerImpl

CP Group Availability Listener

CPGroupAvailabilityListener is notified when the availability of a CP group decreases or it loses the majority completely.

In general, the availability decreases when a CP member becomes unreachable because of process crash, network partition, out of memory, etc. Once a member is declared as unavailable by the Hazelcast’s failure detector, that member is removed from the cluster. If it is also a CP member, CPGroupAvailabilityEvents are fired for each CP group that member belongs to.

As a special case, CPGroupAvailabilityListener has a separate method to report the loss of majority. When the majority of a CP group is lost, that CP group cannot make progress anymore. Even a new CP member cannot join to this CP group since membership changes also go through the Raft consensus algorithm.

When a CP group has lost its majority:

If the group is a non-METADATA CP group, it must be force-destroyed immediately, because it can block the METADATA CP group to perform membership changes on the CP Subsystem.
If the majority of the METADATA CP group permanently crashes, unfortunately it is equivalent to the permanent crash of the majority CP members of the whole CP Subsystem, even though other CP groups are running fine.

The listener interface has methods that are invoked for the following events:

availabilityDecreased: A CP group’s availability decreases, but still has the majority of members available.
majorityLost: A CP group has lost its majority.

The following is an example CPGroupAvailabilityListener class:

public class CPGroupAvailabilityListenerImpl implements CPGroupAvailabilityListener {

    /**
     * Called when a CP group's availability decreases,
     * but still has the majority of members available.
     */
    public void availabilityDecreased(CPGroupAvailabilityEvent event) {
        System.out.println("Availability decreased: " + event);
    }

    /**
     * Called when a CP group has lost its majority.
     */
    public void majorityLost(CPGroupAvailabilityEvent event) {
        System.out.println("Majority Lost: " + event);
    }
}

Registering CP Group Availability Listeners

Similar to CPMembershipListener, a CPGroupAvailabilityListener can be defined in the configuration or can be registered in runtime via the CPSubsystem API.

Below is an example registering the listener in runtime using the CPSubsystem.addGroupAvailabilityListener method:

// Either server or client
HazelcastInstance hazelcastInstance = ...;
hazelcastInstance.getCPSubsystem().addGroupAvailabilityListener(new CPGroupAvailabilityListenerImpl());