Interface PNCounter

All Superinterfaces:

public interface PNCounter extends DistributedObject
PN (Positive-Negative) CRDT counter.

The counter supports adding and subtracting values as well as retrieving the current counter value. Each replica of this counter can perform operations locally without coordination with the other replicas, thus increasing availability. The counter guarantees that whenever two nodes have received the same set of updates, possibly in a different order, their state is identical, and any conflicting updates are merged automatically. If no new updates are made to the shared state, all nodes that can communicate will eventually have the same data.

The updates to this counter are applied locally when invoked on a CRDT replica. A replica can be any hazelcast instance which is not a client or a lite member. The number of replicas in the cluster is determined by the PNCounterConfig.getReplicaCount() configuration value.

When invoking updates from non-replica instance, the invocation is remote. This may lead to indeterminate state - the update may be applied but the response has not been received. In this case, the caller will be notified with a TargetDisconnectedException when invoking from a client or a MemberLeftException when invoked from a member.

The read and write methods provide monotonic read and RYW (read-your-write) guarantees. These guarantees are session guarantees which means that if no replica with the previously observed state is reachable, the session guarantees are lost and the method invocation will throw a ConsistencyLostException. This does not mean that an update is lost. All the updates are part of some replica and will be eventually reflected in the state of all other replicas. This exception just means that you cannot observe your own writes because all replicas that contain your updates are currently unreachable. After you have received a ConsistencyLostException, you can either wait for a sufficiently up-to-date replica to become reachable in which case the session can be continued, or you can reset the session by calling the reset() method. If you have called the reset() method, a new session is started with the next invocation to a CRDT replica.

If the PN Counter is used through a client, the invocations might throw TargetNotMemberException, indicating that the invocation targets are not members of the cluster anymore. That might happen when the client is connected to a new cluster, but the member list is not updated yet, or all the replica members of the PN Counter is dead but the corresponding member list update is not received yet. Upon receiving the exception, one has to wait until the member list is updated on the client side and try again.

NOTE: The CRDT state is kept entirely on non-lite (data) members. If there aren't any and the methods here are invoked on a lite member, they will fail with an NoDataMemberInClusterException.