ClusterSingletonManager

Manages singleton actor instance among all cluster nodes or a group of nodes tagged with a specific role. At most one singleton instance is running at any point in time.

The ClusterSingletonManager is supposed to be started on all nodes, or all nodes with specified role, in the cluster with actorOf. The actual singleton is started on the oldest node by creating a child actor from the supplied singletonProps.

The singleton actor is always running on the oldest member, which can be determined by akka.cluster.Member#isOlderThan. This can change when removing members. A graceful hand over can normally be performed when current oldest node is leaving the cluster. Be aware that there is a short time period when there is no active singleton during the hand-over process.

The cluster failure detector will notice when oldest node becomes unreachable due to things like JVM crash, hard shut down, or network failure. When the crashed node has been removed (via down) from the cluster then a new oldest node will take over and a new singleton actor is created. For these failure scenarios there will not be a graceful hand-over, but more than one active singletons is prevented by all reasonable means. Some corner cases are eventually resolved by configurable timeouts.

You access the singleton actor with actorSelection using the names you have specified when creating the ClusterSingletonManager. You can subscribe to akka.cluster.ClusterEvent.MemberEvent and sort the members by age (akka.cluster.ClusterEvent.Member#isOlderThan) to keep track of oldest member. Alternatively the singleton actor may broadcast its existence when it is started.

Use factory method to create the akka.actor.Props for the actor.

Arguments

singletonProps akka.actor.Props of the singleton actor instance.

singletonName The actor name of the child singleton actor.

terminationMessage When handing over to a new oldest node this terminationMessage is sent to the singleton actor to tell it to finish its work, close resources, and stop. The hand-over to the new oldest node is completed when the singleton actor is terminated. Note that akka.actor.PoisonPill is a perfectly fine terminationMessage if you only need to stop the actor.

role Singleton among the nodes tagged with specified role. If the role is not specified it's a singleton among all nodes in the cluster.

maxHandOverRetries When a node is becoming oldest it sends hand-over request to previous oldest. This is retried with the retryInterval until the previous oldest confirms that the hand over has started, or this maxHandOverRetries limit has been reached. If the retry limit is reached it takes the decision to be the new oldest if previous oldest is unknown (typically removed), otherwise it initiates a new round by throwing akka.contrib.pattern.ClusterSingletonManagerIsStuck and expecting restart with fresh state. For a cluster with many members you might need to increase this retry limit because it takes longer time to propagate changes across all nodes.

maxTakeOverRetries When a oldest node is not oldest any more it sends take over request to the new oldest to initiate the normal hand-over process. This is especially useful when new node joins and becomes oldest immediately, without knowing who was previous oldest. This is retried with the retryInterval until this retry limit has been reached. If the retry limit is reached it initiates a new round by throwing akka.contrib.pattern.ClusterSingletonManagerIsStuck and expecting restart with fresh state. This will also cause the singleton actor to be stopped. maxTakeOverRetries must be less than maxHandOverRetries to ensure that new oldest doesn't start singleton actor before previous is stopped for certain corner cases.