Classic Cluster Singleton
Akka Classic pertains to the original Actor APIs, which have been improved by more type safe and guided Actor APIs. Akka Classic is still fully supported and existing applications can continue to use the classic APIs. It is also possible to use the new Actor APIs together with classic actors in the same ActorSystem, see coexistence. For new projects we recommend using the new Actor API.
For the full documentation of this feature and for new projects see Cluster Singleton.
Module info
The Akka dependencies are available from Akka’s library repository. To access them there, you need to configure the URL for this repository.
To use Cluster Singleton, you must add the following dependency in your project:
- sbt
val AkkaVersion = "2.10.1+3-e17d1cd0-SNAPSHOT" libraryDependencies += "com.typesafe.akka" %% "akka-cluster-tools" % AkkaVersion
- Maven
- Gradle
Project Info: Akka Cluster Tools (classic) | |
---|---|
Artifact | com.typesafe.akka
akka-cluster-tools
2.10.1+3-e17d1cd0-SNAPSHOT
|
JDK versions | Eclipse Temurin JDK 11 Eclipse Temurin JDK 17 Eclipse Temurin JDK 21 |
Scala versions | 2.13.15, 3.3.4 |
JPMS module name | akka.cluster.tools |
License | |
Readiness level |
Since 2.3.0, 2014-03-05
|
Home page | https://akka.io/ |
API documentation | |
Forums | |
Release notes | Akka release notes |
Issues | Github issues |
Sources | https://github.com/akka/akka |
Introduction
For the full documentation of this feature and for new projects see Cluster Singleton - Introduction.
The cluster singleton pattern is implemented by akka.cluster.singleton.ClusterSingletonManager
. It manages one singleton actor instance among all cluster nodes or a group of nodes tagged with a specific role. ClusterSingletonManager
is an actor that is supposed to be started as early as possible on all nodes, or all nodes with specified role, in the cluster. The actual singleton actor is started by the ClusterSingletonManager
on the oldest node by creating a child actor from supplied Props
. ClusterSingletonManager
makes sure that at most one singleton instance is running at any point in time.
You can access the singleton actor by using the provided akka.cluster.singleton.ClusterSingletonProxy
, which will route all messages to the current instance of the singleton. The proxy will keep track of the oldest node in the cluster and resolve the singleton’s ActorRef
by explicitly sending the singleton’s actorSelection
the akka.actor.Identify
message and waiting for it to reply. This is performed periodically if the singleton doesn’t reply within a certain (configurable) time. Given the implementation, there might be periods of time during which the ActorRef
is unavailable, e.g., when a node leaves the cluster. In these cases, the proxy will buffer the messages sent to the singleton and then deliver them when the singleton is finally available. If the buffer is full the ClusterSingletonProxy
will drop old messages when new messages are sent via the proxy. The size of the buffer is configurable and it can be disabled by using a buffer size of 0.
See Cluster Singleton - Potential problems to be aware of.
An Example
Assume that we need one single entry point to an external system. An actor that receives messages from a JMS queue with the strict requirement that only one JMS consumer must exist to make sure that the messages are processed in order. That is perhaps not how one would like to design things, but a typical real-world scenario when integrating with external systems.
Before explaining how to create a cluster singleton actor, let’s define message classes
which will be used by the singleton.- Scala
-
source
object PointToPointChannel { case object UnregistrationOk extends CborSerializable } object Consumer { case object End extends CborSerializable case object GetCurrent extends CborSerializable case object Ping extends CborSerializable case object Pong extends CborSerializable }
- Java
On each node in the cluster you need to start the ClusterSingletonManager
and supply the Props
of the singleton actor, in this case the JMS queue consumer.
- Scala
-
source
system.actorOf( ClusterSingletonManager.props( singletonProps = Props(classOf[Consumer], queue, testActor), terminationMessage = End, settings = ClusterSingletonManagerSettings(system).withRole("worker")), name = "consumer")
- Java
Here we limit the singleton to nodes tagged with the "worker"
role, but all nodes, independent of role, can be used by not specifying withRole
.
We use an application specific terminationMessage
to be able to close the resources before actually stopping the singleton actor. Note that PoisonPill
is a perfectly fine terminationMessage
if you only need to stop the actor.
Here is how the singleton actor handles the terminationMessage
in this example.
- Scala
-
source
case End => queue ! UnregisterConsumer case UnregistrationOk => stoppedBeforeUnregistration = false context.stop(self) case Ping => sender() ! Pong
- Java
With the names given above, access to the singleton can be obtained from any cluster node using a properly configured proxy.
- Scala
-
source
val proxy = system.actorOf( ClusterSingletonProxy.props( singletonManagerPath = "/user/consumer", settings = ClusterSingletonProxySettings(system).withRole("worker")), name = "consumerProxy")
- Java
A more comprehensive sample is available in the tutorial named Distributed workers with Akka and Scala! .
Configuration
For the full documentation of this feature and for new projects see Cluster Singleton - configuration.
Supervision
There are two actors that could potentially be supervised. For the consumer
singleton created above these would be:
- Cluster singleton manager e.g.
/user/consumer
which runs on every node in the cluster - The user actor e.g.
/user/consumer/singleton
which the manager starts on the oldest node
The Cluster singleton manager actor should not have its supervision strategy changed as it should always be running. However it is sometimes useful to add supervision for the user actor. To accomplish this add a parent supervisor actor which will be used to create the ‘real’ singleton instance. Below is an example implementation (credit to this StackOverflow answer)
- Scala
-
source
import akka.actor.{ Actor, Props, SupervisorStrategy } class SupervisorActor(childProps: Props, override val supervisorStrategy: SupervisorStrategy) extends Actor { val child = context.actorOf(childProps, "supervised-child") def receive = { case msg => child.forward(msg) } }
- Java
And used here
- Scala
-
source
import akka.actor.{ PoisonPill, Props } import akka.cluster.singleton.{ ClusterSingletonManager, ClusterSingletonManagerSettings } context.system.actorOf( ClusterSingletonManager.props( singletonProps = Props(classOf[SupervisorActor], props, supervisorStrategy), terminationMessage = PoisonPill, settings = ClusterSingletonManagerSettings(context.system)), name = name)
- Java
Lease
For the full documentation of this feature and for new projects see Cluster Singleton - Lease.