Distributed Publish Subscribe in Cluster

Dependency

To use Distributed Publish Subscribe you must add the following dependency in your project:

sbt

libraryDependencies += "com.typesafe.akka" %% "akka-cluster-tools" % "2.5.32"

Maven

<dependencies>
  <dependency>
    <groupId>com.typesafe.akka</groupId>
    <artifactId>akka-cluster-tools_2.12</artifactId>
    <version>2.5.32</version>
  </dependency>
</dependencies>

Gradle

dependencies {
  implementation "com.typesafe.akka:akka-cluster-tools_2.12:2.5.32"
}

Introduction

How do I send a message to an actor without knowing which node it is running on?

How do I send messages to all actors in the cluster that have registered interest in a named topic?

This pattern provides a mediator actor, akka.cluster.pubsub.DistributedPubSubMediator, that manages a registry of actor references and replicates the entries to peer actors among all cluster nodes or a group of nodes tagged with a specific role.

The DistributedPubSubMediator actor is supposed to be started on all nodes, or all nodes with specified role, in the cluster. The mediator can be started with the DistributedPubSub extension or as an ordinary actor.

The registry is eventually consistent, i.e. changes are not immediately visible at other nodes, but typically they will be fully replicated to all other nodes after a few seconds. Changes are only performed in the own part of the registry and those changes are versioned. Deltas are disseminated in a scalable way to other nodes with a gossip protocol.

Cluster members with status WeaklyUp, will participate in Distributed Publish Subscribe, i.e. subscribers on nodes with WeaklyUp status will receive published messages if the publisher and subscriber are on same side of a network partition.

You can send messages via the mediator on any node to registered actors on any other node.

There a two different modes of message delivery, explained in the sections Publish and Send below.

Publish

This is the true pub/sub mode. A typical usage of this mode is a chat room in an instant messaging application.

Actors are registered to a named topic. This enables many subscribers on each node. The message will be delivered to all subscribers of the topic.

For efficiency the message is sent over the wire only once per node (that has a matching topic), and then delivered to all subscribers of the local topic representation.

You register actors to the local mediator with DistributedPubSubMediator.Subscribe. Successful Subscribe and Unsubscribe is acknowledged with DistributedPubSubMediator.SubscribeAck and DistributedPubSubMediator.UnsubscribeAck replies. The acknowledgment means that the subscription is registered, but it can still take some time until it is replicated to other nodes.

You publish messages by sending DistributedPubSubMediator.Publish message to the local mediator.

Actors are automatically removed from the registry when they are terminated, or you can explicitly remove entries with DistributedPubSubMediator.Unsubscribe.

An example of a subscriber actor:

Scala

copysourceclass Subscriber extends Actor with ActorLogging {
  import DistributedPubSubMediator.{ Subscribe, SubscribeAck }
  val mediator = DistributedPubSub(context.system).mediator
  // subscribe to the topic named "content"
  mediator ! Subscribe("content", self)

  def receive = {
    case s: String =>
      log.info("Got {}", s)
    case SubscribeAck(Subscribe("content", None, `self`)) =>
      log.info("subscribing")
  }
}

Java

copysourcestatic class Subscriber extends AbstractActor {
  LoggingAdapter log = Logging.getLogger(getContext().system(), this);

  public Subscriber() {
    ActorRef mediator = DistributedPubSub.get(getContext().system()).mediator();
    // subscribe to the topic named "content"
    mediator.tell(new DistributedPubSubMediator.Subscribe("content", getSelf()), getSelf());
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(String.class, msg -> log.info("Got: {}", msg))
        .match(DistributedPubSubMediator.SubscribeAck.class, msg -> log.info("subscribed"))
        .build();
  }
}

Subscriber actors can be started on several nodes in the cluster, and all will receive messages published to the “content” topic.

Scala

copysourcerunOn(first) {
  system.actorOf(Props[Subscriber], "subscriber1")
}
runOn(second) {
  system.actorOf(Props[Subscriber], "subscriber2")
  system.actorOf(Props[Subscriber], "subscriber3")
}

Java

copysourcesystem.actorOf(Props.create(Subscriber.class), "subscriber1");
// another node
system.actorOf(Props.create(Subscriber.class), "subscriber2");
system.actorOf(Props.create(Subscriber.class), "subscriber3");

A simple actor that publishes to this “content” topic:

Scala

copysourceclass Publisher extends Actor {
  import DistributedPubSubMediator.Publish
  // activate the extension
  val mediator = DistributedPubSub(context.system).mediator

  def receive = {
    case in: String =>
      val out = in.toUpperCase
      mediator ! Publish("content", out)
  }
}

Java

copysourcestatic class Publisher extends AbstractActor {

  // activate the extension
  ActorRef mediator = DistributedPubSub.get(getContext().system()).mediator();

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            String.class,
            in -> {
              String out = in.toUpperCase();
              mediator.tell(new DistributedPubSubMediator.Publish("content", out), getSelf());
            })
        .build();
  }
}

It can publish messages to the topic from anywhere in the cluster:

Scala

copysourcerunOn(third) {
  val publisher = system.actorOf(Props[Publisher], "publisher")
  later()
  // after a while the subscriptions are replicated
  publisher ! "hello"
}

Java

copysource// somewhere else
ActorRef publisher = system.actorOf(Props.create(Publisher.class), "publisher");
// after a while the subscriptions are replicated
publisher.tell("hello", null);

Topic Groups

Actors may also be subscribed to a named topic with a group id. If subscribing with a group id, each message published to a topic with the sendOneMessageToEachGroup flag set to true is delivered via the supplied RoutingLogic (default random) to one actor within each subscribing group.

If all the subscribed actors have the same group id, then this works just like Send and each message is only delivered to one subscriber.

If all the subscribed actors have different group names, then this works like normal Publish and each message is broadcasted to all subscribers.

Send

This is a point-to-point mode where each message is delivered to one destination, but you still do not have to know where the destination is located. A typical usage of this mode is private chat to one other user in an instant messaging application. It can also be used for distributing tasks to registered workers, like a cluster aware router where the routees dynamically can register themselves.

The message will be delivered to one recipient with a matching path, if any such exists in the registry. If several entries match the path because it has been registered on several nodes the message will be sent via the supplied RoutingLogic (default random) to one destination. The sender of the message can specify that local affinity is preferred, i.e. the message is sent to an actor in the same local actor system as the used mediator actor, if any such exists, otherwise route to any other matching entry.

You register actors to the local mediator with DistributedPubSubMediator.Put. The ActorRef in Put must belong to the same local actor system as the mediator. The path without address information is the key to which you send messages. On each node there can only be one actor for a given path, since the path is unique within one local actor system.

You send messages by sending DistributedPubSubMediator.Send message to the local mediator with the path (without address information) of the destination actors.

Actors are automatically removed from the registry when they are terminated, or you can explicitly remove entries with DistributedPubSubMediator.Remove.

An example of a destination actor:

Scala

copysourceclass Destination extends Actor with ActorLogging {
  import DistributedPubSubMediator.Put
  val mediator = DistributedPubSub(context.system).mediator
  // register to the path
  mediator ! Put(self)

  def receive = {
    case s: String =>
      log.info("Got {}", s)
  }
}

Java

copysourcestatic class Destination extends AbstractActor {
  LoggingAdapter log = Logging.getLogger(getContext().system(), this);

  public Destination() {
    ActorRef mediator = DistributedPubSub.get(getContext().system()).mediator();
    // register to the path
    mediator.tell(new DistributedPubSubMediator.Put(getSelf()), getSelf());
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder().match(String.class, msg -> log.info("Got: {}", msg)).build();
  }
}

Destination actors can be started on several nodes in the cluster, and all will receive messages sent to the path (without address information).

Scala

copysourcerunOn(first) {
  system.actorOf(Props[Destination], "destination")
}
runOn(second) {
  system.actorOf(Props[Destination], "destination")
}

Java

copysourcesystem.actorOf(Props.create(Destination.class), "destination");
// another node
system.actorOf(Props.create(Destination.class), "destination");

A simple actor that sends to the path:

Scala

copysourceclass Sender extends Actor {
  import DistributedPubSubMediator.Send
  // activate the extension
  val mediator = DistributedPubSub(context.system).mediator

  def receive = {
    case in: String =>
      val out = in.toUpperCase
      mediator ! Send(path = "/user/destination", msg = out, localAffinity = true)
  }
}

Java

copysourcestatic class Sender extends AbstractActor {

  // activate the extension
  ActorRef mediator = DistributedPubSub.get(getContext().system()).mediator();

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            String.class,
            in -> {
              String out = in.toUpperCase();
              boolean localAffinity = true;
              mediator.tell(
                  new DistributedPubSubMediator.Send("/user/destination", out, localAffinity),
                  getSelf());
            })
        .build();
  }
}

It can send messages to the path from anywhere in the cluster:

Scala

copysourcerunOn(third) {
  val sender = system.actorOf(Props[Sender], "sender")
  later()
  // after a while the destinations are replicated
  sender ! "hello"
}

Java

copysource// somewhere else
ActorRef sender = system.actorOf(Props.create(Publisher.class), "sender");
// after a while the destinations are replicated
sender.tell("hello", null);

It is also possible to broadcast messages to the actors that have been registered with Put. Send DistributedPubSubMediator.SendToAll message to the local mediator and the wrapped message will then be delivered to all recipients with a matching path. Actors with the same path, without address information, can be registered on different nodes. On each node there can only be one such actor, since the path is unique within one local actor system.

Typical usage of this mode is to broadcast messages to all replicas with the same path, e.g. 3 actors on different nodes that all perform the same actions, for redundancy. You can also optionally specify a property (allButSelf) deciding if the message should be sent to a matching path on the self node or not.

DistributedPubSub Extension

In the example above the mediator is started and accessed with the akka.cluster.pubsub.DistributedPubSub extension. That is convenient and perfectly fine in most cases, but it can be good to know that it is possible to start the mediator actor as an ordinary actor and you can have several different mediators at the same time to be able to divide a large number of actors/topics to different mediators. For example you might want to use different cluster roles for different mediators.

The DistributedPubSub extension can be configured with the following properties:

copysource# Settings for the DistributedPubSub extension
akka.cluster.pub-sub {
  # Actor name of the mediator actor, /system/distributedPubSubMediator
  name = distributedPubSubMediator

  # Start the mediator on members tagged with this role.
  # All members are used if undefined or empty.
  role = ""

  # The routing logic to use for 'Send'
  # Possible values: random, round-robin, broadcast
  routing-logic = random

  # How often the DistributedPubSubMediator should send out gossip information
  gossip-interval = 1s

  # Removed entries are pruned after this duration
  removed-time-to-live = 120s

  # Maximum number of elements to transfer in one message when synchronizing the registries.
  # Next chunk will be transferred in next round of gossip.
  max-delta-elements = 3000

  # When a message is published to a topic with no subscribers send it to the dead letters.
  send-to-dead-letters-when-no-subscribers = on
  
  # The id of the dispatcher to use for DistributedPubSubMediator actors. 
  # If not specified default dispatcher is used.
  # If specified you need to define the settings of the actual dispatcher.
  use-dispatcher = ""
}

It is recommended to load the extension when the actor system is started by defining it in akka.extensions configuration property. Otherwise it will be activated when first used and then it takes a while for it to be populated.

akka.extensions = ["akka.cluster.pubsub.DistributedPubSub"]

Delivery Guarantee

As in Message Delivery Reliability of Akka, message delivery guarantee in distributed pub sub modes is at-most-once delivery. In other words, messages can be lost over the wire.

If you are looking for at-least-once delivery guarantee, we recommend Kafka Akka Streams integration.