Learning Akka Typed from Classic

Akka Classic is the original Actor APIs, which have been improved by more type safe and guided Actor APIs, known as Akka Typed.

If you already know the classic actor APIs and would like to learn Akka Typed, this reference is a good resource. Many concepts are the same and this page tries to highlight differences and how to do certain things in Typed compared to classic.

You should probably learn some of the basics of Akka Typed to see how it looks like before diving into the differences and details described here. A good starting point for that is the IoT example in the Getting Started Guide or the examples shown in Introduction to Actors.

Note that Akka Classic is still fully supported and existing applications can continue to use the classic APIs. It is also possible to use Akka Typed together with classic actors within the same ActorSystem, see coexistence. For new projects we recommend using the new Actor APIs.

Dependencies

The dependencies of the Typed modules are named by adding -typed suffix of the corresponding classic module, with a few exceptions.

For example akka-cluster-typed:

sbt
libraryDependencies += "com.typesafe.akka" %% "akka-cluster-typed" % "2.6-SNAPSHOT"
Maven
<dependency>
  <groupId>com.typesafe.akka</groupId>
  <artifactId>akka-cluster-typed_2.12</artifactId>
  <version>2.6-SNAPSHOT</version>
</dependency>
Gradle
dependencies {
  compile group: 'com.typesafe.akka', name: 'akka-cluster-typed_2.12', version: '2.6-SNAPSHOT'
}

Artifact names:

Classic Typed
akka-actor akka-actor-typed
akka-cluster akka-cluster-typed
akka-cluster-sharding akka-cluster-sharding-typed
akka-cluster-tools akka-cluster-typed
akka-distributed-data akka-cluster-typed
akka-persistence akka-persistence-typed
akka-stream akka-stream-typed
akka-testkit akka-actor-testkit-typed

Cluster Singleton and Distributed Data are included in akka-cluster-typed.

Artifacts not listed in above table don’t have a specific API for Akka Typed.

Package names

The convention of the package names in Akka Typed is to add typed.scaladsl and typed.javadsl to the corresponding Akka classic package name. scaladsl and javadsl is the convention to separate Scala and Java APIs, which is familiar from Akka Streams.

Examples of a few package names:

Classic Typed for Scala Typed for Java
akka.actor akka.actor.typed.scaladsl akka.actor.typed.javadsl
akka.cluster akka.cluster.typed akka.cluster.typed
akka.cluster.sharding akka.cluster.sharding.scaladsl akka.cluster.sharding.javadsl
akka.persistence akka.persistence.typed.scaladsl akka.persistence.typed.javadsl

Actor definition

A classic actor is defined by a class extending akka.actor.Actorakka.actor.AbstractActor.

An actor in Typed is defined by a class extending akka.actor.typed.scaladsl.AbstractBehaviorakka.actor.typed.javadsl.AbstractBehavior.

It’s also possible to define an actor in Typed from functions instead of extending a class. This is called the functional style.

Classic HelloWorld actor:

Scala
import akka.actor.Actor
import akka.actor.ActorLogging
import akka.actor.Props

object HelloWorld {
  final case class Greet(whom: String)
  final case class Greeted(whom: String)

  def props(): Props =
    Props(new HelloWorld)
}

class HelloWorld extends Actor with ActorLogging {
  import HelloWorld._

  override def receive: Receive = {
    case Greet(whom) =>
      log.info("Hello {}!", whom)
      sender() ! Greeted(whom)
  }
}
Java
import akka.actor.AbstractActor;
import akka.actor.Props;
import akka.event.Logging;
import akka.event.LoggingAdapter;

public class HelloWorld extends AbstractActor {

  public static final class Greet {
    public final String whom;

    public Greet(String whom) {
      this.whom = whom;
    }
  }

  public static final class Greeted {
    public final String whom;

    public Greeted(String whom) {
      this.whom = whom;
    }
  }

  public static Props props() {
    return Props.create(HelloWorld.class, HelloWorld::new);
  }

  private final LoggingAdapter log = Logging.getLogger(getContext().getSystem(), this);

  @Override
  public Receive createReceive() {
    return receiveBuilder().match(Greet.class, this::onGreet).build();
  }

  private void onGreet(Greet command) {
    log.info("Hello {}!", command.whom);
    getSender().tell(new Greeted(command.whom), getSelf());
  }
}

Typed HelloWorld actor:

Scala
import akka.actor.typed.ActorRef
import akka.actor.typed.Behavior
import akka.actor.typed.scaladsl.AbstractBehavior
import akka.actor.typed.scaladsl.ActorContext
import akka.actor.typed.scaladsl.Behaviors

object HelloWorld {
  final case class Greet(whom: String, replyTo: ActorRef[Greeted])
  final case class Greeted(whom: String, from: ActorRef[Greet])

  def apply(): Behavior[HelloWorld.Greet] =
    Behaviors.setup(context => new HelloWorld(context))
}

class HelloWorld(context: ActorContext[HelloWorld.Greet]) extends AbstractBehavior[HelloWorld.Greet](context) {
  import HelloWorld._

  override def onMessage(message: Greet): Behavior[Greet] = {
    context.log.info("Hello {}!", message.whom)
    message.replyTo ! Greeted(message.whom, context.self)
    this
  }
}
Java
import akka.actor.typed.ActorRef;
import akka.actor.typed.Behavior;
import akka.actor.typed.javadsl.AbstractBehavior;
import akka.actor.typed.javadsl.ActorContext;
import akka.actor.typed.javadsl.Behaviors;
import akka.actor.typed.javadsl.Receive;

import java.util.HashMap;
import java.util.Map;

public class HelloWorld extends AbstractBehavior<HelloWorld.Greet> {

  public static final class Greet {
    public final String whom;
    public final ActorRef<Greeted> replyTo;

    public Greet(String whom, ActorRef<Greeted> replyTo) {
      this.whom = whom;
      this.replyTo = replyTo;
    }
  }

  public static final class Greeted {
    public final String whom;
    public final ActorRef<Greet> from;

    public Greeted(String whom, ActorRef<Greet> from) {
      this.whom = whom;
      this.from = from;
    }
  }

  public static Behavior<Greet> create() {
    return Behaviors.setup(HelloWorld::new);
  }

  private HelloWorld(ActorContext<Greet> context) {
    super(context);
  }

  @Override
  public Receive<Greet> createReceive() {
    return newReceiveBuilder().onMessage(Greet.class, this::onGreet).build();
  }

  private Behavior<Greet> onGreet(Greet command) {
    getContext().getLog().info("Hello {}!", command.whom);
    command.replyTo.tell(new Greeted(command.whom, getContext().getSelf()));
    return this;
  }
}

Why is it called Behavior and not Actor?

In Typed the Behavior defines how to handle incoming messages and after processing each message it may return a different Behavior to be used for the next message. This means that an actor is started with an initial Behavior and may change Behavior over its lifecycle. This is described more in the section about become.

Note that the Behavior has a type parameter describing the type of messages that it can handle. This information is not defined explicitly for a classic actor.

Links to reference documentation:

actorOf and Props

A classic actor is started with the actorOf method of the ActorContext or ActorSystem.

Corresponding method in Typed is called spawn in the akka.actor.typed.scaladsl.ActorContextakka.actor.typed.javadsl.ActorContext.

There is no spawn in the akka.actor.typed.scaladsl.ActorSystemakka.actor.typed.javadsl.ActorSystem for creating top level actors. Instead, the single to level actor is defined by a user guardian Behavior that is given when starting the ActorSystem. Other actors are started as children of that user guardian actor or children of other actors in the actor hierarchy. This is explained more in ActorSystem.

actorOf takes an akka.actor.Props parameter, which is like a factory for creating the actor instance, and it’s also used when creating a new instance when the actor is restarted. The Props may also define additional properties such as which dispatcher to use for the actor.

The spawn method in Typed creates an actor from a given Behavior instead of via a Props factory. The factory aspect is instead defined via Behaviors.setup when using the object-oriented style with a class extending AbstractBehavior. For the function style there is typically no need for the factory.

Additional properties such as which dispatcher to use for the actor can still be given via an optional akka.actor.typed.Props parameter of the spawn method.

The name parameter of actorOf is optional and if not defined the actor will have a generated name. Corresponding in Typed is achieved with the spawnAnonymous method.

Links to reference documentation:

ActorRef

akka.actor.ActorRef has its correspondence in akka.actor.typed.ActorRef. The difference being that the latter has a type parameter describing which messages the actor can handle. This information is not defined for a classic actor and you can send any type of message to a classic ActorRef even though the actor may not understand it.

ActorSystem

akka.actor.ActorSystem has its correspondence in akka.actor.typed.ActorSystem. One difference is that when creating an ActorSystem in Typed you give it a Behavior that will be used as the top level actor, also known as the user guardian.

It’s from the user guardian you create additional actors for the application and initialize tools like Cluster Sharding. In contrast, such initialization are typically performed from the “outside” after starting a classic ActorSystem.

actorOf of the classic ActorSystem is typically used to create a few (or many) top level actors. The ActorSystem in Typed doesn’t have that capability. Instead, such actors are started as children of the user guardian actor or children of other actors in the actor hierarchy.

become

A classic actor can change its message processing behavior by using become in ActorContext. In Typed this is done by returning a new Behavior after processing a message. The returned Behavior will be used for the next received message.

There is no correspondence to unbecome in Typed. Instead you must explicitly keep track of and return the “previous” Behavior.

Links to reference documentation:

sender

There is no sender()getSender() in Typed. Instead you have to explicitly include an ActorRef representing the sender—or rather representing where to send a reply to—in the messages.

The reason for not having an implicit sender in Typed is that it wouldn’t be possible to know the type for the sender ActorRef[T]ActorRef<T> at compile time. It’s also much better to define this explicitly in the messages as it becomes more clear what the message protocol expects.

Links to reference documentation:

parent

There is no parentgetParent in Typed. Instead you have to explicitly include the ActorRef of the parent as a parameter when constructing the Behavior.

The reason for not having a parent in Typed is that it wouldn’t be possible to know the type for the parent ActorRef[T]ActorRef<T> at compile time without having an additional type parameter in the Behavior. For testing purposes it’s also better to pass in the parent since it can be replaced by a probe or being stubbed out in tests.

Supervision

An important difference is that actors in Typed are by default stopped if an exception is thrown and no supervision strategy is defined while in classic actors they are restarted.

In classic actors the supervision strategy for child actors are defined by overriding the supervisorStrategy method in the parent actor.

In Typed the supervisor strategy is defined by wrapping the Behavior of the child actor with Behaviors.supervise.

The classic BackoffSupervisor is supported by SupervisorStrategy.restartWithBackoff as an ordinary SupervisorStrategy in Typed.

SupervisorStrategy.Escalate isn’t supported in Typed, but similar can be achieved as described in Bubble failures up through the hierarchy.

Links to reference documentation:

Lifcycle hooks

Classic actors have methods preStart, preRestart, postRestart and postStop that can be overridden to act on changes to the actor’s lifecycle.

This is supported with corresponding PreRestart and PostStop signal messages in Typed. There are no PreStart and PostRestart signals because such action can be done from Behaviors.setup or the constructor of the AbstractBehavior class.

Note that classic postStop is called also when the actor is restarted. That is not the case in Typed, only the PreRestart signal is emitted. If you need to do resource cleanup on both restart and stop you have to do that for both PreRestart and PostStop.

Links to reference documentation:

watch

watch and the Terminated message are pretty much the same, with some additional capabilities in Typed.

Terminated is a signal in Typed since it is a different type than the declared message type of the Behavior.

The watchWith method of the ActorContext in Typed can be used to send a message instead of the Terminated signal.

When watching child actors it’s possible to see if the child terminated voluntarily or due to a failure via the ChildFailed signal, which is a subclass of Terminated.

Links to reference documentation:

Stopping

Classic actors can be stopped with the stop method of ActorContext or ActorSystem. In Typed an actor is stopping itself by returning Behaviors.stopped. There is also a stop method in the ActorContext but it can only be used for stopping direct child actors and not any arbitrary actor.

PoisonPill is not supported in Typed. Instead, if you need to request an actor to stop you should define a message that the actor understands and let it return Behaviors.stopped when receiving that message.

Links to reference documentation:

ActorSelection

ActorSelection isn’t supported in Typed. Instead the Receptionist is supposed to be used for finding actors by a registered key.

ActorSelection can be used for sending messages to a path without having an ActorRef of the destination. Note that a Group Router can be used for that.

Links to reference documentation:

ask

The classic ask pattern returns a FutureCompletionStage for the response.

Corresponding ask exists in Typed and is good when the requester itself isn’t an actor. It is located in akka.actor.typed.scaladsl.AskPatternakka.actor.typed.javadsl.AskPattern.

When the requester is an actor it is better to use the ask method of the ActorContext in Typed. It has the advantage of not having to mix FutureCompletionStage callbacks that are running on different threads with actor code.

Links to reference documentation:

pipeTo

pipeTo is typically used together with ask in an actor. The ask method of the ActorContext in Typed removes the need for pipeTo. However, for interactions with other APIs that return FutureCompletionStage it is still useful to send the result as a message to the actor. For this purpose there is a pipeToSelf method in the ActorContext in Typed.

ActorContext.children

The ActorContext has methods children and childgetChildren and getChild to retrieve the ActorRef of started child actors in both Typed and Classic.

The type of the returned ActorRef is unknown, since different types can be used for different children. Therefore, this is not a useful way to lookup children when the purpose is to send messages to them.

Instead of finding children via the ActorContext it’s recommended to use an application specific collection for bookkeeping of children, such as a Map[String, ActorRef[Child.Command]] Map<String, ActorRef<Child.Command>>. It can look like this:

Scala
object Parent {
  sealed trait Command
  case class DelegateToChild(name: String, message: Child.Command) extends Command
  private case class ChildTerminated(name: String) extends Command

  def apply(): Behavior[Command] = {
    def updated(children: Map[String, ActorRef[Child.Command]]): Behavior[Command] = {
      Behaviors.receive { (context, command) =>
        command match {
          case DelegateToChild(name, childCommand) =>
            children.get(name) match {
              case Some(ref) =>
                ref ! childCommand
                context.watchWith(ref, ChildTerminated(name))
                Behaviors.same
              case None =>
                val ref = context.spawn(Child(), name)
                ref ! childCommand
                updated(children + (name -> ref))
            }

          case ChildTerminated(name) =>
            updated(children - name)
        }
      }
    }

    updated(Map.empty)
  }
}
Java
public class Parent extends AbstractBehavior<Parent.Command> {

  public interface Command {}

  public static class DelegateToChild implements Command {
    public final String name;
    public final Child.Command message;

    public DelegateToChild(String name, Child.Command message) {
      this.name = name;
      this.message = message;
    }
  }

  private static class ChildTerminated implements Command {
    final String name;

    ChildTerminated(String name) {
      this.name = name;
    }
  }

  public static Behavior<Command> create() {
    return Behaviors.setup(Parent::new);
  }

  private Map<String, ActorRef<Child.Command>> children = new HashMap<>();

  private Parent(ActorContext<Command> context) {
    super(context);
  }

  @Override
  public Receive<Command> createReceive() {
    return newReceiveBuilder()
        .onMessage(DelegateToChild.class, this::onDelegateToChild)
        .onMessage(ChildTerminated.class, this::onChildTerminated)
        .build();
  }

  private Behavior<Command> onDelegateToChild(DelegateToChild command) {
    ActorRef<Child.Command> ref = children.get(command.name);
    if (ref == null) {
      ref = getContext().spawn(Child.create(), command.name);
      getContext().watchWith(ref, new ChildTerminated(command.name));
      children.put(command.name, ref);
    }
    ref.tell(command.message);
    return this;
  }

  private Behavior<Command> onChildTerminated(ChildTerminated command) {
    children.remove(command.name);
    return this;
  }
}

Remember to remove entries from the Map when the children are terminated. For that purpose it’s convenient to use watchWith, as illustrated in the example above, because then you can include the key to the Map in the termination message. In that way the name of the actor doesn’t have to be the same as identifier used for bookkeeping.

Retrieving the children from the ActorContext can still be useful for some certain cases, such as:

  • see if a child name is in use
  • stopping children
  • the type of the child is well known and unsafeUpcast of the ActorRef is considered “safe enough”

Remote deployment

Starting an actor on a remote node—so called remote deployment—isn’t supported in Typed.

The main reason is that we would discourage this feature since it often result too tight coupling between nodes and undesired failure handling. For example if the node of the parent actor crashes all remote deployed child actors are brought down with it. Sometimes that can be desired but many times it is used without realizing. This can be achieve by other means, such as using watch.

Routers

Routers are provided in Typed, but in a much simplified form compared to the classic routers.

Destinations of group routers are registered in the Receptionist, which makes them Cluster aware and also more dynamic than classic group routers.

Pool routers are only for local actor destinations in Typed, since remote deployment isn’t supported.

Links to reference documentation:

FSM

With classic actors there is explicit support for building Finite State Machines. No support is needed in Akka Typed as it is straightforward to represent FSMs with behaviors.

Links to reference documentation:

Timers

In classic actors you mixin with Timersextend AbstractActorWithTimers to gain access to delayed and periodic scheduling of messages. In Typed you have access to similar capabilities via Behaviors.withTimers.

Links to reference documentation:

Stash

In classic actors you mixin with Stashextend AbstractActorWithStash to gain access to stashing of messages. In Typed you have access to similar capabilities via Behaviors.withStash.

Links to reference documentation:

PersistentActor

The correspondence of the classic PersistentActor is akka.persistence.typed.scaladsl.EventSourcedBehaviorakka.persistence.typed.javadsl.EventSourcedBehavior.

The Typed API is much more guided to facilitate event sourcing best practises. It also has tighter integration with Cluster Sharding.

Links to reference documentation:

Asynchronous Testing

The Test Kits for asynchronous testing are rather similar.

Links to reference documentation:

Synchronous Testing

The Test Kits for synchronous testing are completely different.

Behaviors in Typed can be tested in isolation without having to be packaged into an Actor, tests can run fully synchronously without having to worry about timeouts and spurious failures.

The BehaviorTestKit provides a nice way of unit testing a Behavior in a deterministic way, but it has some limitations to be aware of. Similar limitations exists for synchronous testing of classic actors.

Links to reference documentation:

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