Typed Actors

Akka “Typed Actors”, now replaced by Akka Typed, were an implementation of the Active Objects pattern. Essentially turning method invocations into asynchronous dispatch instead of synchronous that has been the default way since Smalltalk came out.

Typed Actors consist of 2 “parts”, a public interface and an implementation, and if you’ve done any work in “enterprise” Java, this will be very familiar to you. As with normal Actors you have an external API (the public interface instance) that will delegate method calls asynchronously to a private instance of the implementation.

The advantage of Typed Actors vs. Actors is that with TypedActors you have a static contract, and don’t need to define your own messages, the downside is that it places some limitations on what you can do and what you can’t, i.e. you cannot use become/unbecome.

Typed Actors are implemented using JDK Proxies which provide a pretty easy-worked API to intercept method calls.

When to use Typed Actors

Typed actors are nice for bridging between actor systems (the “inside”) and non-actor code (the “outside”), because they allow you to write normal OO-looking code on the outside. Think of them like doors: their practicality lies in interfacing between private sphere and the public, but you don’t want that many doors inside your house, do you? For a longer discussion see this blog post.

A bit more background: TypedActors can easily be abused as RPC, and that is an abstraction which is well-known to be leaky. Hence TypedActors are not what we think of first when we talk about making highly scalable concurrent software easier to write correctly. They have their niche, use them sparingly.

The tools of the trade

Before we create our first Typed Actor we should first go through the tools that we have at our disposal, it’s located in akka.actor.TypedActor.

Scala

copysource
import akka.actor.TypedActor

//Returns the Typed Actor Extension
val extension = TypedActor(system) //system is an instance of ActorSystem

//Returns whether the reference is a Typed Actor Proxy or not
TypedActor(system).isTypedActor(someReference)

//Returns the backing Akka Actor behind an external Typed Actor Proxy
TypedActor(system).getActorRefFor(someReference)

//Returns the current ActorContext,
// method only valid within methods of a TypedActor implementation
val c: ActorContext = TypedActor.context

//Returns the external proxy of the current Typed Actor,
// method only valid within methods of a TypedActor implementation
val s: Squarer = TypedActor.self[Squarer]

//Returns a contextual instance of the Typed Actor Extension
//this means that if you create other Typed Actors with this,
//they will become children to the current Typed Actor.
TypedActor(TypedActor.context)

Java

copysource
// Returns the Typed Actor Extension
TypedActorExtension extension =
    TypedActor.get(system); // system is an instance of ActorSystem

// Returns whether the reference is a Typed Actor Proxy or not
TypedActor.get(system).isTypedActor(someReference);

// Returns the backing Akka Actor behind an external Typed Actor Proxy
TypedActor.get(system).getActorRefFor(someReference);

// Returns the current ActorContext,
// method only valid within methods of a TypedActor implementation
ActorContext context = TypedActor.context();

// Returns the external proxy of the current Typed Actor,
// method only valid within methods of a TypedActor implementation
Squarer sq = TypedActor.<Squarer>self();

// Returns a contextual instance of the Typed Actor Extension
// this means that if you create other Typed Actors with this,
// they will become children to the current Typed Actor.
TypedActor.get(TypedActor.context());

Creating Typed Actors

To create a Typed Actor you need to have one or more interfaces, and one implementation.

The following imports are assumed:

Scala

copysourceimport java.lang.String.{ valueOf => println }

import akka.actor.{ ActorContext, ActorRef, TypedActor, TypedProps }
import akka.routing.RoundRobinGroup
import akka.testkit._

import scala.concurrent.{ Await, Future }
import scala.concurrent.duration._

Java

copysourceimport akka.actor.TypedActor;
import akka.actor.*;
import akka.japi.*;
import akka.dispatch.Futures;

import jdocs.AbstractJavaTest;
import scala.concurrent.Await;
import scala.concurrent.Future;
import scala.concurrent.duration.Duration;
import java.util.concurrent.TimeUnit;

import java.util.List;
import java.util.ArrayList;
import java.util.Random;
import akka.routing.RoundRobinGroup;

Our example interface:

Scala

copysourcetrait Squarer {
  def squareDontCare(i: Int): Unit //fire-forget

  def square(i: Int): Future[Int] //non-blocking send-request-reply

  def squareNowPlease(i: Int): Option[Int] //blocking send-request-reply

  def squareNow(i: Int): Int //blocking send-request-reply

  @throws(classOf[Exception]) //declare it or you will get an UndeclaredThrowableException
  def squareTry(i: Int): Int //blocking send-request-reply with possible exception
}

Java

copysourcestatic interface Squarer {
  void squareDontCare(int i); // fire-forget

  Future<Integer> square(int i); // non-blocking send-request-reply

  Option<Integer> squareNowPlease(int i); // blocking send-request-reply

  int squareNow(int i); // blocking send-request-reply
}

Our example implementation of that interface:

Scala

copysourceclass SquarerImpl(val name: String) extends Squarer {

  def this() = this("default")
  def squareDontCare(i: Int): Unit = i * i //Nobody cares :(

  def square(i: Int): Future[Int] = Future.successful(i * i)

  def squareNowPlease(i: Int): Option[Int] = Some(i * i)

  def squareNow(i: Int): Int = i * i

  def squareTry(i: Int): Int = throw new Exception("Catch me!")
}

Java

copysourceclass SquarerImpl implements Squarer {
  private String name;

  public SquarerImpl() {
    this.name = "default";
  }

  public SquarerImpl(String name) {
    this.name = name;
  }


  public void squareDontCare(int i) {
    int sq = i * i; // Nobody cares :(
  }

  public Future<Integer> square(int i) {
    return Futures.successful(i * i);
  }

  public Option<Integer> squareNowPlease(int i) {
    return Option.some(i * i);
  }

  public int squareNow(int i) {
    return i * i;
  }
}

The most trivial way of creating a Typed Actor instance of our Squarer:

Scala

copysourceval mySquarer: Squarer =
  TypedActor(system).typedActorOf(TypedProps[SquarerImpl]())

Java

copysourceSquarer mySquarer =
    TypedActor.get(system)
        .typedActorOf(new TypedProps<SquarerImpl>(Squarer.class, SquarerImpl.class));

First type is the type of the proxy, the second type is the type of the implementation. If you need to call a specific constructor you do it like this:

Scala

copysourceval otherSquarer: Squarer =
  TypedActor(system).typedActorOf(TypedProps(classOf[Squarer], new SquarerImpl("foo")), "name")

Java

copysourceSquarer otherSquarer =
    TypedActor.get(system)
        .typedActorOf(
            new TypedProps<SquarerImpl>(
                Squarer.class,
                new Creator<SquarerImpl>() {
                  public SquarerImpl create() {
                    return new SquarerImpl("foo");
                  }
                }),
            "name");

Since you supply a Props, you can specify which dispatcher to use, what the default timeout should be used and more. Now, our Squarer doesn’t have any methods, so we’d better add those.

Scala

copysourcetrait Squarer {
  def squareDontCare(i: Int): Unit //fire-forget

  def square(i: Int): Future[Int] //non-blocking send-request-reply

  def squareNowPlease(i: Int): Option[Int] //blocking send-request-reply

  def squareNow(i: Int): Int //blocking send-request-reply

  @throws(classOf[Exception]) //declare it or you will get an UndeclaredThrowableException
  def squareTry(i: Int): Int //blocking send-request-reply with possible exception
}

Java

copysourcestatic interface Squarer {
  void squareDontCare(int i); // fire-forget

  Future<Integer> square(int i); // non-blocking send-request-reply

  Option<Integer> squareNowPlease(int i); // blocking send-request-reply

  int squareNow(int i); // blocking send-request-reply
}

Alright, now we’ve got some methods we can call, but we need to implement those in SquarerImpl.

Scala

copysourceclass SquarerImpl(val name: String) extends Squarer {

  def this() = this("default")
  def squareDontCare(i: Int): Unit = i * i //Nobody cares :(

  def square(i: Int): Future[Int] = Future.successful(i * i)

  def squareNowPlease(i: Int): Option[Int] = Some(i * i)

  def squareNow(i: Int): Int = i * i

  def squareTry(i: Int): Int = throw new Exception("Catch me!")
}

Java

copysourceclass SquarerImpl implements Squarer {
  private String name;

  public SquarerImpl() {
    this.name = "default";
  }

  public SquarerImpl(String name) {
    this.name = name;
  }


  public void squareDontCare(int i) {
    int sq = i * i; // Nobody cares :(
  }

  public Future<Integer> square(int i) {
    return Futures.successful(i * i);
  }

  public Option<Integer> squareNowPlease(int i) {
    return Option.some(i * i);
  }

  public int squareNow(int i) {
    return i * i;
  }
}

Excellent, now we have an interface and an implementation of that interface, and we know how to create a Typed Actor from that, so let’s look at calling these methods.

Method dispatch semantics

Methods returning:

Messages and immutability

While Akka cannot enforce that the parameters to the methods of your Typed Actors are immutable, we strongly recommend that parameters passed are immutable.

One-way message send

Scala

copysourcemySquarer.squareDontCare(10)

Java

copysourcemySquarer.squareDontCare(10);

As simple as that! The method will be executed on another thread; asynchronously.

Request-reply message send

Scala

copysourceval oSquare = mySquarer.squareNowPlease(10) //Option[Int]

Java

copysourceOption<Integer> oSquare = mySquarer.squareNowPlease(10); // Option[Int]

This will block for as long as the timeout that was set in the Props of the Typed Actor, if needed. It will return None if a timeout occurs.

Scala

copysourceval iSquare = mySquarer.squareNow(10) //Int

Java

copysourceint iSquare = mySquarer.squareNow(10); // Int

This will block for as long as the timeout that was set in the Props of the Typed Actor, if needed. It will throw a java.util.concurrent.TimeoutException if a timeout occurs.

Request-reply-with-future message send

Scala

copysourceval fSquare = mySquarer.square(10) //A Future[Int]

Java

copysourceFuture<Integer> fSquare = mySquarer.square(10); // A Future[Int]

This call is asynchronous, and the Future returned can be used for asynchronous composition.

Stopping Typed Actors

Since Akka’s Typed Actors are backed by Akka Actors they must be stopped when they aren’t needed anymore.

Scala

copysourceTypedActor(system).stop(mySquarer)

Java

copysourceTypedActor.get(system).stop(mySquarer);

This asynchronously stops the Typed Actor associated with the specified proxy ASAP.

Scala

copysourceTypedActor(system).poisonPill(otherSquarer)

Java

copysourceTypedActor.get(system).poisonPill(otherSquarer);

This asynchronously stops the Typed Actor associated with the specified proxy after it’s done with all calls that were made prior to this call.

Typed Actor Hierarchies

Since you can obtain a contextual Typed Actor Extension by passing in an ActorContext you can create child Typed Actors by invoking typedActorOf(..) on that.

Scala

copysource//Inside your Typed Actor
val childSquarer: Squarer =
  TypedActor(TypedActor.context).typedActorOf(TypedProps[SquarerImpl]())
//Use "childSquarer" as a Squarer

Java

copysourceSquarer childSquarer =
    TypedActor.get(TypedActor.context())
        .typedActorOf(new TypedProps<SquarerImpl>(Squarer.class, SquarerImpl.class));
// Use "childSquarer" as a Squarer

You can also create a child Typed Actor in regular Akka Actors by giving the ActorContextAbstractActor.ActorContext as an input parameter to TypedActor.get(…).

Supervisor Strategy

By having your Typed Actor implementation class implement TypedActor.Supervisor you can define the strategy to use for supervising child actors, as described in supervision and Fault Tolerance.

Lifecycle callbacks

By having your Typed Actor implementation class implement any and all of the following:

• TypedActor.PreStart
• TypedActor.PostStop
• TypedActor.PreRestart
• TypedActor.PostRestart

You can hook into the lifecycle of your Typed Actor.

Receive arbitrary messages

If your implementation class of your TypedActor extends akka.actor.TypedActor.Receiver, all messages that are not MethodCall instances will be passed into the onReceive-method.

This allows you to react to DeathWatch Terminated-messages and other types of messages, e.g. when interfacing with untyped actors.

Proxying

You can use the typedActorOf that takes a TypedProps and an ActorRef to proxy the given ActorRef as a TypedActor. This is usable if you want to communicate remotely with TypedActors on other machines, pass the ActorRef to typedActorOf.

Lookup & Remoting

Since TypedActors are backed by Akka Actors, you can use typedActorOf to proxy ActorRefs potentially residing on remote nodes.

Scala

copysourceval typedActor: Foo with Bar =
  TypedActor(system).typedActorOf(TypedProps[FooBar], actorRefToRemoteActor)
//Use "typedActor" as a FooBar

Java

copysourceSquarer typedActor =
    TypedActor.get(system)
        .typedActorOf(new TypedProps<Squarer>(Squarer.class), actorRefToRemoteActor);
// Use "typedActor" as a FooBar

copysourcetrait Foo {
  def doFoo(times: Int): Unit = println("doFoo(" + times + ")")
}

trait Bar {
  def doBar(str: String): Future[String] =
    Future.successful(str.toUpperCase)
}

class FooBar extends Foo with Bar

copysourceval awesomeFooBar: Foo with Bar =
  TypedActor(system).typedActorOf(TypedProps[FooBar]())

awesomeFooBar.doFoo(10)
val f = awesomeFooBar.doBar("yes")

TypedActor(system).poisonPill(awesomeFooBar)

Typed Router pattern

Sometimes you want to spread messages between multiple actors. The easiest way to achieve this in Akka is to use a Router, which can implement a specific routing logic, such as smallest-mailbox or consistent-hashing etc.

Routers are not provided directly for typed actors, but it is really easy to leverage an untyped router and use a typed proxy in front of it. To showcase this let’s create typed actors that assign themselves some random id, so we know that in fact, the router has sent the message to different actors:

Scala

copysourcetrait HasName {
  def name(): String
}

class Named extends HasName {
  import scala.util.Random
  private val id = Random.nextInt(1024)

  def name(): String = "name-" + id
}

Java

copysourceinterface HasName {
  String name();
}

class Named implements HasName {
  private int id = new Random().nextInt(1024);

  @Override
  public String name() {
    return "name-" + id;
  }
}

In order to round robin among a few instances of such actors, you can create a plain untyped router, and then facade it with a TypedActor like shown in the example below. This works because typed actors communicate using the same mechanisms as normal actors, and methods calls on them get transformed into message sends of MethodCall messages.

Scala

copysourcedef namedActor(): HasName = TypedActor(system).typedActorOf(TypedProps[Named]())

// prepare routees
val routees: List[HasName] = List.fill(5) { namedActor() }
val routeePaths = routees.map { r =>
  TypedActor(system).getActorRefFor(r).path.toStringWithoutAddress
}

// prepare untyped router
val router: ActorRef = system.actorOf(RoundRobinGroup(routeePaths).props())

// prepare typed proxy, forwarding MethodCall messages to `router`
val typedRouter: HasName =
  TypedActor(system).typedActorOf(TypedProps[Named](), actorRef = router)

println("actor was: " + typedRouter.name()) // name-184
println("actor was: " + typedRouter.name()) // name-753
println("actor was: " + typedRouter.name()) // name-320
println("actor was: " + typedRouter.name()) // name-164

Java

copysource// prepare routees
TypedActorExtension typed = TypedActor.get(system);

Named named1 = typed.typedActorOf(new TypedProps<Named>(Named.class));

Named named2 = typed.typedActorOf(new TypedProps<Named>(Named.class));

List<Named> routees = new ArrayList<Named>();
routees.add(named1);
routees.add(named2);

List<String> routeePaths = new ArrayList<String>();
routeePaths.add(typed.getActorRefFor(named1).path().toStringWithoutAddress());
routeePaths.add(typed.getActorRefFor(named2).path().toStringWithoutAddress());

// prepare untyped router
ActorRef router = system.actorOf(new RoundRobinGroup(routeePaths).props(), "router");

// prepare typed proxy, forwarding MethodCall messages to `router`
Named typedRouter = typed.typedActorOf(new TypedProps<Named>(Named.class), router);

System.out.println("actor was: " + typedRouter.name()); // name-243
System.out.println("actor was: " + typedRouter.name()); // name-614
System.out.println("actor was: " + typedRouter.name()); // name-243
System.out.println("actor was: " + typedRouter.name()); // name-614