§Integrating with Akka

Akka uses the Actor Model to raise the abstraction level and provide a better platform to build correct concurrent and scalable applications. For fault-tolerance it adopts the ‘Let it crash’ model, which has been used with great success in the telecoms industry to build applications that self-heal - systems that never stop. Actors also provide the abstraction for transparent distribution and the basis for truly scalable and fault-tolerant applications.

§The application actor system

Akka 2.0 can work with several containers called ActorSystems. An actor system manages the resources it is configured to use in order to run the actors it contains.

A Play application defines a special actor system to be used by the application. This actor system follows the application life-cycle and restarts automatically when the application restarts.

Note: Nothing prevents you from using another actor system from within a Play application. The provided default actor system is just a convenient way to start a few actors without having to set-up your own.

You can access the default application actor system using the play.libs.Akka helper:

ActorRef myActor = Akka.system().actorOf(new Props(MyActor.class));

§Configuration

The default actor system configuration is read from the Play application configuration file. For example to configure the default dispatcher of the application actor system, add these lines to the conf/application.conf file:

akka.default-dispatcher.fork-join-executor.pool-size-max = 64
akka.actor.debug.receive = on

Note: You can also configure any other actor system from the same file, just provide a top configuration key.

§Converting Akka Future to Play Promise

When you interact asynchronously with an Akka actor you will get a Future object. You can easily convert a Future to a Play Promise by calling the play.libs.F.Promise.wrap() method:

import akka.actor.*;
import play.mvc.*;
import play.libs.Akka;
import play.libs.F.Function;
import play.libs.F.Promise;
import static akka.pattern.Patterns.ask;

public class Application extends Controller {

    public static Result index() {
        ActorRef myActor = Akka.system().actorFor("user/my-actor");
        return async(
            Promise.wrap(ask(myActor, "hello", 1000)).map(
                new Function<Object, Result>() {
                    public Result apply(Object response) {
                        return ok(response.toString());
                    }
                }
            )
        );
    }
}

§Executing a block of code asynchronously

A common use case within Akka is to have some computation performed concurrently without needing the extra utility of an Actor. If you find yourself creating a pool of Actors for the sole reason of performing a calculation in parallel, there is an easier (and faster) way:

import play.libs.F.*;
import play.mvc.*;
import java.util.concurrent.Callable;

import static play.libs.F.Promise.promise;

public class Application extends Controller {
    public static Result index() {
        return async(
            promise(new Function0<Integer>() {
                public Integer apply() {
                    return longComputation();
                }
            }).map(new Function<Integer,Result>() {
                public Result apply(Integer i) {
                return ok("Got " + i);
                }
            })
        );
    }
    public static int longComputation() { return 2;}
}

§Scheduling asynchronous tasks

You can schedule sending messages to actors and executing tasks (functions or Runnable instances). You will get a Cancellable back that you can call cancel on to cancel the execution of the scheduled operation.

For example, to send a message to the testActor every 30 minutes:

Akka.system().scheduler().schedule(
        Duration.create(0, TimeUnit.MILLISECONDS), //Initial delay 0 milliseconds
        Duration.create(30, TimeUnit.MINUTES),     //Frequency 30 minutes
        testActor,
        "tick",
        Akka.system().dispatcher(),
        null
);

Alternatively, to run a block of code ten milliseconds from now:

Akka.system().scheduler().scheduleOnce(
        Duration.create(10, TimeUnit.MILLISECONDS),
        new Runnable() {
            public void run() {
                file.delete();
            }
        },
        Akka.system().dispatcher()
);

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