ScalaAkka

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 which 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 is convenient if you only need to start a few actors without bothering to set-up your own actor system.

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

val myActor = Akka.system.actorOf(Props[MyActor], name = "myactor")

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 it to a Play Promise using the implicit conversion provided in play.api.libs.concurrent._:

import play.api.Play.current
import akka.pattern.ask
import akka.util.Timeout
import akka.util.duration._
import play.api.libs.concurrent._

def index = Action {
  Async {
    implicit  val timeout= Timeout(5.seconds)
    (myActor ? "hello").mapTo[String].asPromise.map { response =>
      Ok(response)      
    }    
  }
}

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:

def index = Action {
  Async {
    Akka.future { longComputation() }.map { result =>
      Ok("Got " + result)    
    }    
  }
}

Scheduling asynchronous tasks

You can schedule sending messages to actors and executing tasks (functions or Runnable). 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(0 seconds, 30 minutes, testActor, "tick")

Note: This example uses implicit conversions defined in akka.util.duration to convert numbers to Duration objects with various time units.

Similarly, to run a block of code ten seconds from now:

Akka.system.scheduler.scheduleOnce(10 seconds) {
  file.delete()
}

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