Question

I’m modeling a simple P2P with Scala and Akka:

class Node() extends Peer with Actor {

  var peers: List[ActorRef] = List()

  def receive = {
    case _register(peer: ActorRef, p: Option[Int]) => {
      println("registering [" + peer + "] for [" + this + "]")
      peers = peer :: peers
    }
  }

}

sealed case class _register(val peer: ActorRef, var p: Option[Int] = None)

and then a simple network:

class Network() extends Actor {

  def this(name: String) = {

    this()

    val system = ActorSystem(name)

    val s1 = system.actorOf(Props(new Node()), name = "s1")
    val s2 = system.actorOf(Props(new Node()), name = "s2")

    val c1 = system.actorOf(Props(new Node()), name = "c1")
    val c2 = system.actorOf(Props(new Node()), name = "c2")
    val c3 = system.actorOf(Props(new Node()), name = "c3")
    val c4 = system.actorOf(Props(new Node()), name = "c4")

    implicit val timeout = Timeout(5 second)

    s1 ? _register(c1)
    s1 ? _register(c2)
    s1 ? _register(c3)
    val lastRegistered = s2 ? _register(c4)
    Await.ready(lastRegistered, timeout.duration)

    println("initialized nodes")
  }
}

The output that I’m getting is always like:

registering [Actor[akka://p2p/user/c1]] for [nl.cwi.crisp.examples.p2p.scala.Node@14b5f4a]
registering [Actor[akka://p2p/user/c2]] for [nl.cwi.crisp.examples.p2p.scala.Node@14b5f4a]
registering [Actor[akka://p2p/user/c3]] for [nl.cwi.crisp.examples.p2p.scala.Node@14b5f4a]
registering [Actor[akka://p2p/user/c4]] for [nl.cwi.crisp.examples.p2p.scala.Node@13c0b53]
[ERROR] [04/10/2012 22:07:04.34] [main-akka.actor.default-dispatcher-1] [akka://main/user/p2p] error while creating actor
java.util.concurrent.TimeoutException: Futures timed out after [5000] milliseconds
    at akka.dispatch.DefaultPromise.ready(Future.scala:834)
    at akka.dispatch.DefaultPromise.ready(Future.scala:811)
    at akka.dispatch.Await$.ready(Future.scala:64)
    at nl.cwi.crisp.examples.p2p.scala.Network.<init>(Node.scala:136)
    at nl.cwi.crisp.examples.p2p.scala.Main$$anonfun$11.apply(Node.scala:164)
    at nl.cwi.crisp.examples.p2p.scala.Main$$anonfun$11.apply(Node.scala:164)
    at akka.actor.ActorCell.newActor(ActorCell.scala:488)
    at akka.actor.ActorCell.create$1(ActorCell.scala:506)
    at akka.actor.ActorCell.systemInvoke(ActorCell.scala:591)
    at akka.dispatch.Mailbox.processAllSystemMessages(Mailbox.scala:191)
    at akka.dispatch.Mailbox.run(Mailbox.scala:160)
    at akka.dispatch.ForkJoinExecutorConfigurator$MailboxExecutionTask.exec(AbstractDispatcher.scala:505)
    at akka.jsr166y.ForkJoinTask.doExec(ForkJoinTask.java:259)
    at akka.jsr166y.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:997)
    at akka.jsr166y.ForkJoinPool.runWorker(ForkJoinPool.java:1495)
    at akka.jsr166y.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:104)

I have followed the documentation of Futures on Akka reference documentation. Exchanging Await.ready with Await.result does not have any effect. The log shows that the last registration has been successful.

How should I fix this?

Answer 1

You are waiting for a message to be returned from the Node actor, but the Node actor does not send a message back to the sender actorRef, so the Future[Any] created by s1 ? _register will never receive a response, so the Future will never be complete. You could add sender ! something from inside the Node receive method to send a response, I’m not sure what something makes sense in this case.