Akka Futures Exceptions

Question

What happens when an actor of a future throws an exception?

According to the Akka documentation at http://doc.akka.io/docs/akka/snapshot/scala/futures.html:

It doesn't matter if an Actor or the dispatcher is completing the Future, if an Exception is caught the Future will contain it instead of a valid result. If a Future does contain an Exception, calling Await.result will cause it to be thrown again so it can be handled properly.

I am not sure this is what I am seeing when running this piece of code:

  class Worker extends Actor {
    def receive = {
      case i: Int => throw new RuntimeException
    }         
  }

  implicit val system = ActorSystem("MySystem")
  val worker = system.actorOf(Props(new Worker), name="worker")
  implicit val timeout = Timeout(5 minutes)
  val future = worker ? 0
  val res = Await.result(future, 10 seconds)

According to the documentation, Await.result should throw the exception again, but what I am getting is a TimeoutException! Can someone clarify on this?

Answer 1

For actors you need to catch the exception and return it as a failure status. Right now you're not returning anything to the sender so you're getting a timeout exception:

class Worker extends Actor {
  def receive = {
    case i: Int => {
      try {
        throw new RuntimeException
        sender ! "Some good result"
      } catch {
        case e: Exception =>
          sender ! akka.actor.Status.Failure(e) // Alert the sender of the failure
          throw e // Alert any supervisor actor of the failure
      }
    }
  }
}

Futures can handle this a little more gracefully since they always send a result, while actors do not (this would give you the same result as above):

  val future = Future {
    throw new RuntimeException
  }