What is the proper way to chain Akka Actor Ask calls?

Question

I'm using the Play Framework with scala. I'm new to scala, akka, and play.

This is my actor system. I'm not sure I'm doing this right, but I have 2 routers. 1 for Actor A, and 1 for Actor B:

  val system = ActorSystem("ActionSystem")
  val actorARouter = system.actorOf(Props[ActionParser].withRouter(
    SmallestMailboxRouter(Runtime.getRuntime.availableProcessors())), name = "actorARouter")
  val actorBRouter = system.actorOf(Props[ActionDispatcher].withRouter(
    SmallestMailboxRouter(Runtime.getRuntime.availableProcessors())), name = "actorBRouter")

This is the current setup that I have: The play framework provides a Controller for me that receives a http rest call with some json. Whenever the Controller receives a rest call, I do an ask sends the json to a router for Actor A. Here is what that looks like:

(actorARouter ? request.body.asJson.get).map {
      case m: controllers.HttpMessages.OK => Ok(m.body)
      case m: controllers.HttpMessages.HttpResponse => Status(m.status)(m.body)
    }

Actor A then parses the json into a Seq of objects and then sends them via an ask to Actor B. Actor B is supposed to eventually process those by sending them to other actors, but for now is just returning generic responses.

The generic responses are being received back by ActorA via the future, then being parsed to JSON and then returned to the Controller via an OK response... or at least that's what is supposed to happen.

What's happening: So what's happening is the controller sends to ActorA, ActorA sends to ActorB. ActorB sends generic responses to ActorA. ActorA parses generic responses into JSON and tries to do sender ! OK(json) but I get a message in the console saying it wasn't delivered as it's a "dead letter". when I debug into it when I look at sender, sender is a reference to the actor akka://ActionSystem/deadLetters

My questions:

Obviously I'm doing something wrong. Maybe I shouldn't be chaining these actors responses together like this. Again, I mentioned I only had plans to further this by having ActorB send out requests to other actors.
When I do an ask in an actor, that doesn't hog that thread and stop it from processing other messages while it's waiting for a response does it?

EDIT: I found out I can save a reference to the sender for later use, and then send to that, and that seems to fix the dead letter problem. But I'm still very uncertain if this is the right way to be doing things. It feels like every time I'm adding another layer of actors 10's of milliseconds are being added onto my response time. Perhaps that's due to other factors though.

pushy pushy · Accepted Answer · 2014-04-02T10:12:35

Without looking at your code, I cannot really comment on what caused the dead letter, from your edit I guess you closed over sender() instead of assigning it to a variable and closing over that.

To respond to your questions:

It is much easier to construct message flows with actors if you only use fire-and-forget messages. The ask Pattern is useful in some cases, but most of the time you should try to avoid it. What you can do instead is to pass the original sender along through your actors by using forward instead of tell. This way a response can be generated by the last actor in your message flow. The first actor only needs the code to handle the response, and does not need to care about generating the response. Nice separation of concerns right there. If you need to aggregate several responses in order to send out a single response afterwards, you can also use a temporary actor that all other actors will send their response to, and that knows the orignal sender. Temporary actors need to be stopped after doing their work.
As far as I know the ask pattern is asynchronous and uses temporary actors internally. However, if you wait for the result of the future in your actor, that will block that actor, and it will not be able to process further messages. A nice method to use the ask pattern is in combination with the pipeTo Pattern, which you can use to send the result of ask to an actor (usally self)

What is the proper way to chain Akka Actor Ask calls?

1 Answers