Scala: Conditional Actor Chaining

Question

I have two actors A and B. A Controller sends a request to actor A. Now actor A return a response of type Try[AbcResponse] . AbcResponse being a case class here. Actor A based on some logic might directly return this response or it might conditionally call another actor B using ask. After manipulating the response from B it would then send the response of type Try[AbcResponse] to the controller.

So what should i do in my actor A to handle this situation. I do not want to put a await in my actor A because that would waste the thread pool and cause slowdowns in the system. How can i efficiently handle this?

Answer 1

You could pass the sender reference in a message to actor B and pipe the response from actor B to self . Obviously actor B in its response would have to pass this reference back to actor A.

import akka.pattern.{ask, pipe}

case class MsgToActorB(..., target: ActorRef)
case class ResponseFromActorB(..., target: ActorRef)

class ActorA extends Actor {
  def receive = {
    case r: Request =>
      val s = sender
      implicit val timeout = Timeout(5 seconds)
      // do something with the request
      if (someCondition)
        s ! Try(AbcResponse(...))
      else
        (actorB ? MsgToActorB(..., s)).mapTo[ResponseFromActorB].pipeTo(self)

    case ResponseFromActorB(..., target) =>
      // do something with the response from B and send a response to the original sender
      target ! Try(AbcResponse(...))
  }
}

While the above approach is safe, it'd be simpler to not use ask as shown below. If you must use ask and if actor B is blocking when it processes a message from actor A, then consider configuring a separate dispatcher as described here .

def receive = {
  case r: Request =>
    val s = sender
    // do something with the request
    if (someCondition)
      s ! Try(AbcResponse(...))
    else
      actorB ! MsgToActorB(..., s)

  case ResponseFromActorB(..., target) =>
    // do something with the response from B and send a response to the original sender
    target ! Try(AbcResponse(...))
}