Scala Cookbook

13.10. Sending a Message to an Actor and Waiting for a Reply

Problem

You have one actor that needs to ask another actor for some information, and needs an immediate reply. (The first actor can’t continue without the information from the second actor.)

Solution

Use the ? or ask methods to send a message to an Akka actor and wait for a reply, as demonstrated in the following example:

import akka.actor._
import akka.pattern.ask
import akka.util.Timeout
import scala.concurrent.{Await, ExecutionContext, Future}
import scala.concurrent.duration._
import scala.language.postfixOps

case object AskNameMessage

class TestActor extends Actor {
  def receive = {
    case AskNameMessage => // respond to the 'ask' request
                           sender ! "Fred"
    case _ => println("that was unexpected")
  }
}

object AskTest extends App {

  // create the system and actor
  val system = ActorSystem("AskTestSystem")
  val myActor = system.actorOf(Props[TestActor], name = "myActor")

  // (1) this is one way to "ask" another actor for information
  implicit val timeout = Timeout(5 seconds)
  val future = myActor ? AskNameMessage
  val result = Await.result(future, timeout.duration).asInstanceOf[String]
  println(result)
  // (2) a slightly different way to ask another actor for information
  val future2: Future[String] = ask(myActor, AskNameMessage).mapTo[String]
  val result2 = Await.result(future2, 1 second)
  println(result2)

  system.shutdown

}

Discussion

Both the ? or ask methods use the Future and Await.result approach demonstrated in Recipe 13.9. The recipe ...