Scala Cookbook

13.3. How to Communicate Between Actors

Problem

You’re building an actor-based application and want to send messages between actors.

Solution

Actors should be sent immutable messages with the ! method.

When an actor receives a message from another actor, it also receives an implicit reference named sender, and it can use that reference to send a message back to the originating actor.

The general syntax to send a message to an actor is:

actorInstance ! message

For example, if you have an actor instance named car, you can send it a start message like this:

car ! "start"

In this case, the message is the String literal start. The car actor should receive this message in a match expression in its receive method, and from there it can send a message back to whoever sent the start message. A simplified version of a receive method for car might look like this:

def receive = {
  case "start" =>
       val result = tryToStart()
       sender ! result
  case _ => // do nothing

}

As mentioned, the sender instance is implicitly made available to your actor. If you just want to send a message back to the code that sent you a message, that’s all you have to do.

Discussion

To demonstrate a more complicated example of actors communicating, the following code shows how to send messages back and forth between Akka actors. It was inspired by the “Ping Pong” threading example in the book by James Gosling et al., The Java Programming Language (Addison-Wesley Professional):

import akka.actor._

case object PingMessage
case object PongMessage ...