Scala Cookbook

13.8. Monitoring the Death of an Actor with watch

Problem

You want an actor to be notified when another actor dies.

Solution

Use the watch method of an actor’s context object to declare that the actor should be notified when an actor it’s monitoring is stopped.

In the following code snippet, the Parent actor creates an actor instance named kenny, and then declares that it wants to “watch” kenny:

class Parent extends Actor {
  val kenny = context.actorOf(Props[Kenny], name = "Kenny")
  context.watch(kenny)
  // more code here ...

(Technically, kenny is an ActorRef instance, but it’s simpler to say “actor.”)

If kenny is killed or stopped, the Parent actor is sent a Terminated(kenny) message. This complete example demonstrates the approach:

package actortests.deathwatch

import akka.actor._

class Kenny extends Actor {
  def receive = {
    case _ => println("Kenny received a message")
  }
}

class Parent extends Actor {
  // start Kenny as a child, then keep an eye on it
  val kenny = context.actorOf(Props[Kenny], name = "Kenny")
  context.watch(kenny)

  def receive = {
    case Terminated(kenny) => println("OMG, they killed Kenny")
    case _ => println("Parent received a message")
  }
}

object DeathWatchTest extends App {

  // create the ActorSystem instance
  val system = ActorSystem("DeathWatchTest")

  // create the Parent that will create Kenny
  val parent = system.actorOf(Props[Parent], name = "Parent")

  // lookup kenny, then kill it
  val kenny = system.actorSelection("/user/Parent/Kenny")
  kenny ! PoisonPill

  Thread.sleep(5000 ...