BackoffSuperVisorApp.scala

package my.akka

import akka.actor.{Actor, ActorRef, ActorSystem, PoisonPill, Props}
import akka.pattern.{Backoff, BackoffSupervisor, ask}

import scala.concurrent.Await
import scala.concurrent.duration._


class Child extends Actor {
  println(s"[Child]: created.         (path = ${this.self.path}, instance = ${this})")

  override def preStart(): Unit = {
    println(s"[Child]: preStart called. (path = ${this.self.path}, instance = ${this})")
    super.preStart()
  }

  override def postStop(): Unit = {
    println(s"[Child]: postStop called. (path = ${this.self.path}, instance = ${this})")
    super.postStop()
  }

  override def preRestart(reason: Throwable, message: Option[Any]): Unit = {
    println(s"[Child]: preRestart called with ($reason, $message). (path = ${this.self.path}, instance = ${this})")
    super.preRestart(reason, message)
  }

  override def postRestart(reason: Throwable): Unit = {
    println(s"[Child]: postRestart called with ($reason). (path = ${this.self.path}, instance = ${this})")
    super.postRestart(reason)
  }

  def receive = {
    case "boom" =>
      throw new Exception("kaboom")
    case "get ref" =>
      sender() ! self
    case a: Any =>
      println(s"[Child]: received ${a}")
  }
}

object Child {
  def props: Props
    = Props(new Child)

  def backOffOnFailureProps: Props
    = BackoffSupervisor.props(
    Backoff.onFailure(
      Child.props,
      childName = "myEcho",
      minBackoff = 1.seconds,
      maxBackoff = 30.seconds,
      randomFactor = 0.2 // adds 20% "noise" to vary the intervals slightly
    ))

  def backOffOnStopProps: Props
  = BackoffSupervisor.props(
    Backoff.onStop(
      Child.props,
      childName = "myEcho",
      minBackoff = 1.seconds,
      maxBackoff = 10.seconds,
      randomFactor = 0.2 // adds 20% "noise" to vary the intervals slightly
    ))
}

object BackoffSuperVisorApp {
  def defaultSuperVisorCase(): Unit = {
    println(
      """
        |default ---------------------------
      """.stripMargin)

    val system = ActorSystem("app")
    try{
      /**
       * Let's see if "hello" message is received by the child
       */
      val child = system.actorOf(Child.props, "child")
      Thread.sleep(100)
      child ! "hello"
      //[Child]: received hello

      /**
       * Now restart the child with an exception within its receive method
       * and see if the `child` ActorRef is still valid (i.e. ActorRef incarnation remains same)
       */
      child ! "boom"
      Thread.sleep(200)

      child ! "hello after normal exception"
      //[Child]: received hello after normal exception

      /**
       * PoisonPill causes the child actor to `Stop`, different from restart.
       * The ActorRef incarnation gets updated.
       */
      child ! PoisonPill
      Thread.sleep(200)

      /**
       * This causes delivery to deadLetter, since the "incarnation" of ActorRef `child` became obsolete
       * after child is "Stopped"
       *
       * An incarnation is tied to an ActorRef (NOT to its internal actor instance)
       * and the same incarnation means "you can keep using the same ActorRef"
       */
      child ! "hello after PoisonPill"
      // [akka://app/user/parent/child-1] Message [java.lang.String] without sender to Actor[akka://app/user/child#-767539042]
      //   was not delivered. [1] dead letters encountered.

      Thread.sleep(200)
    }
    finally{
      system.terminate()
      Thread.sleep(500)
    }
  }

  def backOffOnStopCase(): Unit ={
    println(
      """
        |backoff onStop ---------------------------
      """.stripMargin)

    val system = ActorSystem("app")
    try{
      /**
       * Let's see if "hello" message is forwarded to the child
       * by the backoff supervisor onStop
       */
      implicit val futureTimeout: akka.util.Timeout = 1.second
      val backoffSupervisorActor = system.actorOf(Child.backOffOnStopProps, "child")
      Thread.sleep(100)

      backoffSupervisorActor ! "hello to backoff supervisor" //forwarded to child
      //[Child]: received hello to backoff supervisor

      /**
       * Now "Restart" the child with an exception from its receive method.
       * As with the default supervisory strategy, the `child` ActorRef remains valid. (i.e. incarnation kept same)
       */
      val child = Await.result(backoffSupervisorActor ? "get ref", 1.second).asInstanceOf[ActorRef]
      child ! "boom"
      Thread.sleep(2000)

      child ! "hello to child after normal exception"
      //[Child]: received hello to child after normal exception

      /**
       * Backoff Supervisor can still forward the message
       */
      backoffSupervisorActor ! "hello to backoffSupervisorActor after normal exception"
      //[Child]: received hello to backoffSupervisorActor after normal exception

      Thread.sleep(200)

      /**
       * PoisonPill causes the child actor to `Stop`, different from restart.
       * The `child` ActorRef incarnation gets updated.
       */
      child ! PoisonPill
      Thread.sleep(2000)

      child ! "hello to child ref after PoisonPill"
      //delivered to deadLetters

      /**
       * Backoff Supervisor can forward the message to its child with the new incarnation
       */
      backoffSupervisorActor ! "hello to backoffSupervisorActor after PoisonPill"
      //[Child]: received hello to backoffSupervisorActor after PoisonPill

      Thread.sleep(200)
    }
    finally{
      system.terminate()
      Thread.sleep(500)
    }
  }

  def backOffOnFailureCase(): Unit ={
    println(
      """
        |backoff onFailure ---------------------------
      """.stripMargin)

    val system = ActorSystem("app")
    try{
      /**
       * Let's see if "hello" message is forwarded to the child
       * by the backoff supervisor onFailure
       */
      implicit val futureTimeout: akka.util.Timeout = 1.second
      val backoffSupervisorActor = system.actorOf(Child.backOffOnFailureProps, "child")
      Thread.sleep(100)

      backoffSupervisorActor ! "hello to backoff supervisor" //forwarded to child
      //[Child]: received hello to backoff supervisor

      /**
       * Now "Stop" the child with an exception from its receive method.
       * You'll see the difference between "Restart" and "Stop" from here:
       */
      val child = Await.result(backoffSupervisorActor ? "get ref", 1.second).asInstanceOf[ActorRef]
      child ! "boom"
      Thread.sleep(2000)

      /**
       * Note that this is after normal exception, not after PoisonPill,
       * but child is completely "Stopped" and its ActorRef "incarnation" became obsolete
       *
       * So, the message to the `child` ActorRef is delivered to deadLetters
       */
      child ! "hello to child after normal exception"
      //causes delivery to deadLetter

      /**
       * Backoff Supervisor can still forward the message to the new child ActorRef incarnation
       */
      backoffSupervisorActor ! "hello to backoffSupervisorActor after normal exception"
      //[Child]: received hello to backoffSupervisorActor after normal exception

      /**
       * You can get a new ActorRef which represents the new incarnation
       */
      val newChildRef = Await.result(backoffSupervisorActor ? "get ref", 1.second).asInstanceOf[ActorRef]
      newChildRef ! "hello to new child ref after normal exception"
      //[Child]: received hello to new child ref after normal exception

      Thread.sleep(200)

      /**
       * No matter whether the supervisory strategy is default or backoff,
       * PoisonPill causes the actor to "Stop", not "Restart"
       */
      newChildRef ! PoisonPill
      Thread.sleep(3000)

      newChildRef ! "hello to new child ref after PoisonPill"
      //delivered to deadLetters

      Thread.sleep(200)
    }
    finally{
      system.terminate()
      Thread.sleep(500)
    }
  }

  def main(args: Array[String]): Unit ={
    defaultSuperVisorCase()
    backOffOnStopCase()
    backOffOnFailureCase()
  }
}

output.md

default ---------------------------

[Child]: created.         (path = akka://app/user/child, instance = my.akka.Child@1456efca)
[Child]: preStart called. (path = akka://app/user/child, instance = my.akka.Child@1456efca)
[Child]: received hello
[Child]: preRestart called with (java.lang.Exception: kaboom, Some(boom)). (path = akka://app/user/child, instance = my.akka.Child@1456efca)
[Child]: postStop called. (path = akka://app/user/child, instance = my.akka.Child@1456efca)
[ERROR] [09/18/2017 02:17:30.555] [app-akka.actor.default-dispatcher-2] [akka://app/user/child] kaboom
java.lang.Exception: kaboom
        at my.akka.Child$$anonfun$receive$1.applyOrElse(BackoffSuperVisorApp.scala:35)
        at akka.actor.Actor.aroundReceive(Actor.scala:514)
        at akka.actor.Actor.aroundReceive$(Actor.scala:512)
        at my.akka.Child.aroundReceive(BackoffSuperVisorApp.scala:10)
        at akka.actor.ActorCell.receiveMessage(ActorCell.scala:527)
        at akka.actor.ActorCell.invoke(ActorCell.scala:496)
        at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)
        at akka.dispatch.Mailbox.run(Mailbox.scala:224)
        at akka.dispatch.Mailbox.exec(Mailbox.scala:234)
        at akka.dispatch.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
        at akka.dispatch.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
        at akka.dispatch.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
        at akka.dispatch.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)

[Child]: created.         (path = akka://app/user/child, instance = my.akka.Child@5ec2b9c5)
[Child]: postRestart called with (java.lang.Exception: kaboom). (path = akka://app/user/child, instance = my.akka.Child@5ec2b9c5)
[Child]: preStart called. (path = akka://app/user/child, instance = my.akka.Child@5ec2b9c5)
[Child]: received hello after normal exception
[Child]: postStop called. (path = akka://app/user/child, instance = my.akka.Child@5ec2b9c5)
[INFO] [09/18/2017 02:17:30.955] [app-akka.actor.default-dispatcher-3] [akka://app/user/child] Message [java.lang.String] without sender to Actor[akka://app/user/child#-1104979952] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.

You can see that preRestart and postRestart ARE called with Backoff.onStop.

backoff onStop ---------------------------

[Child]: created.         (path = akka://app/user/child/myEcho, instance = my.akka.Child@6bfb3e83)
[Child]: preStart called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@6bfb3e83)
[Child]: received hello to backoff supervisor
[Child]: preRestart called with (java.lang.Exception: kaboom, Some(boom)). (path = akka://app/user/child/myEcho, instance = my.akka.Child@6bfb3e83)
[ERROR] [09/18/2017 02:17:31.855] [app-akka.actor.default-dispatcher-6] [akka://app/user/child/myEcho] kaboom
java.lang.Exception: kaboom
        at my.akka.Child$$anonfun$receive$1.applyOrElse(BackoffSuperVisorApp.scala:35)
        at akka.actor.Actor.aroundReceive(Actor.scala:514)
        at akka.actor.Actor.aroundReceive$(Actor.scala:512)
        at my.akka.Child.aroundReceive(BackoffSuperVisorApp.scala:10)
        at akka.actor.ActorCell.receiveMessage(ActorCell.scala:527)
        at akka.actor.ActorCell.invoke(ActorCell.scala:496)
        at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)
        at akka.dispatch.Mailbox.run(Mailbox.scala:224)
        at akka.dispatch.Mailbox.exec(Mailbox.scala:234)
        at akka.dispatch.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
        at akka.dispatch.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
        at akka.dispatch.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
        at akka.dispatch.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)

[Child]: postStop called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@6bfb3e83)
[Child]: created.         (path = akka://app/user/child/myEcho, instance = my.akka.Child@4d80e601)
[Child]: postRestart called with (java.lang.Exception: kaboom). (path = akka://app/user/child/myEcho, instance = my.akka.Child@4d80e601)
[Child]: preStart called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@4d80e601)
[Child]: received hello to child after normal exception
[Child]: received hello to backoffSupervisorActor after normal exception
[Child]: postStop called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@4d80e601)
[Child]: created.         (path = akka://app/user/child/myEcho, instance = my.akka.Child@6d3cd821)
[Child]: preStart called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@6d3cd821)
[Child]: received hello to backoffSupervisorActor after PoisonPill
[INFO] [09/18/2017 02:17:36.070] [app-akka.actor.default-dispatcher-7] [akka://app/user/child/myEcho] Message [java.lang.String] without sender to Actor[akka://app/user/child/myEcho#-1620028058] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.
[Child]: postStop called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@6d3cd821)

You can see that preRestart and postRestart are NOT called with Backoff.onFailure.

backoff onFailure ---------------------------

[Child]: created.         (path = akka://app/user/child/myEcho, instance = my.akka.Child@496dd7f5)
[Child]: preStart called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@496dd7f5)
[Child]: received hello to backoff supervisor
[Child]: postStop called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@496dd7f5)
[ERROR] [09/18/2017 02:17:36.938] [app-akka.actor.default-dispatcher-5] [akka://app/user/child/myEcho] kaboom
java.lang.Exception: kaboom
        at my.akka.Child$$anonfun$receive$1.applyOrElse(BackoffSuperVisorApp.scala:35)
        at akka.actor.Actor.aroundReceive(Actor.scala:514)
        at akka.actor.Actor.aroundReceive$(Actor.scala:512)
        at my.akka.Child.aroundReceive(BackoffSuperVisorApp.scala:10)
        at akka.actor.ActorCell.receiveMessage(ActorCell.scala:527)
        at akka.actor.ActorCell.invoke(ActorCell.scala:496)
        at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)
        at akka.dispatch.Mailbox.run(Mailbox.scala:224)
        at akka.dispatch.Mailbox.exec(Mailbox.scala:234)
        at akka.dispatch.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
        at akka.dispatch.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
        at akka.dispatch.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
        at akka.dispatch.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)

[Child]: created.         (path = akka://app/user/child/myEcho, instance = my.akka.Child@2f964777)
[Child]: preStart called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@2f964777)
[Child]: received hello to backoffSupervisorActor after normal exception
[INFO] [09/18/2017 02:17:38.952] [app-akka.actor.default-dispatcher-5] [akka://app/user/child/myEcho] Message [java.lang.String] without sender to Actor[akka://app/user/child/myEcho#-893737626] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.
[Child]: received hello to new child ref after normal exception
[Child]: postStop called. (path = akka://app/user/child/myEcho, instance = my.akka.Child@2f964777)
[INFO] [09/18/2017 02:17:42.182] [app-akka.actor.default-dispatcher-3] [akka://app/user/child/myEcho] Message [java.lang.String] without sender to Actor[akka://app/user/child/myEcho#2043667113] was not delivered. [2] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.