当发送消息后父级停止时，从父级参与者到子级参与者的消息上的死信

Question

考虑下面的代码示例（版本1）。在这里，父角色（ActorA）向子角色（ActorB）发送消息，然后停止自我。由于父角色的自停止，在高负载下，子角色甚至在从邮箱提取邮件之前就会死亡，因此邮件变成了“死信”（参见下面的示例输出1）。

由于某些原因，我无法修改应用程序设计以删除父参与者的自停止。

版本1

import akka.actor.Actor
import akka.actor.OneForOneStrategy
import akka.actor.SupervisorStrategy.Stop
import akka.actor.Props
import akka.actor.ActorSystem

object AkkaTest extends App {

  val system = ActorSystem("AkkaTest")

  for (i <- 1 to 5) {
    system.actorOf(Props[ActorA]) ! i  
  }

}

class ActorA extends Actor {

  def receive = {
    case i: Int => { 
      context.actorOf(Props[ActorB]) ! i
      context.stop(self)
    }
  }

  override def postStop = println("ActorA - stopped")

}

class ActorB extends Actor {

  def receive = {
    case i: Int => {
      println("ActorB - processing msg - " + i)
    }
  }

  override def postStop = println("ActorB - stopped")

}

样本输出1

ActorB - processing msg - 2
ActorB - processing msg - 1
ActorB - stopped
ActorB - stopped
ActorB - stopped
ActorB - stopped
ActorB - processing msg - 3
ActorB - stopped
ActorA - stopped
ActorA - stopped
[INFO] [09/09/2014 08:26:56.101] [AkkaTest-akka.actor.default-dispatcher-6] [akka://AkkaTest/user/$e/$a] Message [java.lang.Integer] from Actor[akka://AkkaTest/user/$e#-289783076] to Actor[akka://AkkaTest/user/$e/$a#-86921027] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.
[INFO] [09/09/2014 08:26:56.101] [AkkaTest-akka.actor.default-dispatcher-3] [akka://AkkaTest/user/$d/$a] Message [java.lang.Integer] from Actor[akka://AkkaTest/user/$d#-1255514179] to Actor[akka://AkkaTest/user/$d/$a#402128903] 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'.
ActorA - stopped
ActorA - stopped
ActorA - stopped

现在考虑下面的代码修改（版本2）。通过将消息确认从子参与者引入到父参与者，然后在收到此确认时自动停止父参与者，可以消除死信（请参阅示例输出2）。

版本2

import akka.actor.Actor
import akka.actor.OneForOneStrategy
import akka.actor.SupervisorStrategy.Stop
import akka.actor.Props
import akka.actor.ActorSystem

object AkkaTest extends App {

  val system = ActorSystem("AkkaTest")

  for (i <- 1 to 5) {
    system.actorOf(Props[ActorA]) ! i  
  }

}

class ActorA extends Actor {

  def receive = {
    case i: Int => context.actorOf(Props[ActorB]) ! i
    case m: MsgAck => context.stop(self) 
  }

  override def postStop = println("ActorA - stopped")

}

class ActorB extends Actor {

  def receive = {
    case i: Int => {
      sender ! MsgAck()
      println("ActorB - processing msg - " + i)
    }
  }

  override def postStop = println("ActorB - stopped")

}

case class MsgAck()

样本输出2

ActorB - processing msg - 4
ActorB - processing msg - 2
ActorB - processing msg - 5
ActorB - processing msg - 3
ActorB - processing msg - 1
ActorB - stopped
ActorB - stopped
ActorB - stopped
ActorB - stopped
ActorB - stopped
ActorA - stopped
ActorA - stopped
ActorA - stopped
ActorA - stopped
ActorA - stopped

现在我的问题是，有没有其他方法可以达到同样的效果？我的意思是扔掉死信。

Answer 1

当一个父级被停止时，它的所有子级都将被停止。这就是死信的原因。确保没有死信的唯一方法是确保父母活着，直到孩子收到消息。除了ack消息之外，我想不出任何其他方法。