[翻译]AKKA笔记 - DEATHWATCH -7

当我们说Actor生命周期的时候,我们能看到Actor能被很多种方式停掉（用ActorSystem.stop或ActorContext.stop或发送一个PoisonPill - 也有一个kill和gracefulstop）。

无论Actor是怎么死的，有些情况一些系统中的其他actor想要知道。让我们举一个Actor与数据库交互的例子 - 我们叫它RepositoryActor。很明显，会有一些其他actor会向这个RepositoryActor发送消息。这些有“兴趣”的Actor很愿意留个eye或者看（watch）这个actor关闭时的消息。这个在Actor里面叫DeathWatch。这个用来watch和unwatch的方法就是ActorContext.watch和ActorContext.unwatch。如果已经监视了，这些watcher会在Actor关闭时收到一个Terminated消息，并可以很舒服的加到他们的receive功能中。

不像Supervision有一个严格的父子继承关系，任何Actor都可以watch任何ActorSystem中的Actor。

让我们看下。

代码

QUOTEREPOSITORYACTOR

1.我们的QueryRepositoryActor格言查询Actor保存了一个quote的列表并且在收到一个QuoteRepositoryRequest时随机返回一条。

1. 他记录了收到消息的个数，如果收到超过3个消息，他用PoisonPill把自己杀掉

这没啥神奇的。

package me.rerun.akkanotes.deathwatch

import akka.actor.{PoisonPill, Actor, ActorLogging, actorRef2Scala}
import me.rerun.akkanotes.protocols.QuoteRepositoryProtocol._
import scala.util.Random

class QuoteRepositoryActor() extends Actor with ActorLogging {

  val quotes = List(
    "Moderation is for cowards",
    "Anything worth doing is worth overdoing",
    "The trouble is you think you have time",
    "You never gonna know if you never even try")

  var repoRequestCount:Int=1

  def receive = {

    case QuoteRepositoryRequest => {

      if (repoRequestCount>3){
        self!PoisonPill
      }
      else {
        //Get a random Quote from the list and construct a response
        val quoteResponse = QuoteRepositoryResponse(quotes(Random.nextInt(quotes.size)))

        log.info(s"QuoteRequest received in QuoteRepositoryActor. Sending response to Teacher Actor $quoteResponse")
        repoRequestCount=repoRequestCount+1
        sender ! quoteResponse
      }

    }

  }

}

TEACHERACTORWATCHER

一样的，TeacherActorWatcher也没啥神奇的，除了他创建了一个QuoteRepositoryActor并且用context.watch观察。

package me.rerun.akkanotes.deathwatch

import akka.actor.{Terminated, Props, Actor, ActorLogging}
import me.rerun.akkanotes.protocols.TeacherProtocol.QuoteRequest
import me.rerun.akkanotes.protocols.QuoteRepositoryProtocol.QuoteRepositoryRequest

class TeacherActorWatcher extends Actor with ActorLogging {

  val quoteRepositoryActor=context.actorOf(Props[QuoteRepositoryActor], "quoteRepositoryActor")
  context.watch(quoteRepositoryActor)

  def receive = {
    case QuoteRequest => {
      quoteRepositoryActor ! QuoteRepositoryRequest
    }
    case Terminated(terminatedActorRef)=>{
      log.error(s"Child Actor {$terminatedActorRef} Terminated")
    }
  }
}

测试CASE

这里会有点意思。我从来没想过这个可以被测试。akka-testkit。我们会分析下这三个测试CASE：

1. 断言如果观察到已经收到Terminated消息

QuoteRepositoryActor应该在收到第四条消息时给测试case发送一条Terminated消息。前三条应该是可以的。

"A QuoteRepositoryActor" must {
    ...
    ...
    ...

    "send back a termination message to the watcher on 4th message" in {
      val quoteRepository=TestActorRef[QuoteRepositoryActor]

      val testProbe=TestProbe()
      testProbe.watch(quoteRepository) //Let's watch the Actor

      within (1000 millis) {
        var receivedQuotes = List[String]()
        (1 to 3).foreach(_ => quoteRepository ! QuoteRepositoryRequest)
        receiveWhile() {
          case QuoteRepositoryResponse(quoteString) => {
            receivedQuotes = receivedQuotes :+ quoteString
          }
        }

        receivedQuotes.size must be (3)
        println(s"receiveCount ${receivedQuotes.size}")

        //4th message
        quoteRepository!QuoteRepositoryRequest
        testProbe.expectTerminated(quoteRepository)  //Expect a Terminated Message
      }
    }

2.如果没有观察（watched/unwatched）到则断言没收到Terminated消息

事实上，我们做这个只是演示下context.unwatch。如果我们移掉testProbe.watch和testProbe.unwatch这行，则测试case会运行的很正常。

    "not send back a termination message on 4th message if not watched" in {
      val quoteRepository=TestActorRef[QuoteRepositoryActor]

      val testProbe=TestProbe()
      testProbe.watch(quoteRepository) //watching

      within (1000 millis) {
        var receivedQuotes = List[String]()
        (1 to 3).foreach(_ => quoteRepository ! QuoteRepositoryRequest)
        receiveWhile() {
          case QuoteRepositoryResponse(quoteString) => {
            receivedQuotes = receivedQuotes :+ quoteString
          }
        }

        testProbe.unwatch(quoteRepository) //not watching anymore
        receivedQuotes.size must be (3)
        println(s"receiveCount ${receivedQuotes.size}")

        //4th message
        quoteRepository!QuoteRepositoryRequest
        testProbe.expectNoMsg() //Not Watching. No Terminated Message
      }
    }

3. 在TeacherActorWatcher中断言收到了Terminated消息

我们订阅了EventStream并通过检查一个特殊的日志消息来断言termination。

   "end back a termination message to the watcher on 4th message to the TeacherActor" in {

      //This just subscribes to the EventFilter for messages. We have asserted all that we need against the QuoteRepositoryActor in the previous testcase
      val teacherActor=TestActorRef[TeacherActorWatcher]

      within (1000 millis) {
        (1 to 3).foreach (_=>teacherActor!QuoteRequest) //this sends a message to the QuoteRepositoryActor

        EventFilter.error (pattern="""Child Actor .* Terminated""", occurrences = 1).intercept{
          teacherActor!QuoteRequest //Send the dangerous 4th message
        }
      }
    }

EventFilter中的pattern属性，没啥奇怪的，需要一个正则表达式。正则pattern="""Child Actor .* Terminated"""用来匹配一条格式是Child Actor {Actor[akka://TestUniversityMessageSystem/user/$$d/quoteRepositoryActor#-1905987636]} Terminated日志信息。

Github###

与往常一样，代码在github。看下deathwatch的包。

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