在上一篇讨论中我们谈到了监管:在Akka中就是一种直属父子监管树结构,父级Actor负责处理直属子级Actor产生的异常。当时我们把BackoffSupervisor作为父子监管方式的其中一种。实际上BackoffSupervisor与定义了supervisorStrategy的Actor有所不同。我们应该把BackoffSupervisor看作是一个一体化的Actor。当然,它的实现方式还是由一对父子Actor组成。监管策略(SupervisorStrategy)是在BackoffSupervisor的内部实现的。从外表上BackoffSupervisor就像是一个Actor,运算逻辑是在子级Actor中定义的,所谓的父级Actor除监管之外没有任何其它功能,我们甚至没有地方定义父级Actor的功能,它的唯一功能是转发收到的信息给子级,是嵌入BackoffSupervisor里的。所以我们虽然发送消息给BackoffSupervisor,但实际上是在与它的子级交流。我们看看下面这个例子:
package backoffSupervisorDemo import akka.actor._ import akka.pattern._ import backoffSupervisorDemo.InnerChild.TestMessage import scala.concurrent.duration._ object InnerChild { case class TestMessage(msg: String) class ChildException extends Exception def props = Props[InnerChild] } class InnerChild extends Actor with ActorLogging { import InnerChild._ override def receive: Receive = { case TestMessage(msg) => //模拟子级功能 log.info(s"Child received message: ${msg}") } } object Supervisor { def props: Props = { //在这里定义了监管策略和child Actor构建 def decider: PartialFunction[Throwable, SupervisorStrategy.Directive] = { case _: InnerChild.ChildException => SupervisorStrategy.Restart } val options = Backoff.onFailure(InnerChild.props, "innerChild", 1 second, 5 seconds, 0.0) .withManualReset .withSupervisorStrategy( OneForOneStrategy(maxNrOfRetries = 5, withinTimeRange = 5 seconds)( decider.orElse(SupervisorStrategy.defaultDecider) ) ) BackoffSupervisor.props(options) } } //注意:下面是Supervisor的父级,不是InnerChild的父级 object ParentalActor { case class SendToSupervisor(msg: InnerChild.TestMessage) case class SendToInnerChild(msg: InnerChild.TestMessage) case class SendToChildSelection(msg: InnerChild.TestMessage) def props = Props[ParentalActor] } class ParentalActor extends Actor with ActorLogging { import ParentalActor._ //在这里构建子级Actor supervisor val supervisor = context.actorOf(Supervisor.props,"supervisor") supervisor ! BackoffSupervisor.getCurrentChild //要求supervisor返回当前子级Actor var innerChild: Option[ActorRef] = None //返回的当前子级ActorRef val selectedChild = context.actorSelection("/user/parent/supervisor/innerChild") override def receive: Receive = { case BackoffSupervisor.CurrentChild(ref) => //收到子级Actor信息 innerChild = ref case SendToSupervisor(msg) => supervisor ! msg case SendToChildSelection(msg) => selectedChild ! msg case SendToInnerChild(msg) => innerChild foreach(child => child ! msg) } } object BackoffSupervisorDemo extends App { import ParentalActor._ val testSystem = ActorSystem("testSystem") val parent = testSystem.actorOf(ParentalActor.props,"parent") Thread.sleep(1000) //wait for BackoffSupervisor.CurrentChild(ref) received parent ! SendToSupervisor(TestMessage("Hello message 1 to supervisor")) parent ! SendToInnerChild(TestMessage("Hello message 2 to innerChild")) parent ! SendToChildSelection(TestMessage("Hello message 3 to selectedChild")) scala.io.StdIn.readLine() testSystem.terminate() }
在上面的例子里我们分别向supervisor,innerChild,selectedChild发送消息。但所有消息都是由InnerChild响应的,如下:
[INFO] [05/29/2017 16:11:48.167] [testSystem-akka.actor.default-dispatcher-2] [akka://testSystem/user/parent/supervisor/innerChild] Child received message: Hello message 1 to supervisor [INFO] [05/29/2017 16:11:48.177] [testSystem-akka.actor.default-dispatcher-2] [akka://testSystem/user/parent/supervisor/innerChild] Child received message: Hello message 2 to innerChild [INFO] [05/29/2017 16:11:48.179] [testSystem-akka.actor.default-dispatcher-3] [akka://testSystem/user/parent/supervisor/innerChild] Child received message: Hello message 3 to selectedChild
上面我们向supervisor发送了一个BackoffSupervisor.GetCurrentChild消息用来获取子级Actor。BackoffSupervisor是这样处理下面几个特殊消息的:
private[akka] trait HandleBackoff { this: Actor ? def childProps: Props def childName: String def reset: BackoffReset var child: Option[ActorRef] = None var restartCount = 0 import BackoffSupervisor._ import context.dispatcher override def preStart(): Unit = startChild() def startChild(): Unit = { if (child.isEmpty) { child = Some(context.watch(context.actorOf(childProps, childName))) } } def handleBackoff: Receive = { case StartChild ? startChild() reset match { case AutoReset(resetBackoff) ? val _ = context.system.scheduler.scheduleOnce(resetBackoff, self, ResetRestartCount(restartCount)) case _ ? // ignore } case Reset ? reset match { case ManualReset ? restartCount = 0 case msg ? unhandled(msg) } case ResetRestartCount(current) ? if (current == restartCount) { restartCount = 0 } case GetRestartCount ? sender() ! RestartCount(restartCount) case GetCurrentChild ? sender() ! CurrentChild(child) case msg if child.contains(sender()) ? // use the BackoffSupervisor as sender context.parent ! msg case msg ? child match { case Some(c) ? c.forward(msg) case None ? context.system.deadLetters.forward(msg) } } }
在handleBackoff函数里可以找到这些消息的处理方式。
在构建上面例子里的Supervisor的Props时定义了监管策略(SupervisorStrategy)对InnerChild产生的异常ChildException进行Restart处理。我们调整一下InnerChild代码来随机产生一些异常:
object InnerChild { case class TestMessage(msg: String) class ChildException(val errmsg: TestMessage) extends Exception object CException { //for pattern match of class with parameter def apply(msg: TestMessage) = new ChildException(msg) def unapply(cex: ChildException) = Some(cex.errmsg) } def props = Props[InnerChild] } class InnerChild extends Actor with ActorLogging { import InnerChild._ context.parent ! BackoffSupervisor.Reset //reset backoff counts override def receive: Receive = { case TestMessage(msg) => //模拟子级功能 if (Random.nextBoolean()) //任意产生异常 throw new ChildException(TestMessage(msg)) else log.info(s"Child received message: ${msg}") } }
我们用Random.nextBoolean来任意产生一些异常。注意:我们同时把ChildException改成了一个带参数的class,因为我们可能需要在重启之前获取造成异常的消息,如下:
def decider: PartialFunction[Throwable, SupervisorStrategy.Directive] = { case InnerChild.CException(tmsg) => println(s"Message causing exception: ${tmsg.msg}") //we can extract message here SupervisorStrategy.Restart }
所有信息发给supervisor就行了:
class ParentalActor extends Actor with ActorLogging { import ParentalActor._ //在这里构建子级Actor supervisor val supervisor = context.actorOf(Supervisor.props,"supervisor") override def receive: Receive = { case [email protected] _ => supervisor ! msg } } object BackoffSupervisorDemo extends App { import ParentalActor._ import InnerChild._ val testSystem = ActorSystem("testSystem") val parent = testSystem.actorOf(ParentalActor.props,"parent") parent ! TestMessage("Hello message 1 to supervisor") parent ! TestMessage("Hello message 2 to supervisor") parent ! TestMessage("Hello message 3 to supervisor") parent ! TestMessage("Hello message 4 to supervisor") parent ! TestMessage("Hello message 5 to supervisor") parent ! TestMessage("Hello message 6 to supervisor") scala.io.StdIn.readLine() testSystem.terminate() }
运行后发现在出现异常后所有消息都变成了DeadLetter:
[INFO] [05/29/2017 18:22:11.689] [testSystem-akka.actor.default-dispatcher-5] [akka://testSystem/user/parent/supervisor/innerChild] Message [backoffSupervisorDemo.InnerChild$TestMessage] from Actor[akka://testSystem/user/parent#2140150413] to Actor[akka://testSystem/user/parent/supervisor/innerChild#-1047097634] 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‘. ....
这也证明了BackoffSupervisor具有不同的Restart处理方式,好像是直接终止InnerChild而非正常的挂起,销毁了ActorRef和邮箱,所以在完成启动之前发给InnerChild的消息都被导入DeadLetter队列了。也就是说不但错过造成异常的消息,而是跳过了下面启动时间段内所有的消息。
下面我们来解决失踪消息的问题:首先是如何重新发送造成异常的消息,我们可以在监管策略中重启前发送:
def decider: PartialFunction[Throwable, SupervisorStrategy.Directive] = { case InnerChild.CException(tmsg) => println(s"Message causing exception: ${tmsg.msg}") //we can extract message here BackoffSupervisorDemo.sendToParent(tmsg) //resend message SupervisorStrategy.Restart }
在BackoffSupervisorDemo里先声明sendToParent函数:
def sendToParent(msg: TestMessage) = parent ! msg
然后再想办法把DeadLetter捞出来。我们可以用Akka的eventStream来订阅DeadLetter类型消息:
object DeadLetterMonitor { def props(parentRef: ActorRef) = Props(new DeadLetterMonitor(parentRef)) } class DeadLetterMonitor(receiver: ActorRef) extends Actor with ActorLogging { import InnerChild._ import context.dispatcher override def receive: Receive = { case DeadLetter(msg,sender,_) => //wait till InnerChild finishes restart then resend context.system.scheduler.scheduleOnce(1 second,receiver,msg.asInstanceOf[TestMessage]) } } object BackoffSupervisorDemo extends App { import ParentalActor._ import InnerChild._ def sendToParent(msg: TestMessage) = parent ! msg val testSystem = ActorSystem("testSystem") val parent = testSystem.actorOf(ParentalActor.props,"parent") val deadLetterMonitor = testSystem.actorOf(DeadLetterMonitor.props(parent),"dlmonitor") testSystem.eventStream.subscribe(deadLetterMonitor,classOf[DeadLetter]) //listen to DeadLetter parent ! TestMessage("Hello message 1 to supervisor") parent ! TestMessage("Hello message 2 to supervisor") parent ! TestMessage("Hello message 3 to supervisor") parent ! TestMessage("Hello message 4 to supervisor") parent ! TestMessage("Hello message 5 to supervisor") parent ! TestMessage("Hello message 6 to supervisor") scala.io.StdIn.readLine() testSystem.terminate() }
试运算后显示InnerChild成功处理了所有6条消息。
下面是本次讨论的完整示范代码:
package backoffSupervisorDemo import akka.actor._ import akka.pattern._ import scala.util.Random import scala.concurrent.duration._ object InnerChild { case class TestMessage(msg: String) class ChildException(val errmsg: TestMessage) extends Exception object CException { //for pattern match of class with parameter def apply(msg: TestMessage) = new ChildException(msg) def unapply(cex: ChildException) = Some(cex.errmsg) } def props = Props[InnerChild] } class InnerChild extends Actor with ActorLogging { import InnerChild._ context.parent ! BackoffSupervisor.Reset //reset backoff counts override def receive: Receive = { case TestMessage(msg) => //模拟子级功能 if (Random.nextBoolean()) //任意产生异常 throw new ChildException(TestMessage(msg)) else log.info(s"Child received message: ${msg}") } } object Supervisor { def props: Props = { //在这里定义了监管策略和child Actor构建 def decider: PartialFunction[Throwable, SupervisorStrategy.Directive] = { case InnerChild.CException(tmsg) => println(s"Message causing exception: ${tmsg.msg}") //we can extract message here BackoffSupervisorDemo.sendToParent(tmsg) //resend message SupervisorStrategy.Restart } val options = Backoff.onFailure(InnerChild.props, "innerChild", 1 second, 5 seconds, 0.0) .withManualReset .withSupervisorStrategy( OneForOneStrategy(maxNrOfRetries = 5, withinTimeRange = 5 seconds)( decider.orElse(SupervisorStrategy.defaultDecider) ) ) BackoffSupervisor.props(options) } } //注意:下面是Supervisor的父级,不是InnerChild的父级 object ParentalActor { case class SendToSupervisor(msg: InnerChild.TestMessage) case class SendToInnerChild(msg: InnerChild.TestMessage) case class SendToChildSelection(msg: InnerChild.TestMessage) def props = Props[ParentalActor] } class ParentalActor extends Actor with ActorLogging { import ParentalActor._ //在这里构建子级Actor supervisor val supervisor = context.actorOf(Supervisor.props,"supervisor") override def receive: Receive = { case [email protected] _ => supervisor ! msg } } object DeadLetterMonitor { def props(parentRef: ActorRef) = Props(new DeadLetterMonitor(parentRef)) } class DeadLetterMonitor(receiver: ActorRef) extends Actor with ActorLogging { import InnerChild._ import context.dispatcher override def receive: Receive = { case DeadLetter(msg,sender,_) => //wait till InnerChild finishes restart then resend context.system.scheduler.scheduleOnce(1 second,receiver,msg.asInstanceOf[TestMessage]) } } object BackoffSupervisorDemo extends App { import ParentalActor._ import InnerChild._ def sendToParent(msg: TestMessage) = parent ! msg val testSystem = ActorSystem("testSystem") val parent = testSystem.actorOf(ParentalActor.props,"parent") val deadLetterMonitor = testSystem.actorOf(DeadLetterMonitor.props(parent),"dlmonitor") testSystem.eventStream.subscribe(deadLetterMonitor,classOf[DeadLetter]) //listen to DeadLetter parent ! TestMessage("Hello message 1 to supervisor") parent ! TestMessage("Hello message 2 to supervisor") parent ! TestMessage("Hello message 3 to supervisor") parent ! TestMessage("Hello message 4 to supervisor") parent ! TestMessage("Hello message 5 to supervisor") parent ! TestMessage("Hello message 6 to supervisor") scala.io.StdIn.readLine() testSystem.terminate() }