akka-typed(1) - actor生命周期管理

   akka-typed的actor从创建、启用、状态转换、停用、监视等生命周期管理方式和akka-classic还是有一定的不同之处。这篇我们就介绍一下akka-typed的actor生命周期管理。

每一种actor都是通过定义它的行为属性behavior形成模版，然后由对上一层的父辈actor用spawn方法产生actor实例的。产生的actor实例加入一个系统的由上至下树形结构，直接在spawn产生自己的父辈之下。akka-typed的守护guardian-actor，即根部root-actor是通过在定义ActorSystem时指定并产生的。如下：

    val config = ConfigFactory.load("application.conf")
    val man: ActorSystem[GreetStarter.Command] = ActorSystem(GreetStarter(), "greetDemo",config)
    man ! GreetStarter.RepeatedGreeting("Tiger",1.seconds)

在某种意义上，这个ActorSystem实例man就代表root-actor。我们可以向man发送消息然后由GreetStarter的behavior用自己的ActorContext进行spawn,stop,watch及分派计算任务等，其实就是一个程序的集线器：

  object GreetStarter {
    import Messages._
    def apply(): Behavior[SayHi] = {
      Behaviors.setup { ctx =>
        val props = DispatcherSelector.fromConfig("akka.actor.default-blocking-io-dispatcher")
        val helloActor = ctx.spawn(HelloActor(), "hello-actor",props)
        val greeter = ctx.spawn(Greeter(helloActor), "greeter")
        ctx.watch(greeter)
        ctx.watchWith(helloActor,StopWorker("something happend"))
        Behaviors.receiveMessage { who =>
          if (who.name == "stop") {
            ctx.stop(helloActor)
            ctx.stop(greeter)
            Behaviors.stopped
          } else {
            greeter ! who
            Behaviors.same
          }
        }
      }
    }
  }

但是，总有时候我们需要在root-actor的ActorContext之外来进行一些制造、使用actor的操作。下面这个官方文档上的例子是很好的示范：

import akka.actor.typed.Behavior
import akka.actor.typed.SpawnProtocol
import akka.actor.typed.scaladsl.Behaviors
import akka.actor.typed.scaladsl.LoggerOps

object HelloWorldMain {
  def apply(): Behavior[SpawnProtocol.Command] =
    Behaviors.setup { context =>
      // Start initial tasks
      // context.spawn(...)

      SpawnProtocol()
    }
}

object Main extends App {
implicit val system: ActorSystem[SpawnProtocol.Command] =
  ActorSystem(HelloWorldMain(), "hello")

// needed in implicit scope for ask (?)
import akka.actor.typed.scaladsl.AskPattern._
implicit val ec: ExecutionContext = system.executionContext
implicit val timeout: Timeout = Timeout(3.seconds)

val greeter: Future[ActorRef[HelloWorld.Greet]] =
  system.ask(SpawnProtocol.Spawn(behavior = HelloWorld(), name = "greeter", props = Props.empty, _))

val greetedBehavior = Behaviors.receive[HelloWorld.Greeted] { (context, message) =>
  context.log.info2("Greeting for {} from {}", message.whom, message.from)
  Behaviors.stopped
}

val greetedReplyTo: Future[ActorRef[HelloWorld.Greeted]] =
  system.ask(SpawnProtocol.Spawn(greetedBehavior, name = "", props = Props.empty, _))

for (greeterRef <- greeter; replyToRef <- greetedReplyTo) {
  greeterRef ! HelloWorld.Greet("Akka", replyToRef)
}
...
}

可以看到所有操作都在actor框架之外进行的。这个SpawnProtocol本身就是一个actor，如下：

object SpawnProtocol {

...
  final case class Spawn[T](behavior: Behavior[T], name: String, props: Props, replyTo: ActorRef[ActorRef[T]])
      extends Command
...
  def apply(): Behavior[Command] =
    Behaviors.receive { (ctx, msg) =>
      msg match {
        case Spawn(bhvr, name, props, replyTo) =>
          val ref =
            if (name == null || name.equals(""))
              ctx.spawnAnonymous(bhvr, props)
            else {

              @tailrec def spawnWithUniqueName(c: Int): ActorRef[Any] = {
                val nameSuggestion = if (c == 0) name else s"$name-$c"
                ctx.child(nameSuggestion) match {
                  case Some(_) => spawnWithUniqueName(c + 1) // already taken, try next
                  case None    => ctx.spawn(bhvr, nameSuggestion, props)
                }
              }

              spawnWithUniqueName(0)
            }
          replyTo ! ref
          Behaviors.same
      }
    }

}

外界通过发送Spawn消息来指定产生新的actor。

actor的状态切换就是从一种behavior转到另一种behavior。我们可以自定义behavior或者用现成的Behaviors.???。如果只是涉及内部变量变化，那么可以直接生成带着变量的当前behavior，如下：

object HelloWorldBot {

  def apply(max: Int): Behavior[HelloWorld.Greeted] = {
    bot(0, max)
  }

  private def bot(greetingCounter: Int, max: Int): Behavior[HelloWorld.Greeted] =
    Behaviors.receive { (context, message) =>
      val n = greetingCounter + 1
      context.log.info2("Greeting {} for {}", n, message.whom)
      if (n == max) {
        Behaviors.stopped
      } else {
        message.from ! HelloWorld.Greet(message.whom, context.self)
        bot(n, max)
      }
    }
}

actor停用可以由直属父辈actor的ActorContext.stop或者自身的Behaviors.stopped来实现。Behaviors.stopped可以带入一个清理函数。在actor完全停止之前进行一些清理操作： 

object MasterControlProgram {
  sealed trait Command
  final case class SpawnJob(name: String) extends Command
  case object GracefulShutdown extends Command

  // Predefined cleanup operation
  def cleanup(log: Logger): Unit = log.info("Cleaning up!")

  def apply(): Behavior[Command] = {
    Behaviors
      .receive[Command] { (context, message) =>
        message match {
          case SpawnJob(jobName) =>
            context.log.info("Spawning job {}!", jobName)
            context.spawn(Job(jobName), name = jobName)
            Behaviors.same
          case GracefulShutdown =>
            context.log.info("Initiating graceful shutdown...")
            // perform graceful stop, executing cleanup before final system termination
            // behavior executing cleanup is passed as a parameter to Actor.stopped
            Behaviors.stopped { () =>
              cleanup(context.system.log)
            }
        }
      }
      .receiveSignal {
        case (context, PostStop) =>
          context.log.info("Master Control Program stopped")
          Behaviors.same
      }
  }
}

实际上一个actor转入停用stop状态可以在另一个作为监视actor的receiveSignal获取，如下：

  object GreetStarter {
    import Messages._
    def apply(): Behavior[SayHi] = {
      Behaviors.setup { ctx =>
        val props = DispatcherSelector.fromConfig("akka.actor.default-blocking-io-dispatcher")
        val helloActor = ctx.spawn(HelloActor(), "hello-actor",props)
        val greeter = ctx.spawn(Greeter(helloActor), "greeter")
        ctx.watch(greeter)
        ctx.watchWith(helloActor,StopWorker("something happend"))
        Behaviors.receiveMessage { who =>
          if (who.name == "stop") {
            ctx.stop(helloActor)
            ctx.stop(greeter)
            Behaviors.stopped
          } else {
            greeter ! who
            Behaviors.same
          }
        }.receiveSignal {
            case (context, Terminated(ref)) =>
              context.log.info("{} stopped!", ref.path.name)
              Behaviors.same
          }
      }
    }
  }

下面是.receiveSignal函数及其捕获的Signal消息：

  trait Receive[T] extends Behavior[T] {
    def receiveSignal(onSignal: PartialFunction[(ActorContext[T], Signal), Behavior[T]]): Behavior[T]
  }



trait Signal

/**
 * Lifecycle signal that is fired upon restart of the Actor before replacing
 * the behavior with the fresh one (i.e. this signal is received within the
 * behavior that failed).
 */
sealed abstract class PreRestart extends Signal
case object PreRestart extends PreRestart {
  def instance: PreRestart = this
}

/**
 * Lifecycle signal that is fired after this actor and all its child actors
 * (transitively) have terminated. The [[Terminated]] signal is only sent to
 * registered watchers after this signal has been processed.
 */
sealed abstract class PostStop extends Signal
// comment copied onto object for better hints in IDEs
/**
 * Lifecycle signal that is fired after this actor and all its child actors
 * (transitively) have terminated. The [[Terminated]] signal is only sent to
 * registered watchers after this signal has been processed.
 */
case object PostStop extends PostStop {
  def instance: PostStop = this
}

object Terminated {
  def apply(ref: ActorRef[Nothing]): Terminated = new Terminated(ref)
  def unapply(t: Terminated): Option[ActorRef[Nothing]] = Some(t.ref)
}