月份：2014年8月

我们在前面介绍Actor系统时说过每个Actor都是其子Actor的管理员，并且每个Actor定义了发生错误时的管理策略，策略一旦定义好，之后不能修改，就像是Actor系统不可分割的一部分。
实用错误处理
首先我们来看一个例子来显示一种处理数据存储错误的情况，这是现实中一个应用可能出现的典型错误。当然实际的应用可能针对数据源不存在时有不同的处理，这里我们使用重新连接的处理方法。
下面是例子的源码，比较长，需要仔细阅读，最好是实际运行，参考日志来理解：

import akka.actor._
import akka.actor.SupervisorStrategy._
import scala.concurrent.duration._
import akka.util.Timeout
import akka.event.LoggingReceive
import akka.pattern.{ask, pipe}
import com.typesafe.config.ConfigFactory

/**
 * Runs the sample
 */
object FaultHandlingDocSample extends App {

  import Worker._

  val config = ConfigFactory.parseString( """
      akka.loglevel = "DEBUG"
      akka.actor.debug {
      receive = on
      lifecycle = on
      }
      """)

  val system = ActorSystem("FaultToleranceSample", config)
  val worker = system.actorOf(Props[Worker], name = "worker")
  val listener = system.actorOf(Props[Listener], name = "listener")
  // start the work and listen on progress
  // note that the listener is used as sender of the tell,
  // i.e. it will receive replies from the worker
  worker.tell(Start, sender = listener)
}

/**
 * Listens on progress from the worker and shuts down the system when enough
 * work has been done.
 */
class Listener extends Actor with ActorLogging {

  import Worker._

  // If we don’t get any progress within 15 seconds then the service is unavailable
  context.setReceiveTimeout(15 seconds)

  def receive = {
    case Progress(percent) =>
      log.info("Current progress: {} %", percent)
      if (percent >= 100.0) {
        log.info("That’s all, shutting down")
        context.system.shutdown()
      }
    case ReceiveTimeout =>
      // No progress within 15 seconds, ServiceUnavailable
      log.error("Shutting down due to unavailable service")
      context.system.shutdown()
  }
}

object Worker {

  case object Start

  case object Do

  final case class Progress(percent: Double)

}

/**
 * Worker performs some work when it receives the ‘Start‘ message.
 * It will continuously notify the sender of the ‘Start‘ message
 * of current ‘‘Progress‘‘. The ‘Worker‘ supervise the ‘CounterService‘.
 */
class Worker extends Actor with ActorLogging {

  import Worker._
  import CounterService._

  implicit val askTimeout = Timeout(5 seconds)
  // Stop the CounterService child if it throws ServiceUnavailable
  override val supervisorStrategy = OneForOneStrategy() {
    case _: CounterService.ServiceUnavailable => Stop
  }
  // The sender of the initial Start message will continuously be notified
  // about progress
  var progressListener: Option[ActorRef] = None
  val counterService = context.actorOf(Props[CounterService], name = "counter")
  val totalCount = 51

  import context.dispatcher

  // Use this Actors’ Dispatcher as ExecutionContext
  def receive = LoggingReceive {
    case Start if progressListener.isEmpty =>
      progressListener = Some(sender())
      context.system.scheduler.schedule(Duration.Zero, 1 second, self, Do)
    case Do =>
      counterService ! Increment(1)
      counterService ! Increment(1)
      counterService ! Increment(1)
      // Send current progress to the initial sender
      counterService ? GetCurrentCount map {
        case CurrentCount(_, count) => Progress(100.0 * count / totalCount)
      } pipeTo progressListener.get
  }
}

object CounterService {

  final case class Increment(n: Int)

  case object GetCurrentCount

  final case class CurrentCount(key: String, count: Long)

  class ServiceUnavailable(msg: String) extends RuntimeException(msg)

  private case object Reconnect

}

/**
 * Adds the value received in ‘Increment‘ message to a persistent
 * counter. Replies with ‘CurrentCount‘ when it is asked for ‘CurrentCount‘.
 * ‘CounterService‘ supervise ‘Storage‘ and ‘Counter‘.
 */
class CounterService extends Actor {

  import CounterService._
  import Counter._
  import Storage._

  // Restart the storage child when StorageException is thrown.
  // After 3 restarts within 5 seconds it will be stopped.
  override val supervisorStrategy = OneForOneStrategy(maxNrOfRetries = 3,
    withinTimeRange = 5 seconds) {
    case _: Storage.StorageException => Restart
  }
  val key = self.path.name
  var storage: Option[ActorRef] = None
  var counter: Option[ActorRef] = None
  var backlog = IndexedSeq.empty[(ActorRef, Any)]
  val MaxBacklog = 10000

  import context.dispatcher

  // Use this Actors’ Dispatcher as ExecutionContext
  override def preStart() {
    initStorage()
  }

  /**
   * The child storage is restarted in case of failure, but after 3 restarts,
   * and still failing it will be stopped. Better to back-off than continuously
   * failing. When it has been stopped we will schedule a Reconnect after a delay.
   * Watch the child so we receive Terminated message when it has been terminated.
   */
  def initStorage() {
    storage = Some(context.watch(context.actorOf(Props[Storage], name = "storage")))
    // Tell the counter, if any, to use the new storage
    counter foreach {
      _ ! UseStorage(storage)
    }
    // We need the initial value to be able to operate
    storage.get ! Get(key)
  }

  def receive = LoggingReceive {
    case Entry(k, v) if k == key && counter == None =>
      // Reply from Storage of the initial value, now we can create the Counter
      val c = context.actorOf(Props(classOf[Counter], key, v))
      counter = Some(c)
      // Tell the counter to use current storage
      c ! UseStorage(storage)
      // and send the buffered backlog to the counter
      for ((replyTo, msg) <- backlog) c.tell(msg, sender = replyTo)
      backlog = IndexedSeq.empty
    case msg@Increment(n) => forwardOrPlaceInBacklog(msg)

    case msg@GetCurrentCount => forwardOrPlaceInBacklog(msg)
    case Terminated(actorRef) if Some(actorRef) == storage =>
      // After 3 restarts the storage child is stopped.
      // We receive Terminated because we watch the child, see initStorage.
      storage = None
      // Tell the counter that there is no storage for the moment
      counter foreach {
        _ ! UseStorage(None)
      }
      // Try to re-establish storage after while
      context.system.scheduler.scheduleOnce(10 seconds, self, Reconnect)
    case Reconnect =>
      // Re-establish storage after the scheduled delay
      initStorage()
  }

  def forwardOrPlaceInBacklog(msg: Any) {
    // We need the initial value from storage before we can start delegate to
    // the counter. Before that we place the messages in a backlog, to be sent
    // to the counter when it is initialized.
    counter match {
      case Some(c) => c forward msg
      case None =>
        if (backlog.size >= MaxBacklog)
          throw new ServiceUnavailable(
            "CounterService not available, lack of initial value")
        backlog :+= (sender() -> msg)
    }
  }
}

object Counter {

  final case class UseStorage(storage: Option[ActorRef])

}

/**
 * The in memory count variable that will send current
 * value to the ‘Storage‘, if there is any storage
 * available at the moment.
 */
class Counter(key: String, initialValue: Long) extends Actor {

  import Counter._
  import CounterService._
  import Storage._

  var count = initialValue
  var storage: Option[ActorRef] = None

  def receive = LoggingReceive {
    case UseStorage(s) =>
      storage = s
      storeCount()
    case Increment(n) =>
      count += n
      storeCount()
    case GetCurrentCount =>
      sender() ! CurrentCount(key, count)
  }

  def storeCount() {
    // Delegate dangerous work, to protect our valuable state.
    // We can continue without storage.
    storage foreach {
      _ ! Store(Entry(key, count))
    }
  }
}

object DummyDB {

  import Storage.StorageException

  private var db = Map[String, Long]()

  @throws(classOf[StorageException])
  def save(key: String, value: Long): Unit = synchronized {
    if (11 <= value && value <= 14)
      throw new StorageException("Simulated store failure " + value)
    db += (key -> value)
  }

  @throws(classOf[StorageException])
  def load(key: String): Option[Long] = synchronized {
    db.get(key)
  }
}

object Storage {

  final case class Store(entry: Entry)

  final case class Get(key: String)

  final case class Entry(key: String, value: Long)

  class StorageException(msg: String) extends RuntimeException(msg)

}

/**
 * Saves key/value pairs to persistent storage when receiving ‘Store‘ message.
 * Replies with current value when receiving ‘Get‘ message.
 * Will throw StorageException if the underlying data store is out of order.
 */
class Storage extends Actor {

  import Storage._

  val db = DummyDB

  def receive = LoggingReceive {
    case Store(Entry(key, count)) => db.save(key, count)
    case Get(key) => sender() ! Entry(key, db.load(key).getOrElse(0L))
  }
}

这个例子定义了五个Actor，分别是Worker, Listener, CounterService ,Counter 和 Storage,下图给出了系统正常运行时的流程（无错误发生的情况）：

其中Worker是CounterService的父Actor（管理员），CounterService是Counter和Storage的父Actor（管理员）图中浅红色，白色代表引用，其中Worker引用了Listener，Listener也引用了Worker，它们之间不存在父子关系，同样Counter也引用了Storage，但Counter不是Storage的管理员。

正常流程如下：

下图给出系统出错的情况，例子中Worker和CounterService作为管理员分别定义了两个管理策略，Worker在收到CounterService 的ServiceUnaviable上终止CounterService的运行，而CounterService在收到StorageException时重启Storage。

出错时的流程

这里给出运行的一个日志供参考。