Akka/Scala example source code file (FaultHandling.scala)

This example Akka source code file (FaultHandling.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Akka and Scala source code examples by using tags.

All credit for the original source code belongs to akka.io; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Akka tags/keywords

actorcontext, actorref, boolean, collection, concurrent, decider, duration, int, iterable, mutable, supervisorstrategy, throwable, time, unit, utilities

The FaultHandling.scala Akka example source code

/**
 * Copyright (C) 2009-2014 Typesafe Inc. <http://www.typesafe.com>
 */
package akka.actor

import language.implicitConversions
import java.lang.{ Iterable ⇒ JIterable }
import java.util.concurrent.TimeUnit
import akka.japi.Util.immutableSeq
import scala.collection.mutable.ArrayBuffer
import scala.collection.immutable
import scala.concurrent.duration.Duration
import akka.event.Logging.LogEvent
import akka.event.Logging.Error
import akka.event.Logging.Warning
import scala.util.control.NonFatal

/**
 * INTERNAL API
 */
private[akka] sealed trait ChildStats

/**
 * INTERNAL API
 */
private[akka] case object ChildNameReserved extends ChildStats

/**
 * ChildRestartStats is the statistics kept by every parent Actor for every child Actor
 * and is used for SupervisorStrategies to know how to deal with problems that occur for the children.
 */
final case class ChildRestartStats(child: ActorRef, var maxNrOfRetriesCount: Int = 0, var restartTimeWindowStartNanos: Long = 0L)
  extends ChildStats {

  def uid: Int = child.path.uid

  //FIXME How about making ChildRestartStats immutable and then move these methods into the actual supervisor strategies?
  def requestRestartPermission(retriesWindow: (Option[Int], Option[Int])): Boolean =
    retriesWindow match {
      case (Some(retries), _) if retries < 1 ⇒ false
      case (Some(retries), None)             ⇒ { maxNrOfRetriesCount += 1; maxNrOfRetriesCount <= retries }
      case (x, Some(window))                 ⇒ retriesInWindowOkay(if (x.isDefined) x.get else 1, window)
      case (None, _)                         ⇒ true
    }

  private def retriesInWindowOkay(retries: Int, window: Int): Boolean = {
    /*
     * Simple window algorithm: window is kept open for a certain time
     * after a restart and if enough restarts happen during this time, it
     * denies. Otherwise window closes and the scheme starts over.
     */
    val retriesDone = maxNrOfRetriesCount + 1
    val now = System.nanoTime
    val windowStart =
      if (restartTimeWindowStartNanos == 0) {
        restartTimeWindowStartNanos = now
        now
      } else restartTimeWindowStartNanos
    val insideWindow = (now - windowStart) <= TimeUnit.MILLISECONDS.toNanos(window)
    if (insideWindow) {
      maxNrOfRetriesCount = retriesDone
      retriesDone <= retries
    } else {
      maxNrOfRetriesCount = 1
      restartTimeWindowStartNanos = now
      true
    }
  }
}

/**
 * Implement this interface in order to configure the supervisorStrategy for
 * the top-level guardian actor (`/user`). An instance of this class must be
 * instantiable using a no-arg constructor.
 */
trait SupervisorStrategyConfigurator {
  def create(): SupervisorStrategy
}

final class DefaultSupervisorStrategy extends SupervisorStrategyConfigurator {
  override def create(): SupervisorStrategy = SupervisorStrategy.defaultStrategy
}

final class StoppingSupervisorStrategy extends SupervisorStrategyConfigurator {
  override def create(): SupervisorStrategy = SupervisorStrategy.stoppingStrategy
}

trait SupervisorStrategyLowPriorityImplicits { this: SupervisorStrategy.type ⇒

  /**
   * Implicit conversion from `Seq` of Cause-Directive pairs to a `Decider`. See makeDecider(causeDirective).
   */
  implicit def seqCauseDirective2Decider(trapExit: Iterable[CauseDirective]): Decider = makeDecider(trapExit)
  // the above would clash with seqThrowable2Decider for empty lists
}

object SupervisorStrategy extends SupervisorStrategyLowPriorityImplicits {
  sealed trait Directive

  /**
   * Resumes message processing for the failed Actor
   */
  case object Resume extends Directive

  /**
   * Discards the old Actor instance and replaces it with a new,
   * then resumes message processing.
   */
  case object Restart extends Directive

  /**
   * Stops the Actor
   */
  case object Stop extends Directive

  /**
   * Escalates the failure to the supervisor of the supervisor,
   * by rethrowing the cause of the failure, i.e. the supervisor fails with
   * the same exception as the child.
   */
  case object Escalate extends Directive

  /**
   * Java API: Returning this directive resumes message processing for the failed Actor
   */
  def resume = Resume

  /**
   * Java API: Returning this directive discards the old Actor instance and replaces it with a new,
   * then resumes message processing.
   */
  def restart = Restart

  /**
   * Java API: Returning this directive stops the Actor
   */
  def stop = Stop

  /**
   * Java API: Returning this directive escalates the failure to the supervisor of the supervisor,
   * by rethrowing the cause of the failure, i.e. the supervisor fails with
   * the same exception as the child.
   */
  def escalate = Escalate

  /**
   * When supervisorStrategy is not specified for an actor this
   * [[Decider]] is used by default in the supervisor strategy.
   * The child will be stopped when [[akka.actor.ActorInitializationException]],
   * [[akka.actor.ActorKilledException]], or [[akka.actor.DeathPactException]] is
   * thrown. It will be restarted for other `Exception` types.
   * The error is escalated if it's a `Throwable`, i.e. `Error`.
   */
  final val defaultDecider: Decider = {
    case _: ActorInitializationException ⇒ Stop
    case _: ActorKilledException         ⇒ Stop
    case _: DeathPactException           ⇒ Stop
    case _: Exception                    ⇒ Restart
  }

  /**
   * When supervisorStrategy is not specified for an actor this
   * is used by default. OneForOneStrategy with decider defined in
   * [[#defaultDecider]].
   */
  final val defaultStrategy: SupervisorStrategy = {
    OneForOneStrategy()(defaultDecider)
  }

  /**
   * This strategy resembles Erlang in that failing children are always
   * terminated (one-for-one).
   */
  final val stoppingStrategy: SupervisorStrategy = {
    def stoppingDecider: Decider = {
      case _: Exception ⇒ Stop
    }
    OneForOneStrategy()(stoppingDecider)
  }

  /**
   * Implicit conversion from `Seq` of Throwables to a `Decider`.
   * This maps the given Throwables to restarts, otherwise escalates.
   */
  implicit def seqThrowable2Decider(trapExit: immutable.Seq[Class[_ <: Throwable]]): Decider = makeDecider(trapExit)

  type Decider = PartialFunction[Throwable, Directive]
  type JDecider = akka.japi.Function[Throwable, Directive]
  type CauseDirective = (Class[_ <: Throwable], Directive)

  /**
   * Decider builder which just checks whether one of
   * the given Throwables matches the cause and restarts, otherwise escalates.
   */
  def makeDecider(trapExit: immutable.Seq[Class[_ <: Throwable]]): Decider = {
    case x ⇒ if (trapExit exists (_ isInstance x)) Restart else Escalate
  }

  /**
   * Decider builder which just checks whether one of
   * the given Throwables matches the cause and restarts, otherwise escalates.
   */
  def makeDecider(trapExit: JIterable[Class[_ <: Throwable]]): Decider = makeDecider(immutableSeq(trapExit))

  /**
   * Decider builder for Iterables of cause-directive pairs, e.g. a map obtained
   * from configuration; will sort the pairs so that the most specific type is
   * checked before all its subtypes, allowing carving out subtrees of the
   * Throwable hierarchy.
   */
  def makeDecider(flat: Iterable[CauseDirective]): Decider = {
    val directives = sort(flat)

    { case x ⇒ directives collectFirst { case (c, d) if c isInstance x ⇒ d } getOrElse Escalate }
  }

  /**
   * Converts a Java Decider into a Scala Decider
   */
  def makeDecider(func: JDecider): Decider = { case x ⇒ func(x) }

  /**
   * Sort so that subtypes always precede their supertypes, but without
   * obeying any order between unrelated subtypes (insert sort).
   *
   * INTERNAL API
   */
  private[akka] def sort(in: Iterable[CauseDirective]): immutable.Seq[CauseDirective] =
    (new ArrayBuffer[CauseDirective](in.size) /: in) { (buf, ca) ⇒
      buf.indexWhere(_._1 isAssignableFrom ca._1) match {
        case -1 ⇒ buf append ca
        case x  ⇒ buf insert (x, ca)
      }
      buf
    }.to[immutable.IndexedSeq]

  private[akka] def withinTimeRangeOption(withinTimeRange: Duration): Option[Duration] =
    if (withinTimeRange.isFinite && withinTimeRange >= Duration.Zero) Some(withinTimeRange) else None

  private[akka] def maxNrOfRetriesOption(maxNrOfRetries: Int): Option[Int] =
    if (maxNrOfRetries < 0) None else Some(maxNrOfRetries)

  private[akka] val escalateDefault = (_: Any) ⇒ Escalate
}

/**
 * An Akka SupervisorStrategy is the policy to apply for crashing children.
 *
 * <b>IMPORTANT:</b>
 *
 * You should not normally need to create new subclasses, instead use the
 * existing [[akka.actor.OneForOneStrategy]] or [[akka.actor.AllForOneStrategy]],
 * but if you do, please read the docs of the methods below carefully, as
 * incorrect implementations may lead to “blocked” actor systems (i.e.
 * permanently suspended actors).
 */
abstract class SupervisorStrategy {

  import SupervisorStrategy._

  /**
   * Returns the Decider that is associated with this SupervisorStrategy.
   * The Decider is invoked by the default implementation of `handleFailure`
   * to obtain the Directive to be applied.
   */
  def decider: Decider

  /**
   * This method is called after the child has been removed from the set of children.
   * It does not need to do anything special. Exceptions thrown from this method
   * do NOT make the actor fail if this happens during termination.
   */
  def handleChildTerminated(context: ActorContext, child: ActorRef, children: Iterable[ActorRef]): Unit

  /**
   * This method is called to act on the failure of a child: restart if the flag is true, stop otherwise.
   */
  def processFailure(context: ActorContext, restart: Boolean, child: ActorRef, cause: Throwable, stats: ChildRestartStats, children: Iterable[ChildRestartStats]): Unit

  /**
   * This is the main entry point: in case of a child’s failure, this method
   * must try to handle the failure by resuming, restarting or stopping the
   * child (and returning `true`), or it returns `false` to escalate the
   * failure, which will lead to this actor re-throwing the exception which
   * caused the failure. The exception will not be wrapped.
   *
   * This method calls [[akka.actor.SupervisorStrategy#logFailure]], which will
   * log the failure unless it is escalated. You can customize the logging by
   * setting [[akka.actor.SupervisorStrategy#loggingEnabled]] to `false` and
   * do the logging inside the `decider` or override the `logFailure` method.
   *
   * @param children is a lazy collection (a view)
   */
  def handleFailure(context: ActorContext, child: ActorRef, cause: Throwable, stats: ChildRestartStats, children: Iterable[ChildRestartStats]): Boolean = {
    val directive = decider.applyOrElse(cause, escalateDefault)
    directive match {
      case d @ Resume ⇒
        logFailure(context, child, cause, d)
        resumeChild(child, cause)
        true
      case d @ Restart ⇒
        logFailure(context, child, cause, d)
        processFailure(context, true, child, cause, stats, children)
        true
      case d @ Stop ⇒
        logFailure(context, child, cause, d)
        processFailure(context, false, child, cause, stats, children)
        true
      case d @ Escalate ⇒
        logFailure(context, child, cause, d)
        false
    }
  }

  /**
   * Logging of actor failures is done when this is `true`.
   */
  protected def loggingEnabled: Boolean = true

  /**
   * Default logging of actor failures when
   * [[akka.actor.SupervisorStrategy#loggingEnabled]] is `true`.
   * `Escalate` failures are not logged here, since they are supposed
   * to be handled at a level higher up in the hierarchy.
   * `Resume` failures are logged at `Warning` level.
   * `Stop` and `Restart` failures are logged at `Error` level.
   */
  def logFailure(context: ActorContext, child: ActorRef, cause: Throwable, decision: Directive): Unit =
    if (loggingEnabled) {
      val logMessage = cause match {
        case e: ActorInitializationException if e.getCause ne null ⇒ e.getCause.getMessage
        case e ⇒ e.getMessage
      }
      decision match {
        case Resume   ⇒ publish(context, Warning(child.path.toString, getClass, logMessage))
        case Escalate ⇒ // don't log here
        case _        ⇒ publish(context, Error(cause, child.path.toString, getClass, logMessage))
      }
    }

  // logging is not the main purpose, and if it fails there’s nothing we can do
  private def publish(context: ActorContext, logEvent: LogEvent): Unit =
    try context.system.eventStream.publish(logEvent) catch { case NonFatal(_) ⇒ }

  /**
   * Resume the previously failed child: <b>do never apply this to a child which
   * is not the currently failing child</b>. Suspend/resume needs to be done in
   * matching pairs, otherwise actors will wake up too soon or never at all.
   */
  final def resumeChild(child: ActorRef, cause: Throwable): Unit = child.asInstanceOf[InternalActorRef].resume(causedByFailure = cause)

  /**
   * Restart the given child, possibly suspending it first.
   *
   * <b>IMPORTANT:</b>
   *
   * If the child is the currently failing one, it will already have been
   * suspended, hence `suspendFirst` must be false. If the child is not the
   * currently failing one, then it did not request this treatment and is
   * therefore not prepared to be resumed without prior suspend.
   */
  final def restartChild(child: ActorRef, cause: Throwable, suspendFirst: Boolean): Unit = {
    val c = child.asInstanceOf[InternalActorRef]
    if (suspendFirst) c.suspend()
    c.restart(cause)
  }

}

/**
 * Applies the fault handling `Directive` (Resume, Restart, Stop) specified in the `Decider`
 * to all children when one fails, as opposed to [[akka.actor.OneForOneStrategy]] that applies
 * it only to the child actor that failed.
 *
 * @param maxNrOfRetries the number of times a child actor is allowed to be restarted, negative value means no limit,
 *   if the limit is exceeded the child actor is stopped
 * @param withinTimeRange duration of the time window for maxNrOfRetries, Duration.Inf means no window
 * @param decider mapping from Throwable to [[akka.actor.SupervisorStrategy.Directive]], you can also use a
 *   [[scala.collection.immutable.Seq]] of Throwables which maps the given Throwables to restarts, otherwise escalates.
 * @param loggingEnabled the strategy logs the failure if this is enabled (true), by default it is enabled
 */
case class AllForOneStrategy(
  maxNrOfRetries: Int = -1,
  withinTimeRange: Duration = Duration.Inf,
  override val loggingEnabled: Boolean = true)(val decider: SupervisorStrategy.Decider)
  extends SupervisorStrategy {

  import SupervisorStrategy._

  /**
   * Java API
   */
  def this(maxNrOfRetries: Int, withinTimeRange: Duration, decider: SupervisorStrategy.JDecider, loggingEnabled: Boolean) =
    this(maxNrOfRetries, withinTimeRange, loggingEnabled)(SupervisorStrategy.makeDecider(decider))

  /**
   * Java API
   */
  def this(maxNrOfRetries: Int, withinTimeRange: Duration, decider: SupervisorStrategy.JDecider) =
    this(maxNrOfRetries, withinTimeRange)(SupervisorStrategy.makeDecider(decider))

  /**
   * Java API
   */
  def this(maxNrOfRetries: Int, withinTimeRange: Duration, trapExit: JIterable[Class[_ <: Throwable]]) =
    this(maxNrOfRetries, withinTimeRange)(SupervisorStrategy.makeDecider(trapExit))

  /**
   * Java API: compatible with lambda expressions
   * This is an EXPERIMENTAL feature and is subject to change until it has received more real world testing.
   */
  def this(maxNrOfRetries: Int, withinTimeRange: Duration, decider: SupervisorStrategy.Decider) =
    this(maxNrOfRetries = maxNrOfRetries, withinTimeRange = withinTimeRange)(decider)

  /**
   * Java API: compatible with lambda expressions
   * This is an EXPERIMENTAL feature and is subject to change until it has received more real world testing.
   */
  def this(loggingEnabled: Boolean, decider: SupervisorStrategy.Decider) =
    this(loggingEnabled = loggingEnabled)(decider)

  /**
   * Java API: compatible with lambda expressions
   * This is an EXPERIMENTAL feature and is subject to change until it has received more real world testing.
   */
  def this(decider: SupervisorStrategy.Decider) =
    this()(decider)

  /*
   *  this is a performance optimization to avoid re-allocating the pairs upon
   *  every call to requestRestartPermission, assuming that strategies are shared
   *  across actors and thus this field does not take up much space
   */
  private val retriesWindow = (maxNrOfRetriesOption(maxNrOfRetries), withinTimeRangeOption(withinTimeRange).map(_.toMillis.toInt))

  def handleChildTerminated(context: ActorContext, child: ActorRef, children: Iterable[ActorRef]): Unit = ()

  def processFailure(context: ActorContext, restart: Boolean, child: ActorRef, cause: Throwable, stats: ChildRestartStats, children: Iterable[ChildRestartStats]): Unit = {
    if (children.nonEmpty) {
      if (restart && children.forall(_.requestRestartPermission(retriesWindow)))
        children foreach (crs ⇒ restartChild(crs.child, cause, suspendFirst = (crs.child != child)))
      else
        for (c ← children) context.stop(c.child)
    }
  }
}

/**
 * Applies the fault handling `Directive` (Resume, Restart, Stop) specified in the `Decider`
 * to the child actor that failed, as opposed to [[akka.actor.AllForOneStrategy]] that applies
 * it to all children.
 *
 * @param maxNrOfRetries the number of times a child actor is allowed to be restarted, negative value means no limit,
 *   if the limit is exceeded the child actor is stopped
 * @param withinTimeRange duration of the time window for maxNrOfRetries, Duration.Inf means no window
 * @param decider mapping from Throwable to [[akka.actor.SupervisorStrategy.Directive]], you can also use a
 *   [[scala.collection.immutable.Seq]] of Throwables which maps the given Throwables to restarts, otherwise escalates.
 * @param loggingEnabled the strategy logs the failure if this is enabled (true), by default it is enabled
 */
case class OneForOneStrategy(
  maxNrOfRetries: Int = -1,
  withinTimeRange: Duration = Duration.Inf,
  override val loggingEnabled: Boolean = true)(val decider: SupervisorStrategy.Decider)
  extends SupervisorStrategy {

  /**
   * Java API
   */
  def this(maxNrOfRetries: Int, withinTimeRange: Duration, decider: SupervisorStrategy.JDecider, loggingEnabled: Boolean) =
    this(maxNrOfRetries, withinTimeRange, loggingEnabled)(SupervisorStrategy.makeDecider(decider))

  /**
   * Java API
   */
  def this(maxNrOfRetries: Int, withinTimeRange: Duration, decider: SupervisorStrategy.JDecider) =
    this(maxNrOfRetries, withinTimeRange)(SupervisorStrategy.makeDecider(decider))

  /**
   * Java API
   */
  def this(maxNrOfRetries: Int, withinTimeRange: Duration, trapExit: JIterable[Class[_ <: Throwable]]) =
    this(maxNrOfRetries, withinTimeRange)(SupervisorStrategy.makeDecider(trapExit))

  /**
   * Java API: compatible with lambda expressions
   * This is an EXPERIMENTAL feature and is subject to change until it has received more real world testing.
   */
  def this(maxNrOfRetries: Int, withinTimeRange: Duration, decider: SupervisorStrategy.Decider) =
    this(maxNrOfRetries = maxNrOfRetries, withinTimeRange = withinTimeRange)(decider)

  /**
   * Java API: compatible with lambda expressions
   * This is an EXPERIMENTAL feature and is subject to change until it has received more real world testing.
   */
  def this(loggingEnabled: Boolean, decider: SupervisorStrategy.Decider) =
    this(loggingEnabled = loggingEnabled)(decider)

  /**
   * Java API: compatible with lambda expressions
   * This is an EXPERIMENTAL feature and is subject to change until it has received more real world testing.
   */
  def this(decider: SupervisorStrategy.Decider) =
    this()(decider)

  /*
   *  this is a performance optimization to avoid re-allocating the pairs upon
   *  every call to requestRestartPermission, assuming that strategies are shared
   *  across actors and thus this field does not take up much space
   */
  private val retriesWindow = (
    SupervisorStrategy.maxNrOfRetriesOption(maxNrOfRetries),
    SupervisorStrategy.withinTimeRangeOption(withinTimeRange).map(_.toMillis.toInt))

  def handleChildTerminated(context: ActorContext, child: ActorRef, children: Iterable[ActorRef]): Unit = ()

  def processFailure(context: ActorContext, restart: Boolean, child: ActorRef, cause: Throwable, stats: ChildRestartStats, children: Iterable[ChildRestartStats]): Unit = {
    if (restart && stats.requestRestartPermission(retriesWindow))
      restartChild(child, cause, suspendFirst = false)
    else
      context.stop(child) //TODO optimization to drop child here already?
  }
}