Classic Fault Tolerance

@@includeincludes.md { #actor-api } For the full documentation of this feature and for new projects see @ref:fault tolerance.

Dependency

@@@note The Akka dependencies are available from Akka’s secure library repository. To access them you need to use a secure, tokenized URL as specified at https://account.akka.io/token. @@@

The concept of fault tolerance relates to actors, so in order to use these make sure to depend on actors.

@@dependency[sbt,Maven,Gradle] { bomGroup=com.typesafe.akka bomArtifact=akka-bom_$scala.binary.version$ bomVersionSymbols=AkkaVersion symbol1=AkkaVersion value1="$akka.version$" group="com.typesafe.akka" artifact="akka-actor_$scala.binary.version$" version=AkkaVersion }

Introduction

As explained in @ref:Actor Systems each actor is the supervisor of its children, and as such each actor defines fault handling supervisor strategy. This strategy cannot be changed afterwards as it is an integral part of the actor system’s structure.

Fault Handling in Practice

First, let us look at a sample that illustrates one way to handle data store errors, which is a typical source of failure in real world applications. Of course it depends on the actual application what is possible to do when the data store is unavailable, but in this sample we use a best effort re-connect approach.

Read the following source code. The inlined comments explain the different pieces of the fault handling and why they are added. It is also highly recommended to run this sample as it is easy to follow the log output to understand what is happening at runtime.

Creating a Supervisor Strategy

The following sections explain the fault handling mechanism and alternatives in more depth.

For the sake of demonstration let us consider the following strategy:

Scala : @@snip FaultHandlingDocSpec.scala { #strategy }

Java : @@snip FaultHandlingTest.java { #strategy }

We have chosen a few well-known exception types in order to demonstrate the application of the fault handling directives described in @ref:supervision. First off, it is a one-for-one strategy, meaning that each child is treated separately (an all-for-one strategy works very similarly, the only difference is that any decision is applied to all children of the supervisor, not only the failing one). In the above example, 10 and @scala[1 minute]@java[Duration.create(1, TimeUnit.MINUTES)] are passed to the maxNrOfRetries and withinTimeRange parameters respectively, which means that the strategy restarts a child up to 10 restarts per minute. The child actor is stopped if the restart count exceeds maxNrOfRetries during the withinTimeRange duration.

Also, there are special values for these parameters. If you specify:

• -1 to maxNrOfRetries, and @scala[Duration.Inf]@java[Duration.Inf()] to withinTimeRange
  • then the child is always restarted without any limit
• -1 to maxNrOfRetries, and a non-infinite Duration to withinTimeRange
  • maxNrOfRetries is treated as 1
• a non-negative number to maxNrOfRetries and @scala[Duration.Inf]@java[Duration.Inf()] to withinTimeRange
  • withinTimeRange is treated as infinite duration (i.e.) no matter how long it takes, once the restart count exceeds maxNrOfRetries, the child actor is stopped

The match statement which forms the bulk of the body
@scala[is of type Decider which is a PartialFunction[Throwable, Directive].] @java[consists of PFBuilder returned by DeciderBuilder's match method, where the builder is finished by the build method.] This is the piece which maps child failure types to their corresponding directives.

@@@ note

If the strategy is declared inside the supervising actor (as opposed to @scala[within a companion object]@java[a separate class]) its decider has access to all internal state of the actor in a thread-safe fashion, including obtaining a reference to the currently failed child (available as the @scala[sender]@java[getSender()] of the failure message).

@@@

Default Supervisor Strategy

Escalate is used if the defined strategy doesn't cover the exception that was thrown.

When the supervisor strategy is not defined for an actor the following exceptions are handled by default:

• ActorInitializationException will stop the failing child actor
• ActorKilledException will stop the failing child actor
• DeathPactException will stop the failing child actor
• Exception will restart the failing child actor
• Other types of Throwable will be escalated to parent actor

If the exception escalate all the way up to the root guardian it will handle it in the same way as the default strategy defined above.

@@@ div { .group-scala }

You can combine your own strategy with the default strategy:

@@snip FaultHandlingDocSpec.scala { #default-strategy-fallback }

@@@

Stopping Supervisor Strategy

Closer to the Erlang way is the strategy to stop children when they fail and then take corrective action in the supervisor when DeathWatch signals the loss of the child. This strategy is also provided pre-packaged as SupervisorStrategy.stoppingStrategy with an accompanying StoppingSupervisorStrategy configurator to be used when you want the "/user" guardian to apply it.

Logging of Actor Failures

By default the SupervisorStrategy logs failures unless they are escalated. Escalated failures are supposed to be handled, and potentially logged, at a level higher in the hierarchy.

Log levels can be controlled by providing a Decider and using the appropriate decision methods accepting a LogLevel on SupervisorStrategy.

You can mute the default logging of a SupervisorStrategy by setting loggingEnabled to false when instantiating it. Customized logging can be done inside the Decider. Note that the reference to the currently failed child is available as the sender when the SupervisorStrategy is declared inside the supervising actor.

You may also customize the logging in your own SupervisorStrategy implementation by overriding the logFailure method.

Supervision of Top-Level Actors

Toplevel actors means those which are created using system.actorOf(), and they are children of the @ref:User Guardian. There are no special rules applied in this case, the guardian applies the configured strategy.

Test Application

The following section shows the effects of the different directives in practice, where a test setup is needed. First off, we need a suitable supervisor:

Scala : @@snip FaultHandlingDocSpec.scala { #supervisor }

Java : @@snip FaultHandlingTest.java { #supervisor }

This supervisor will be used to create a child, with which we can experiment:

Scala : @@snip FaultHandlingDocSpec.scala { #child }

Java : @@snip FaultHandlingTest.java { #child }

The test is easier by using the utilities described in @scala[@ref:Testing Actor Systems]@java[@ref:TestKit], where TestProbe provides an actor ref useful for receiving and inspecting replies.

Scala : @@snip FaultHandlingDocSpec.scala { #testkit }

Java : @@snip FaultHandlingTest.java { #testkit }

Let us create actors:

Scala : @@snip FaultHandlingDocSpec.scala { #create }

Java : @@snip FaultHandlingTest.java { #create }

The first test shall demonstrate the Resume directive, so we try it out by setting some non-initial state in the actor and have it fail:

Scala : @@snip FaultHandlingDocSpec.scala { #resume }

Java : @@snip FaultHandlingTest.java { #resume }

As you can see the value 42 survives the fault handling directive. Now, if we change the failure to a more serious NullPointerException, that will no longer be the case:

Scala : @@snip FaultHandlingDocSpec.scala { #restart }

Java : @@snip FaultHandlingTest.java { #restart }

And finally in case of the fatal IllegalArgumentException the child will be terminated by the supervisor:

Scala : @@snip FaultHandlingDocSpec.scala { #stop }

Java : @@snip FaultHandlingTest.java { #stop }

Up to now the supervisor was completely unaffected by the child’s failure, because the directives set did handle it. In case of an Exception, this is not true anymore and the supervisor escalates the failure.

Scala : @@snip FaultHandlingDocSpec.scala { #escalate-kill }

Java : @@snip FaultHandlingTest.java { #escalate-kill }

The supervisor itself is supervised by the top-level actor provided by the ActorSystem, which has the default policy to restart in case of all Exception cases (with the notable exceptions of ActorInitializationException and ActorKilledException). Since the default directive in case of a restart is to kill all children, we expected our poor child not to survive this failure.

In case this is not desired (which depends on the use case), we need to use a different supervisor which overrides this behavior.

Scala : @@snip FaultHandlingDocSpec.scala { #supervisor2 }

Java : @@snip FaultHandlingTest.java { #supervisor2 }

With this parent, the child survives the escalated restart, as demonstrated in the last test:

Scala : @@snip FaultHandlingDocSpec.scala { #escalate-restart }

Java : @@snip FaultHandlingTest.java { #escalate-restart }

Delayed restarts for classic actors

The supervision strategy to restart a classic actor only provides immediate restart. In some cases that will only trigger the same failure right away and giving things a bit of time before restarting is required to actually resolve the failure.

The akka.pattern.BackoffSupervisor implements the so-called exponential backoff supervision strategy, starting a child actor again when it fails, each time with a growing time delay between restarts.

This pattern is useful when the started actor fails <a id="^1" href="#1">[1]</a> because some external resource is not available, and we need to give it some time to start-up again. One of the prime examples when this is useful is when a @ref:PersistentActor fails (by stopping) with a persistence failure - which indicates that the database may be down or overloaded, in such situations it makes most sense to give it a little bit of time to recover before the persistent actor is started.

<a id="1" href="#^1">[1]</a> A failure can be indicated in two different ways; by an actor stopping or crashing.

Supervision strategies

There are two basic supervision strategies available for backoff:

• 'On failure': The supervisor will terminate and then start the supervised actor if it crashes. If the supervised actor stops normally (e.g. through context.stop), the supervisor will be terminated and no further attempt to start the supervised actor will be done.
• 'On stop': The supervisor will terminate and then start the supervised actor if it terminates in any way (consider this for PersistentActor since they stop on persistence failures instead of crashing)

To note that this supervision strategy does not restart the actor but rather stops and starts it. Be aware of it if you use @scala[Stash trait’s] @java[AbstractActorWithStash] in combination with the backoff supervision strategy. The preRestart hook will not be executed if the supervised actor fails or stops and you will miss the opportunity to unstash the messages.

Sharding

If the 'on stop' strategy is used for sharded actors a final termination message should be configured and used to terminate the actor on passivation. Otherwise the supervisor will just stop and start the actor again.

The termination message is configured with:

@@snip BackoffSupervisorDocSpec.scala { #backoff-sharded }

And must be used for passivation:

@@snip BackoffSupervisorDocSpec.scala { #backoff-sharded-passivation }

Simple backoff

The following snippet shows how to create a backoff supervisor which will start the given echo actor after it has stopped because of a failure, in increasing intervals of 3, 6, 12, 24 and finally 30 seconds:

Scala : @@snip BackoffSupervisorDocSpec.scala { #backoff-stop }

Java : @@snip BackoffSupervisorDocTest.java { #backoff-stop }

Using a randomFactor to add a little bit of additional variance to the backoff intervals is highly recommended, in order to avoid multiple actors re-start at the exact same point in time, for example because they were stopped due to a shared resource such as a database going down and re-starting after the same configured interval. By adding additional randomness to the re-start intervals the actors will start in slightly different points in time, thus avoiding large spikes of traffic hitting the recovering shared database or other resource that they all need to contact.

The akka.pattern.BackoffSupervisor actor can also be configured to stop and start the actor after a delay when the actor crashes and the supervision strategy decides that it should restart.

The following snippet shows how to create a backoff supervisor which will start the given echo actor after it has crashed because of some exception, in increasing intervals of 3, 6, 12, 24 and finally 30 seconds:

Scala : @@snip BackoffSupervisorDocSpec.scala { #backoff-fail }

Java : @@snip BackoffSupervisorDocTest.java { #backoff-fail }

Customization

The akka.pattern.BackoffOnFailureOptions and akka.pattern.BackoffOnRestartOptions can be used to customize the behavior of the back-off supervisor actor. Options are:

• withAutoReset: The backoff is reset if no failure/stop occurs within the duration. This is the default behaviour with minBackoff as default value
• withManualReset: The child must send BackoffSupervisor.Reset to its backoff supervisor (parent)
• withSupervisionStrategy: Sets a custom OneForOneStrategy (as each backoff supervisor only has one child). The default strategy uses the akka.actor.SupervisorStrategy.defaultDecider which stops and starts the child on exceptions.
• withMaxNrOfRetries: Sets the maximum number of retries until the supervisor will give up (-1 is default which means no limit of retries). Note: This is set on the supervision strategy, so setting a different strategy resets the maxNrOfRetries.
• withReplyWhileStopped: By default all messages received while the child is stopped are forwarded to dead letters. With this set, the supervisor will reply to the sender instead.

Only available on BackoffOnStopOptions:

• withDefaultStoppingStrategy: Sets a OneForOneStrategy with the stopping decider that stops the child on all exceptions.
• withFinalStopMessage: Allows to define a predicate to decide on finally stopping the child (and supervisor). Used for passivate sharded actors - see above.

Some examples:

@@snip BackoffSupervisorDocSpec.scala { #backoff-custom-stop }

The above code sets up a back-off supervisor that requires the child actor to send a akka.pattern.BackoffSupervisor.Reset message to its parent when a message is successfully processed, resetting the back-off. It also uses a default stopping strategy, any exception will cause the child to stop.

@@snip BackoffSupervisorDocSpec.scala { #backoff-custom-fail }

The above code sets up a back-off supervisor that stops and starts the child after back-off if MyException is thrown, any other exception will be escalated. The back-off is automatically reset if the child does not throw any errors within 10 seconds.