56 lines
2.7 KiB
ReStructuredText
56 lines
2.7 KiB
ReStructuredText
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.. _mailbox-acking:
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Mailbox with Explicit Acknowledgement
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=====================================
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When an Akka actor is processing a message and an exception occurs, the normal
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behavior is for the actor to drop that message, and then continue with the next
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message after it has been restarted. This is in some cases not the desired
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solution, e.g. when using failure and supervision to manage a connection to an
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unreliable resource; the actor could after the restart go into a buffering mode
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(i.e. change its behavior) and retry the real processing later, when the
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unreliable resource is back online.
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One way to do this is by sending all messages through the supervisor and
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buffering them there, acknowledging successful processing in the child; another
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way is to build an explicit acknowledgement mechanism into the mailbox. The
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idea with the latter is that a message is reprocessed in case of failure until
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the mailbox is told that processing was successful.
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The pattern is implemented `here
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<@github@/akka-contrib/src/main/scala/akka/contrib/mailbox/PeekMailbox.scala>`_.
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A demonstration of how to use it (although for brevity not a perfect example)
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is the following:
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.. includecode:: @contribSrc@/src/test/scala/akka/contrib/mailbox/PeekMailboxSpec.scala
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:include: demo
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:exclude: business-logic-elided
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Running this application (try it in the Akka sources by saying
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``sbt akka-contrib/test:run``) may produce the following output (note the
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processing of “World” on lines 2 and 16):
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.. code-block:: none
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Hello
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World
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[ERROR] [12/17/2012 16:28:36.581] [MySystem-peek-dispatcher-5] [akka://MySystem/user/myActor] DONTWANNA
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java.lang.Exception: DONTWANNA
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at akka.contrib.mailbox.MyActor.doStuff(PeekMailbox.scala:105)
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at akka.contrib.mailbox.MyActor$$anonfun$receive$1.applyOrElse(PeekMailbox.scala:98)
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at akka.actor.ActorCell.receiveMessage(ActorCell.scala:425)
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at akka.actor.ActorCell.invoke(ActorCell.scala:386)
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at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:230)
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at akka.dispatch.Mailbox.run(Mailbox.scala:212)
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at akka.dispatch.ForkJoinExecutorConfigurator$MailboxExecutionTask.exec(AbstractDispatcher.scala:502)
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at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:262)
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at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:975)
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at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1478)
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at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:104)
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World
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Normally one would want to make processing idempotent (i.e. it does not matter
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if a message is processed twice) or ``context.become`` a different behavior
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upon restart; the above example included the ``println(msg)`` call just to
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demonstrate the re-processing.
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