102 lines
4.5 KiB
ReStructuredText
102 lines
4.5 KiB
ReStructuredText
.. _howto-java:
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######################
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HowTo: Common Patterns
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######################
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This section lists common actor patterns which have been found to be useful,
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elegant or instructive. Anything is welcome, example topics being message
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routing strategies, supervision patterns, restart handling, etc. As a special
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bonus, additions to this section are marked with the contributor’s name, and it
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would be nice if every Akka user who finds a recurring pattern in his or her
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code could share it for the profit of all. Where applicable it might also make
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sense to add to the ``akka.pattern`` package for creating an `OTP-like library
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<http://www.erlang.org/doc/man_index.html>`_.
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You might find some of the patterns described in the Scala chapter of
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:ref:`howto-scala` useful even though the example code is written in Scala.
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Scheduling Periodic Messages
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============================
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This pattern describes how to schedule periodic messages to yourself in two different
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ways.
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The first way is to set up periodic message scheduling in the constructor of the actor,
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and cancel that scheduled sending in ``postStop`` or else we might have multiple registered
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message sends to the same actor.
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.. note::
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With this approach the scheduled periodic message send will be restarted with the actor on restarts.
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This also means that the time period that elapses between two tick messages during a restart may drift
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off based on when you restart the scheduled message sends relative to the time that the last message was
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sent, and how long the initial delay is. Worst case scenario is ``interval`` plus ``initialDelay``.
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.. includecode:: code/docs/pattern/SchedulerPatternTest.java#schedule-constructor
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The second variant sets up an initial one shot message send in the ``preStart`` method
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of the actor, and the then the actor when it receives this message sets up a new one shot
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message send. You also have to override ``postRestart`` so we don't call ``preStart``
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and schedule the initial message send again.
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.. note::
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With this approach we won't fill up the mailbox with tick messages if the actor is
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under pressure, but only schedule a new tick message when we have seen the previous one.
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.. includecode:: code/docs/pattern/SchedulerPatternTest.java#schedule-receive
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Single-Use Actor Trees with High-Level Error Reporting
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======================================================
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*Contributed by: Rick Latrine*
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A nice way to enter the actor world from java is the use of Patterns.ask().
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This method starts a temporary actor to forward the message and collect the result from the actor to be "asked".
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In case of errors within the asked actor the default supervision handling will take over.
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The caller of Patterns.ask() will *not* be notified.
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If that caller is interested in such an exception, he must make sure that the asked actor replies with Status.Failure(Throwable).
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Behind the asked actor a complex actor hierarchy might be spawned to accomplish asynchronous work.
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Then supervision is the established way to control error handling.
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Unfortunately the asked actor must know about supervision and must catch the exceptions.
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Such an actor is unlikely to be reused in a different actor hierarchy and contains crippled try/catch blocks.
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This pattern provides a way to encapsulate supervision and error propagation to the temporary actor.
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Finally the promise returned by Patterns.ask() is fulfilled as a failure, including the exception.
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Let's have a look at the example code:
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.. includecode:: code/docs/pattern/SupervisedAsk.java
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In the askOf method the SupervisorCreator is sent the user message.
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The SupervisorCreator creates a SupervisorActor and forwards the message.
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This prevents the actor system from overloading due to actor creations.
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The SupervisorActor is responsible to create the user actor, forwards the message, handles actor termination and supervision.
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Additionally the SupervisorActor stops the user actor if execution time expired.
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In case of an exception the supervisor tells the temporary actor which exception was thrown.
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Afterwards the actor hierarchy is stopped.
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Finally we are able to execute an actor and receive the results or exceptions.
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.. includecode:: code/docs/pattern/SupervisedAskSpec.java
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Template Pattern
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================
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*Contributed by: N. N.*
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This is an especially nice pattern, since it does even come with some empty example code:
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.. includecode:: code/docs/pattern/JavaTemplate.java
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:include: all-of-it
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:exclude: uninteresting-stuff
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.. note::
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Spread the word: this is the easiest way to get famous!
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Please keep this pattern at the end of this file.
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