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From "Ales Dolecek (JIRA)" <j...@apache.org>
Subject [jira] Issue Comment Edited: (SCXML-112) Wrong behavior if event is triggered from EventDispatcher
Date Thu, 16 Jul 2009 13:32:14 GMT

    [ https://issues.apache.org/jira/browse/SCXML-112?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12731934#action_12731934
] 

Ales Dolecek edited comment on SCXML-112 at 7/16/09 6:31 AM:
-------------------------------------------------------------

Oh and probably the easiest solution:

4) throw IllegalStateException if the EventDispatcher tries to re-enter the loop (although
it will not help with the non-deterministic behavior)

BTW: Same probably holds for triggering events from <invoke> or listeners

      was (Author: alesd):
    Oh and probably the easiest solution:

4) throw IllegalStateException if the EventDispatcher tries to re-enter the loop

BTW: Same probably holds for triggering events from <invoke> or listeners
  
> Wrong behavior if event is triggered from EventDispatcher
> ---------------------------------------------------------
>
>                 Key: SCXML-112
>                 URL: https://issues.apache.org/jira/browse/SCXML-112
>             Project: Commons SCXML
>          Issue Type: Bug
>            Reporter: Ales Dolecek
>         Attachments: Dispatcher.java, test1.xml, test2.xml
>
>
> Method SCMLExecutor#triggerEvents is synchronized which is too naive way to ensure that
events are processed in order they arrive.
> Since the callback to EventDispatcher is made by thread that holds lock on SCMLExecutor
triggering event from the dispatcher allows this thread to re-enter the event handling.
> Note: This has nothing to do with multi-threading since there is only one thread.
> Simple queue might e sufficient but in multi-threaded application would make the first
thread to enter #triggerEvents process events from other threads queued while the method is
running.
> Another approach might be to call EventDispatcher in separate thread,but it has it's
drawbacks too:
> a) changes single-threaded application into-multithreaded which might break assumtions
made by unaware user
> b) might start too many threads
> c) Java does not guarantee that threads waiting for lock will get it in same order as
they arrive => might result in non-deterministic behavior of SCXML interpretation
> Possible solutions would be:
> 1) add queue and allow the "first" thread server all events - might be just fine since
multi-threaded applications might create dedicate thread just for SCXML imterpretation
> 2) add queue and block threads after they queue event - so they "wait" until execute
their event become first in queue
> 3) factor out the queue event queue management and allow for "pluggable" strategy

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