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https://issues.apache.org/jira/browse/SCXML-112?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12732923#action_12732923
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Rahul Akolkar commented on SCXML-112:
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I'm afraid externalizing the queue is best for the 0.x line.
Your feedback is appreciated. Some comments:
This involves years of code evolution, but to summarize quickly (not in any
order):
* SCXMLExecutor is the primary API for clients
* SCInstance is the backing store for internal state
* Status exposes the state clients most want to know about
* SCXMLSemantics is the crux of the SCXML algorithm (extension point for users
to have different algorithm if and when it makes sense)
Much of that separation is useful IMO.
The #triggerEvents() behavior is different from a series of triggerEvent()
calls, but that was intentional when it was done (multiple active regions).
With many of the changes in the spec, its certainly on the cards to create a
version of the SCXMLExecutor for the next major line of development that
internalizes the external events queue that feeds the main loop for processing
events (there'd be a inner loop for internal events) and makes other tweaks to
the algorithm as needed. But the idea won't be to drastically change any API /
contracts.
> 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
> Fix For: 1.0
>
> 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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