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https://issues.apache.org/jira/browse/CAY-1864?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13818732#comment-13818732
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Andrus Adamchik commented on CAY-1864:
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It's been a while since I used Cayenne built-in pool for connection pooling.
While it is entirely safe to use in production, it is intentionally simple and
doesn't have many of the bells and whistles of the alternatives, e.g. [1] - the
one that I am using now.
Having said that, I have no objections to improving the algorithm, such as
adding fair ordering, if it doesn't add much overhead of its own.
Question on the use of Deque - in our case we seem to just need a queue, not a
deque?
Also I feel that we may get a bigger bang for the buck if we drop traditional
Java synchronization approach and start using concurrent utils. E.g. Semaphore
[2] already supports fair ordering of threads.
[1] http://tomcat.apache.org/tomcat-7.0-doc/jdbc-pool.html
[2] http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Semaphore.html
> Process threads waiting for an available DB connection in the order they came
> in
> --------------------------------------------------------------------------------
>
> Key: CAY-1864
> URL: https://issues.apache.org/jira/browse/CAY-1864
> Project: Cayenne
> Issue Type: Improvement
> Components: Core Library
> Affects Versions: 3.2M1
> Reporter: John Huss
> Assignee: John Huss
> Attachments:
> 0001-CAY-1864-Process-threads-waiting-for-an-available-DB.patch
>
>
> Currently when all the DB connections in the pool are used up threads will
> wait for available connection. As connections become available they are
> given to essentially random threads, not the thread that was waiting first.
> This causes somewhat erratic behavior since some requests will come in and
> get a connection right away even though others have been waiting for a long
> time, eventually causing more threads to throw exceptions than may have been
> necessary if requests were processed in order.
> Particularly in cases with large but brief spikes in activity - errors may be
> wholly avoided if requests are processed in order where otherwise they could
> produce many failures. I have observed this in my testing.
> I have a patch I am testing out. It creates a queue to keep track of the
> waiting threads and then grant them connections in order.
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