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From "Andrus Adamchik (JIRA)" <>
Subject [jira] [Commented] (CAY-1864) Process threads waiting for an available DB connection in the order they came in
Date Mon, 11 Nov 2013 06:43:17 GMT


Andrus Adamchik commented on CAY-1864:

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.


> Process threads waiting for an available DB connection in the order they came in
> --------------------------------------------------------------------------------
>                 Key: CAY-1864
>                 URL:
>             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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