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From "Jerry Cwiklik (JIRA)" <...@uima.apache.org>
Subject [jira] Created: (UIMA-1902) UIMA AS should send an ACK message to a client for each request
Date Fri, 15 Oct 2010 14:19:32 GMT
UIMA AS should send an ACK message to a client for each request

                 Key: UIMA-1902
                 URL: https://issues.apache.org/jira/browse/UIMA-1902
             Project: UIMA
          Issue Type: Bug
          Components: Async Scaleout
    Affects Versions: 2.3AS
            Reporter: Jerry Cwiklik
            Assignee: Jerry Cwiklik

UIMA AS service checks for existence of a reply queue before processing an incoming request.
This is done to prevent wasting cycles processing a request if it is known that the client
has gone away. Current approach is based on checking broker's queue inventory for presence
of a temp reply queue provided in a request msg.  The framework code uses JMX connection to
a broker to perform the queue lookup. If the lookup fails, the framework concludes that the
client has gone away, places the reply queue in a list of destinations known to be dead and
throws away the request msg. Any subsequent msg containing the same reply queue is automatically
thrown away since the destination is marked as dead. Occasionally, the JMX lookup fails even
though (as reported by some users) the client is alive at the time the lookup was done. It
seems that perhaps under load JMX fails apart, reporting false positive result ( queue not
present). The failed lookup results in a dropped request msg and ultimately in a client timeout
(or possible hang if the client is not using timout). 

Modify the implementation to stop using JMX to perform lookups. Instead, before any processing
takes place, send a small msg (ACK) back to the client via JMS. If the send fails, report
the problem, drop the request and wait for the next request. Also, dont add failed destination
to the list of clients known to be dead. Each request should be subject to an ACK no matter
what happened previously.

The new approach, although generating more jms traffic, has some advantages:

1) A client is immediately notified when its request has been picked by a service and is ready
to be processed.
2) A client "knows" which service is processing each request
3) A client can use a different timeout strategy since it knows precisely when its request
is ready to be processed.  

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