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From "Eugene Rodos (JIRA)" <>
Subject [jira] Created: (AMQ-2651) prefetchExtension off-by-1 for transacted consumers with prefetchSize > 0
Date Mon, 15 Mar 2010 21:23:44 GMT
prefetchExtension off-by-1 for transacted consumers with prefetchSize > 0

                 Key: AMQ-2651
             Project: ActiveMQ
          Issue Type: Bug
          Components: Broker
    Affects Versions: 5.3.0
            Reporter: Eugene Rodos

Linked to forum discussion:


I've searched the forum and JIRA and have noticed that the prefetchExtension in PrefetchSubscription
has caused grief before.  However, I think there's still a problem.

First, I understand the purpose of the prefetchExtension for the case when prefetchSize =
0.  It allows messages to be dispatched to the consumer when the consumer requests them (i.e.
polls them).  However, I don't really understand the purpose of the prefetchExtension for
the cases when prefetchSize > 0.  If the consumer has set the prefetchSize to x, shouldn't
it always receive only x messages at a time?  Why is the prefetchSize being extended?  I see
that it is only extended in the case when the message delivery is transacted but I still don't
understand why this is necessary.

In any case, assuming the prefetchExtension is necessary in the case of prefetchSize >
0 and transacted message delivery, I think the calculation of the prefetchExtension in this
case has an off-by-1 error.  I am attaching a junit test that illustrates this problem.

The test is basically acting like a JCA Resource Adapter and sets up an asynchronous message
listener by creating a ConnectionConsumer with a maxMessages value of 1.  This translates
into a PrefetchSubscription with a prefetchSize of 1.  The test then sends 3 messages.  The
purpose of the 1st message is to trigger the prefetch extension logic in PrefetchSubscription
upon the message's ack.  When this happens, the prefetchExtension is set to 1 and the prefetchSize
essentially becomes 2.  I believe this is incorrect and is the off-by-1 error (assuming the
prefetchExtension is required at all in this case).  The 2nd message is simulated to be a
long running message so that when the 3rd message is dispatched, the 2nd message is still
being processed.  When the 3rd message is dispatched, because the prefetchSize has been extended,
it will try to deliver it even though the processing of the 2nd message has not been completed
and even though maxMessages on the ConnectionConsumer was specified as 1.

Now, if I only have 1 ServerSession in the ServerSessionPool, this behavior seems to violate
the JMS Spec since the maxMessages parameter used during ConnectionConsumer creation is defined
as "the maximum number of messages that can be assigned to a server session at one time" and
ActiveMQ is trying to assign 2 messages to the same ServerSession at the same time. 

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