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https://issues.apache.org/activemq/browse/AMQ-2651?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Gary Tully resolved AMQ-2651.
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Fix Version/s: 5.4.0
Resolution: Fixed
resolved in r935954
policyEntry.usePrefetchExtension allows the prefetch extension behaviur to be
disabled. When false, and with prefetch of 1 or X, a new message will only be
dispatched when the message is acked, rather than when it is delivered. When
false, a client ack consumer or transacted consumer will only ever get
prefetch (X) number messages.
code example of it disabled from the test:
{code}
PolicyMap policyMap = new PolicyMap();
PolicyEntry defaultEntry = new PolicyEntry();
// ensure prefetch is exact. only delivery next when current is acked
defaultEntry.setUsePrefetchExtension(false);
policyMap.setDefaultEntry(defaultEntry);
broker.setDestinationPolicy(policyMap);{code}
> prefetchExtension off-by-1 for transacted consumers with prefetchSize > 0
> -------------------------------------------------------------------------
>
> Key: AMQ-2651
> URL: https://issues.apache.org/activemq/browse/AMQ-2651
> Project: ActiveMQ
> Issue Type: Bug
> Components: Broker
> Affects Versions: 5.3.0
> Reporter: Eugene Rodos
> Assignee: Gary Tully
> Fix For: 5.4.0
>
> Attachments: OnePrefetchAsyncConsumerTest.java,
> UsePrefetchExtension_Option.patch
>
>
> Linked to forum discussion:
> http://old.nabble.com/prefetchExtension-off-by-1-for-transacted-consumers-with-prefetchSize-%3E-0-ts27866123.html
> Hi,
> 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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