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https://issues.apache.org/activemq/browse/AMQ-2567?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=56909#action_56909
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Gary Tully commented on AMQ-2567:
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so the prefetchExtension is the mechanism by which a message is dispatched to a
consumer with prefetch=0. Each receive() sends a pull if there are no messages
which increments the prefetchExtension so another message is dispatched. With a
prefetch of 0, you should only be able to get one message at a time.
I think a deliveredAck may be the problem, a delivered ack uses the
prefetchextension to ensure that within a transaction it is possible to read
past the prefetch limit. If prefetch=0, a pulling consumer, it should not do
that. Check to see if you are getting an ack with ackType=0.
if so, either the logic with a standard ack needs to be duplicated with a
deliveredAck or the delivery ack needs to be suppressed on the consumer.
> Zero Prefetch not working
> -------------------------
>
> Key: AMQ-2567
> URL: https://issues.apache.org/activemq/browse/AMQ-2567
> Project: ActiveMQ
> Issue Type: Bug
> Components: Broker
> Affects Versions: 5.3.0
> Environment: Unix/Windows, Java 1.6
> Reporter: Rudolf Janz
> Attachments: ZeroPrefetchConsumerTest.java
>
>
> I have problems using the zero prefetch. The consumer starts prefetching
> messages after some receives.
>
> Our use case is, that we have a large computation which is split into smaller
> sub jobs. These jobs are sent via ActiveMQ to some processing nodes on
> different machines. The duration of jobs differs very much (10s to some
> minutes). The jobs are sent by decreasing estimated computation time. If one
> of the consumers which receives a large job prefetches some other jobs, these
> will be processed later. In the meantime the remaining consumers are idle,
> and the total computation time is much longer than necessary.
>
> I have modified the existing ZeroPrefetchConsumerTest to test for the problem
> (I have removed the other test methods).
>
> Two consumers (C1 and C2) are instantiated. These messages are sent:
> 1,2,3,4,5,6,7,8,9
>
> C1 reads 4 times, receives 1,2,3,4 -> this is correct
>
> now C2 reads, it receives 8 not 5, which is the next message in the queue.
> The reason is, that C1 prefetched 5, 6, 7, that should not have
> happened.(sometimes C1 only prefetches 5,6)
>
> The problem can be seen in the JMX Console as well, after a while, the first
> consumer has more than one dispatched message and the queue has an
> InflightCount of 3, although there are only two consumers!
>
> The last version that we used was 4.1.1, that worked.
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