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From "Yuriy Sidelnikov (JIRA)" <>
Subject [jira] [Created] (AMQ-4107) Message order can be broken for Topic under a high load when topicPrefetch=1 and comsumer is slow
Date Mon, 15 Oct 2012 08:10:03 GMT
Yuriy Sidelnikov created AMQ-4107:

             Summary: Message order can be broken for Topic under a high load when topicPrefetch=1
and comsumer is slow
                 Key: AMQ-4107
             Project: ActiveMQ
          Issue Type: Bug
          Components: Transport
    Affects Versions: 5.6.0
            Reporter: Yuriy Sidelnikov

For <amq:policyEntry topic=">" producerFlowControl="true" memoryLimit="30mb" topicPrefetch="1"

Short excerpt from TopicSubscription class:
public void add(MessageReference node) throws Exception {

              if (!isFull() && matched.isEmpty()  && !isSlave()) {
            // if maximumPendingMessages is set we will only discard messages which
            // have not been dispatched (i.e. we allow the prefetch buffer to be filled)
            dispatch(node);                   <- Second message will go this way and might
be dispatched sooner than first one.
        } else {
if (matched.tryAddMessageLast(node, 10)) {    <- first message will be put in the VMCursor
queue and might be dispatched later 
 dispatchMatched();   <- First message won't be dispatched immediately because !isFull()
is still false
Possible scenario as I can see it from logs:
1. First message has arrived and !isFull() is false because consumer didn't take some previous
message yet.
2. First message will be processed by tryAddMessageLast in VMPendingMessageCursor class and
will be dispatched very lately because !isFull() is still false.
3. Meanwhile consumer reads some previous message and !isFull() will return true.    
4. Second message will be dispatched immediately and might be first to be delivered. 
5. Then first message is dispatched.
6. Message order is broken.

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