[jira] [Commented] (AMQ-4107) Message order can be broken for Topic under a high load when topicPrefetch=1 and comsumer is slow

["Tim Bain (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AMQ-4107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14149593#comment-14149593 ] 

Tim Bain commented on AMQ-4107:
-------------------------------

Will this patch only be applied if a unit test is added for it?

> Message order can be broken for Topic under a high load when topicPrefetch=1 and comsumer is slow
> -------------------------------------------------------------------------------------------------
>
>                 Key: AMQ-4107
>                 URL: https://issues.apache.org/jira/browse/AMQ-4107
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: Transport
>    Affects Versions: 5.6.0
>            Reporter: Yuriy Sidelnikov
>         Attachments: 4107.diff, ActiveMQ-5.9-AMQ-4107.patch
>
>
> For <amq:policyEntry topic=">" producerFlowControl="true" memoryLimit="30mb" {color:red}topicPrefetch="1"{color} blockedProducerWarningInterval="30">
> Short excerpt from TopicSubscription class:
> public void add(MessageReference node) throws Exception {
> …..
>               if ({color:red}!isFull(){color} && 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)
>        {color:red}dispatch(node);{color}                   <- Second message will go this way and might be dispatched sooner than first one.
>             setSlowConsumer(false);
>         } else {
> …….
> if ({color:red}matched.tryAddMessageLast(node, 10)){color} {    <- first message will be put in the VMCursor queue and might be dispatched later 
>         
> break;
>                         }
> .....
>  {color:red}dispatchMatched();{color}   <- 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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