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From "Wei Wei (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AMQ-4052) When consumers was killed and restarted frequently, some messages could not be consumed by consumer but they were still in pending messages.
Date Wed, 19 Sep 2012 10:44:07 GMT

    [ https://issues.apache.org/jira/browse/AMQ-4052?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13458554#comment-13458554
] 

Wei Wei commented on AMQ-4052:
------------------------------

I tested against the apache-activemq-5.7-fuse-20120912.001718-11 code and builds in these
two days and found that the problem still exists.

Reason:
In method PrefetchSubscription#dispatchPending(), pending.remove() made the message removed
from the pending, but if canDispatch(node) returns false, the method PrefetchSubscription#dispatch(node)
was not called and the message could not put to dispatched list, so the message was not sent
to client and can not put back to Queue#redeliveredWaitingDispatch. 
When canDispatch(node) return false? If the server received the acknowledge commmand but did
not received the transaction commit command, node.isAcked() returns true and node.isDropped()
returns false, the result is false.

org.apache.activemq.broker.region.QueueSubscription
  protected boolean canDispatch(MessageReference n) throws IOException {
    boolean result = true;
    QueueMessageReference node = (QueueMessageReference)n;
    if ((node.isAcked()) || (node.isDropped())) {
      result = false;
    }
    result = (result) && ((isBrowser()) || (node.lock(this)));
    return result;
  }
                
> When consumers was killed and restarted frequently, some messages could not be consumed
by consumer but they were still in pending messages.
> --------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: AMQ-4052
>                 URL: https://issues.apache.org/jira/browse/AMQ-4052
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: Broker
>    Affects Versions: 5.6.0
>         Environment: CentOS release 5.6 (Final) java version "1.6.0_30"
>            Reporter: Wei Wei
>
> The phenomenon of the bug:
> When consumers was killed and restarted frequently, some messages could not be consumed
by consumer but they were still in pending messages.
> Reason:
> When consumer consumes messages in transaction, it sends ack command and transaction
commit command separately, if the consumer was killed between ack command and transaction
commit command, in other words the Broker only received the ack command but did not receive
the transaction commit command, this lead to the message in PrefetchSubscription’s dispatched
list was set ack true, but was not removed from dispatched list. When the consumer was killed,
Queue#removeSubscription() was called, in this method, PrefetchSubscription#remove() was called
to put PrefetchSubscription’s pending list and dispatched list to Queue’s redeliveredWaitingDispatch
list. After that, when Queue#doDispatch() was called, the redeliveredWaitingDispatch list
was transferred to Queue#doActualDispatch () method to dispatch the messages to consumer again.
In Queue#doActualDispatch () method, if all consumers isFull() method returns true, the target
is null and the expression “interestCount > 0” become the only condition to judge if
the message should be put back to the  redeliveredWaitingDispatch, but now the message’s
isAcked() method returns true, this lead to interestCount is 0, and then the message was not
put back to redeliveredWaitingDispatch, but it was not consumed and was still in pagedInMessages.

> Solution:
> In PrefetchSubscription#remove() method, set ack status to false for all messages in
dispatched list. The code as follows:
> {code}
>     public List<MessageReference> remove(ConnectionContext context, Destination
destination) throws Exception {
>         List<MessageReference> resultList = new ArrayList<MessageReference>();
>         synchronized(pendingLock) {
>             super.remove(context, destination);
>             // Here is a potential problem concerning Inflight stat:
>             // Messages not already committed or rolled back may not be removed from
dispatched list at the moment
>             // Except if each commit or rollback callback action comes before remove
of subscriber.
>             resultList.addAll(pending.remove(context, destination));
>             // Synchronized to DispatchLock
>             synchronized(dispatchLock) {
>                 ArrayList<MessageReference> references = new ArrayList<MessageReference>();
>                 for (MessageReference msgRef : dispatched) {
>                     if( msgRef.getRegionDestination() == destination) {
>                         references.add(msgRef);
>                         if (msgRef instanceof QueueMessageReference) {
>                         	QueueMessageReference ref = (QueueMessageReference) msgRef;
>                         	ref.setAcked(false);
>                         }
>                     }
>                 }
>                 resultList.addAll(references);
>                 destination.getDestinationStatistics().getDispatched().subtract(references.size());
>                 destination.getDestinationStatistics().getInflight().subtract(references.size());
>                 dispatched.removeAll(references);
>             }
>         }
>         
>         return resultList;
>     }
> {code}
> The solution can solve the problem, I want to know if the solution can lead to other
problems.

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