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From "james strachan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (AMQ-930) Session 'consumers' hashtable susceptible to invalid operation exception
Date Fri, 22 Sep 2006 10:05:23 GMT
    [ https://issues.apache.org/activemq/browse/AMQ-930?page=comments#action_36991 ] 
            
james strachan commented on AMQ-930:
------------------------------------

I don't quite understand the reason for this patch - I wonder if you could help explain it?

Session.DispatchAsyncMessages() is only ever called in a separate thread pool via a call to
ThreadPool.QueueUserWorkItem(new WaitCallback(session.DispatchAsyncMessages) in the MessageConsumer.

The idea is that only 1 thread at once calls this method for all consumers created by the
session. (In NMS, just like in JMS, messages are only dispatched from one session at once
to however many consumers it has - rather than dispatching to multiple consumers in parallel).

Am wondering if a better fix is just to ensure that the collection ("consumer.Values") is
completely thread safe to avoid the concurrent modification errors you are seeing. I'm working
on a patch right now - will commit it shortly - am wondering if this is a better approach?




> Session 'consumers' hashtable susceptible to invalid operation exception
> ------------------------------------------------------------------------
>
>                 Key: AMQ-930
>                 URL: https://issues.apache.org/activemq/browse/AMQ-930
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: NMS (C# client)
>    Affects Versions: incubation
>         Environment: Windows XP, NMS API running against a AMQ 4.0.2 provider
>            Reporter: Bryan Schmidt
>
> In a multithreaded environment that reuses the Session object, the following exception
is thrown:
> Invalid operation exception with the text: "Collection was modified; enumeration operation
may not execute."
> The exception is thrown when iterating through Session's consumers.Values collection.
 It appears that the lock is being ignored.  Spinning up a new thread fixes the problem:
> --- Session.cs, DispatchAsyncMessages method ---
>         public void DispatchAsyncMessages(object state)
>         {
>             // lets iterate through each consumer created by this session
>             // ensuring that they have all pending messages dispatched
>             lock (this)
>             {
>                 // lets ensure that only 1 thread dispatches messages in a consumer at
once
>                 foreach (MessageConsumer consumer in consumers.Values)
>                 {
>                     ThreadPool.QueueUserWorkItem(new WaitCallback(consumer.DispatchAsyncMessages));
>                 }
>             }
>         }

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