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From Tim Bain <tb...@alumni.duke.edu>
Subject Re: how to safely delete a cms::MessageListener object
Date Fri, 16 Jan 2015 17:12:19 GMT
That sounds way better than what I'd proposed.

On Fri, Jan 16, 2015 at 9:18 AM, Timothy Bish <tabish121@gmail.com> wrote:

> On 01/15/2015 09:16 PM, artnaseef wrote:
>
>> How about the following instead?  As Tim pointed out, the consumer close
>> won't return until no more calls to onMessage are guaranteed.  I believe
>> that also means it won't return while onMessage() is actively being
>> called.
>>
>> @Tim - can you confirm?
>>
>
> It should be the case that onMessage must always complete prior to close()
> returning, unless you do something silly like call close from onMessage.
> There are two locks at play here that prevent things from going wrong, one
> on the listener and one on the dispatch queue.  This is why it is also safe
> to just call setMessageListener(null) on the consumer as it will also not
> return until no onMessage calls are in progress.  During the time there is
> no set message listener the messages will pile up in the prefetch queue and
> dispatch would start again once a new listener is added.
>
>
>
>> If that's correct, the following should do the job effectively:
>>
>> ON MESSAGE
>>      LOCK semaphore
>>      IF shutdown started
>>      THEN
>>         ABORT
>>      END IF
>>
>>      SET processing active
>>      UNLOCK semaphore
>>
>>      DO process message
>>
>>      LOCK semaphore
>>        SET processing NOT active
>>      UNLOCK semaphore
>>
>>
>> ON LISTENER SHUTDOWN
>>      LOCK semaphore
>>        SET shutdown started
>>      UNLOCK semaphore
>>
>>      DO close consumer
>>
>>      SET not done
>>      WHILE not done
>>        LOCK semaphore
>>          IF processing NOT active
>>          THEN
>>            SET done
>>          END IF
>>        UNLOCK semaphore
>>
>>        IF NOT done
>>        THEN
>>          DELAY // short sleep; note there are methods to eliminate the
>> sleep
>> and use a notification; I don't recall them for C++ right now
>>        END IF
>>      END WHILE
>>
>>      DO delete message listener
>>
>>
>> That last part about the delay can be solved with another semaphore as
>> well.
>> Also, if the part up to "ABORT" in the listener can be done safely even
>> after the listener is deleted, then this works even if the consumer close
>> can return while the call to onMessage() is still active.
>>
>>
>>
>> --
>> View this message in context: http://activemq.2283324.n4.
>> nabble.com/how-to-safely-delete-a-cms-MessageListener-
>> object-tp4689939p4690003.html
>> Sent from the ActiveMQ - User mailing list archive at Nabble.com.
>>
>>
>
> --
> Tim Bish
> Sr Software Engineer | RedHat Inc.
> tim.bish@redhat.com | www.redhat.com
> skype: tabish121 | twitter: @tabish121
> blog: http://timbish.blogspot.com/
>
>

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