On 01/17/2012 10:28 PM, Gang, Litao wrote:
when a message in a queue is marked as consumed? Ideally it should be right before the received(.) callback is invoked. But from what I see looks like it is not.
Assuming auto-ack is in use (and is set to 1), the messages will be acknowledged immediately *after* the received callback. The reason for this is that should the client process crash just as the received() call is being made, it may not get time to process the message.
As a workaround, can I call some routine to tell the broker to remove the message on queue?
Yes, you can turn off auto-ack and do the accept yourself. That way you can do it before processing the message if desired, or as part of any poisoned letter check.
The simplest way to do this is probably to use accept() method on the associated Subscription object. This will be synchronous as well, meaning that the call won't return until the broker has received the accept. The auto-ack property on the SubscriptionSettings can be set to 0 to turn off automatic accepts.
(Another pattern I'll mention just for completeness, is to use LocalQueue rather than MessageListener. Here you explicitly pop off the next message in order to process it and the accept is issued at that point, i.e. generally before any processing. This is similar I believe to JMS where messages are accepted on returned from the listener callback if in use, or on returning from a receive() call if not.)
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