RE: Non-destructive listener?

> From: Nicolaus.Bauman-SWCg6hSX5lit/[email protected]
> 
> > > From: Nicolaus.Bauman-SWCg6hSX5lit/[email protected]
> > > 
> > > I totally agree, my needs are much better suited for topics. 
> > > Alas, I don't have that choice.
> > > 
> > > One last question: the approach you describe works 
> because while the 
> > > browser scans for a message on the queue, it locks the messages 
> > > until the scan is complete. Is that a correct assumption? If so, 
> > > then, yeah, that does sound VERY resource-intensive.
> > > 
> > > Thanks so much for such a clear and succinct response.
> > 
> > The browser doesn't lock the queue. The approach works because the 
> > browser is used to scan the queue, and individual messages are 
> > subsequently synchronously consumed using a QueueReceiver.
> > 
> > E.g, suppose the queue contains messages A, B, and C.
> > Using a queue browser, you determine you want to consume message B.
> > So you construct a QueueReceiver with a selector:
> >   "JMSMessageID='" + B.getJMSMessageID() + "'"
> > and consume the message using the receive() method.
> > That leaves messages A and C in the queue.
> > If message D is subsequently added to the queue, a subsequent scan 
> > with a browser will see messages, A, C, and D.
> > 
> > Its resource intensive in that you need to continuously 
> scan the queue 
> > to pick up new messages.
> > 
> > -Tim
> 
> First of all, thank you for taking time to explain this. I 
> can't remember when I have gotten better support from any 
> software community, let alone individual, on any product or 
> system I've been concerned with. This is so cool. 
> Furthermore, if the community support is already this good, 
> one can only guess at how much better the actual engineering 
> implementation is.
> 
> Secondly, now perhaps I'm just as thick as a whale omlet, but 
> what you describe seems to indicate that the browser can 
> _still_ miss a message if another Listener is listening on 
> the queue. Is this a wrong assumption?
> 

Yes, the browser will miss messages if there is one or more
receivers consuming from the queue. A browser can also see
messages that have already been consumed, as it is working on
a (partial) snapshot of the queue.

If you want your browser to see all messages, then you 
need to use the browser to drive all message consumption i.e,
only create a consumer for message, selecting on its JMSMessageID
once it has been browsed. 

-Tim





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