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From Gary Tully <gary.tu...@gmail.com>
Subject Re: prefetchExtension off-by-1 for transacted consumers with prefetchSize > 0
Date Fri, 19 Mar 2010 10:16:56 GMT
Fair point. From the perspective of the ResourceAdapter where there are
explicit expectations this makes good sense. The prefetch should be written
in stone.

>From a regular transacted consumers perspective, where the prefetch is
largely hidden, avoiding a hang for large transactions is good because a
hang would be hard to diagnose. I agree it could be considered user/error
but it is difficult to flag it as such.

It may make sense to make the (extend the prefetch for a
transaction) behavior configurable such that it can be disabled for the RA
or off by default and enabled in the event that large transactions hang.
Thinking more, off by default is probably the best approach as transactions
that exceed the prefetch should be rare and the RAR behavior would match
expectations.

On 18 March 2010 22:05, rodos77 <eugene.rodos@nexj.com> wrote:

>
> I don't really get this.  If you extend the prefetchSize to more than what
> it
> was configured to be by the maxMessages parameter in the
> Connection.createConnectionConsumer() call, you are violating the JMS spec.
> The maxMessages parameter is defined as "the maximum number of messages
> that
> can be assigned to a server session at one time", so how can you make it
> bigger than what it was set to by the ConnectionConsumer?!
>
> ActiveMQ Resource Adapter has 2 parameters in its ActivationSpec that
> control the maxMessages argument used in the
> Connection.createConnectionConsumer() call: maxSessions and
> maxMessagesPerSessions.  The maxMessages argument is set to their product.
> This makes sense.  If a user configures ActiveMQ to have x maxSessions and
> y
> maxMessagesPerSessions, maxMessages (and consequentially the prefetchSize)
> should be set to x*y.  But why should this ever be extended?  Shouldn't it
> be expected that only a maximum of x*y messages be delivered at the same
> time?  If a user wants to process z <= x*y messages in the same tx, that
> should not pose any problem as the RA will have x ServerSessions each
> capable of processing y messages at the same time.  But if the user wants
> to
> process z > x*y messages at the same time after explicitly configuring
> ActiveMQ to only deliver a maximum of x*y messages at the same time, this
> is
> a user error/problem.  ActiveMQ should not be violating the JMS Spec to
> accommodate this.  What is the point of having those properties in the 1st
> place if you are going to extend them and deliver as many messages as
> available?
>
>
>
> Gary Tully wrote:
> >
> > The prefetch extension in the transacted case is necessary for the case
> > where a transacted consumer wants to consumer more than prefetch messages
> > in
> > a single transaction. Without the extension, the broker would see a full
> > prefetch and not dispatch any more and a receive could block.
> >
> > On 15 March 2010 21:31, rodos77 <eugene.rodos@nexj.com> wrote:
> >
> >>
> >> JIRA AMQ-2651 opened.
> >>
> >> I would be happy to provide a patch but as I stated in my original post,
> >> I
> >> don't fully understand the need for prefetchExtension in the case of a
> >> transacted, asynchronous (prefetchSize > 0) consumer.  If somebody could
> >> explain it, that would perhaps give me the required knowledge to come up
> >> with a patch.
> >>
> >> The way I see it now, I would get rid of the prefetchExtension (in the
> >> above
> >> mentioned case) all together, but again I'm probably missing some piece
> >> of
> >> information and I don't want to cause any side effects.  Barring that, I
> >> think there is an off-by-1 error in its calculation but I need this
> >> confirmed in order to create a proper patch.
> >> --
> >> View this message in context:
> >>
> http://old.nabble.com/prefetchExtension-off-by-1-for-transacted-consumers-with-prefetchSize-%3E-0-tp27866123p27910673.html
> >> Sent from the ActiveMQ - User mailing list archive at Nabble.com.
> >>
> >>
> >
> >
> > --
> > http://blog.garytully.com
> >
> > Open Source Integration
> > http://fusesource.com
> >
> >
>
> --
> View this message in context:
> http://old.nabble.com/prefetchExtension-off-by-1-for-transacted-consumers-with-prefetchSize-%3E-0-tp27866123p27950855.html
> Sent from the ActiveMQ - User mailing list archive at Nabble.com.
>
>


-- 
http://blog.garytully.com

Open Source Integration
http://fusesource.com

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