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From Alan Conway <acon...@redhat.com>
Subject Re: c++ messaging api: a map message example
Date Wed, 05 Aug 2009 13:05:40 GMT
On 08/05/2009 02:45 AM, Gordon Sim wrote:
>> This looks messy:
>> listener.subscribed(session.subscribe("message_queue", listener));
>>
>> Informing the listener can be done as part of the implementation of
>> session.subscribe(), it doesn't
>> need to be left to the user.
>
> I'll have a think about this.

My main objection to the line above is the fact that listener is mentioned 
twice. What does it mean if you use two different listeners? I think its an 
error, in which case the API should be modified to make that error impossibl, 
e.g. by doing the whole thing atomically in sesision.susbcribe. This also avoids 
the error where the user forgets to call listener.subscribed(). I'm assuming 
here that part of the semantics of MessageListener is that it will get a call to 
subscribed() each time it is subscribed.

>> We need to sort out our threading model in this new API. The critical
>> thing is to allow many sessions to be served by the same thread or
>> thread pool pool.
>
> I agree that this is an important use case. I'll go in to this in more
> detail shortly.
>
>> We also want to integrate that with our own client side poller, and of
>> course keep the current model for backwards compatibility.
>>
>> How about providing two alternatives:
>> Session {
>> void dispatch(); // Calling thread dispatches session.
>> // OR
>> void activate(); // Session is dispatched by qpid's own thread pool.
>> Does not block.
>> void deactivate(); // Stop dispatching in qpid thread pool, blocks
>> till current operation completes.
>> void wait(); // Wait for activated session to be closed, or the last
>> subscription to be cancelled.
>> }
>>
>> I think that covers the majority of cases. The other case that has
>> been mentioned is providing a selectable fd to dispatch to qpid from
>> someone elses select/poll/epoll loop. I'd say that's an addition for
>> the 1.1 release of the new API, not necessarily for 1.0.
>
> Agree (on both points!). Thanks again for all the comments and suggestions!

The commnet on void wait(); above should say "wait for session to be 
deactivated, which occurs when session is closed, last subscription is cancelled 
or deactivate() is called on the session."

Thinking: if we auto-deactivate on subscription cancel, should we auto-activate 
on subscribe (if the sesssion was previously activated() rather than dispatched())?

Thinking some more: maintaining a thread pool in global variables is going to 
create a lot of start-up and shut down ordering issues. Since this is a new API 
I think it might be a good idea to introduce a qpid::Qpid object which can 
encapsulate the thread pool and any other context that is shared between all 
connections. So the examples would go

main() {
   Qpid qpid;
   Connection c = qpid.open(...);
   // Qpid dtor cleans up here.
}

That lets us do all our init & shut-down inside main and avoids problems of 
static ctor/dtor ordering. That also gives a place to configure things like the 
client-side thread pool size etc.

   qpid.setWorkerThreads(10); // 10 worker threads in the pool.

We could also provide built-in argc/argv processing:
main (int argc, char**argv) {
   Qpid qpid(argc, argv);
   // Here argc/argv have been modified to remove all --qpid arguments.
}

That gives an out-of-the-box way for customers to write configurable clients. 
For the client side I think we'd prefix all the options with --qpid- to avoid 
clashes with user options. e.g. --qpid-worker-threads etc.

Above ideas shamelessly stolen from CORBA. There is a significant maintenance 
advantage to keeping it all in main() in terms of lost time chasing sporadic 
shutdown bugs.

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