Open Stack

Kinda! The queue has a name, but that name has no bearing on the set of
messages received.

If you create a queue called "MyCustomNotificationQueue", you can bind that
to the "notifications" exchange using the "notifications.info" routing key.

(I'm guessing some of the names here.. I know AMQP, and not the specific
naming nova uses!)

Nova just happens to use the same queue name and routing key. I believe
this is causing the confusion.

Does this make sense?

Anyway - The RabbitMQ docs probably explain it better than I..
http://www.rabbitmq.com/tutorials/tutorial-five-python.html

Thanks,
Kiall


On Wed, May 9, 2012 at 11:30 AM, Day, Phil <philip.day at hp.com> wrote:

> OK, get that so far – so both consumers need to declare and use the same
> exchange.****
>
> ** **
>
> But If I understand the next step right, to get multiple consumers of
>  info notification messages they would all need to create separate “
> notifications.info” queues into that exchange.    And isn’t that exactly
> what Nova currently does to create a shared queue ?****
>
> ** **
>
> Phil****
>
> ** **
>
> *From:* Kiall Mac Innes [mailto:kiall at managedit.ie]
> *Sent:* 09 May 2012 10:51
> *To:* Day, Phil
> *Cc:* openstack at lists.launchpad.net; Russell Bryant; Doug Hellmann
>
> *Subject:* Re: [Openstack] [nova] why does notification use a "topic"
> exchange instead of "fanout"?****
>
> ** **
>
> Your own queue listener should attempt to declare the exchange, using the
> same settings as Nova does. ****
>
> If the exchange exists, its a noop. Otherwise it's created for you.****
>
> After that, if you start up Nova, it will do the same and reuse your
> exchange.****
>
> Obviously this works both ways, and either nova or your code can declare
> the exchange.****
>
> AMQP is designed to be a configuration-less protocol, where resources are
> configured by the first consumer attempting to use them.****
>
> Thanks,
> Kiall****
>
> Sent from my phone.****
>
> On May 9, 2012 9:52 a.m., "Day, Phil" <philip.day at hp.com> wrote:****
>
> Hi Doug,****
>
>  ****
>
> > I think you missed my main point, which was that a topic exchange does
> > not impose a limitation that only one client can consume a given
> > notification.  That's only true if each client is consuming from the
> > same queue bound to the exchange.****
>
>  ****
>
> So just to be clear, if I understand you correctly within the nova
> service/rpc abstraction layers the code is set up so that all services do
> bind to the same queue, and hence we get the round-robin delivery.****
>
> But, if someone wanted to write a separate notification consumer so that
> they didn’t block anyone else from seeing the same messages then they (the
> consumer) should create a new queue on the existing topic exchange.****
>
>  ****
>
> Is that correct – and is there any worked example of doing this ?****
>
>  ****
>
> I thought within the nova code both the exchange and topic queues were set
> up by the consumer (so for example all compute_managers try to create the
> “compute” exchange and topic queue, but its only created by the first one
> and the others connect to the same queue).   In that context I’m finding it
> hard to see how to change this model to have multiple “notify.info” topic
> queues into the same exchange ?****
>
>  ****
>
> Cheers,****
>
> Phil****
>
>  ****
>
>  ****
>
>  ****
>
>  ****
>
> *From:* openstack-bounces+philip.day=hp.com at lists.launchpad.net [mailto:
> openstack-bounces+philip.day=hp.com at lists.launchpad.net] *On Behalf Of *Doug
> Hellmann
> *Sent:* 08 May 2012 23:34
> *To:* Russell Bryant
> *Cc:* openstack at lists.launchpad.net
> *Subject:* Re: [Openstack] [nova] why does notification use a "topic"
> exchange instead of "fanout"?****
>
>  ****
>
>  ****
>
> On Tue, May 8, 2012 at 6:04 PM, Russell Bryant <rbryant at redhat.com> wrote:
> ****
>
> On 05/08/2012 05:59 PM, Doug Hellmann wrote:
> >     Here is a relevant section pulled out of the amqp 0-9-1 spec:
> >
> >        3.1.3.3 The Topic Exchange Type
> >
> >        The topic exchange type works as follows:
> >
> >            1. A message queue binds to the exchange using a routing
> >               pattern, P.
> >            2. A publisher sends the exchange a message with the routing
> >               key R.
> >            3. The message is passed to the message queue if R matches P.
> >
> >        The routing key used for a topic exchange MUST consist of zero or
> >        more words delimited by dots. Each word may contain the letters
> A-Z
> >        and a-z and digits 0-9.
> >
> >        The routing pattern follows the same rules as the routing key with
> >        the addition that * matches a single word, and # matches zero or
> >        more words. Thus the routing pattern *.stock.# matches the routing
> >        keys usd.stock and eur.stock.db but not stock.nasdaq.
> >
> >     In nova, for a given topic such as 'scheduler', all of the consumers
> are
> >     binding to the same queue on the topic exchange, resulting in
> >     round-robin delivery to each of the consumers.  If instead you make a
> >     new queue, you can get your own copy of each message.
> >
> >     There is an additional benefit of using a topic exchange here.  The
> >     topic used for notifications is 'notifications.<priority>'.  That
> means
> >     that when you create your queue, you can set it up to receive all
> >     notifications, or only notifications of a certain priority.
> >
> >
> > Topic exchanges make a lot of sense for messages that should only be
> > consumed once, such as tasks. Notifications are different. Lots of
> > different clients might want to know that some event happened in the
> > system. The way things are in Nova today, they can't. The first client
> > who consumes a notification message will prevent all of the other
> > clients from seeing that message at all.****
>
> I think you missed my main point, which was that a topic exchange does
> not impose a limitation that only one client can consume a given
> notification.  That's only true if each client is consuming from the
> same queue bound to the exchange.****
>
>  ****
>
> Yes, that wasn't obvious from any of the kombu documentation I've seen so
> far. I'll keep looking.****
>
>  ****
>
> Thanks,****
>
> Doug****
>
>  ****
>
>
> > I can change Nova's notification system to use a fanout exchange (in
> > impl_kombu.py changing the exchange type used by NotifyPublisher), but
> > before I submit a patch I want to make sure the current implementation
> > using a topic exchange wasn't selected deliberately for some reason.****
>
> I think using a fanout exchange would be a downgrade.  As I mentioned
> before, a topic exchange allows you to create a queue to get all
> notifications or only notifications of a specific priority.  If the
> exchange type is changed to fanout, it's everybody gets everything, and
> that's it.
>
> --
> Russell Bryant****
>
>  ****
>
>
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