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[Russell Bryant <rbryant@xxxxxxxxxx>]

On 05/08/2012 04:19 PM, Doug Hellmann wrote:
> I'm working on some code to monitor notification messages coming from
> nova by listening directly on the AMQP queue. Unfortunately, nova is
> using a topic exchange so it seems I can't safely consume the messages
> or they might not go to some other client that is expecting to see them
> (especially some other part of nova that is expecting to receive the
> notification). It seems like notifications should be available to more
> than one subscriber at a time, but I thought I would ask for background
> before I started working on a patch to change it.
> 
> Is there a particular reason a topic exchange was used for notifications?

I believe you can still get what you want (but not by using nova.rpc as
your client).

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.

-- 
Russell Bryant