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. > 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 _______________________________________________ Mailing list: https://launchpad.net/~openstack Post to : openstack@lists.launchpad.net Unsubscribe : https://launchpad.net/~openstack More help : https://help.launchpad.net/ListHelp