[openstack-dev] [OSLO] Comments/Questions on Messaging Wiki

William Henry whenry at redhat.com
Fri Jul 12 21:58:13 UTC 2013

Woops the wiki I am referring to is: 



----- Original Message -----

> Hi all,

> I've been reading through the Messaging Wiki and have some comments. Not
> criticisms, just comments and questions.
> I have found this to be a very useful document. Thanks.

> 1. "There are multiple backend transport drivers which implement the API
> semantics using different messaging systems - e.g. RabbitMQ, Qpid, ZeroMQ.
> While both sides of a connection must use the same transport driver
> configured in the same way, the API avoids exposing details of transports so
> that code written using one transport should work with any other transport."

> The good news for AMQP 1.0 users is that technically "boths sides of the
> connection" do not have to use same transport driver. In pre-AMQP 1.0 days
> this was the case. But today interoperability between AMQP 1.0
> implementations has been demonstrated.

> 2. I notice under the RPC concepts section that you mention Exchanges as a
> container in which topics are scoped. Is this exchange a pre AMQP 1.0
> artifact or just a general term for oslo.messaging that is loosely based on
> the pre-AMQP 1.0 artifact called an Exchange? i.e. are you assuming that
> messaging implementations have something called an exchange? Or do you mean
> that messaging implementations can scope a topic and in oslo we call that
> scoping an exchange?

> 3. Some messaging nomenclature: The way the wiki describes RPC " Invoke
> Method on One of Multiple Servers " is more like a queue than a topic. In
> messaging a queue is something that multiple consumers can attach to and one
> of them gets and services a message/request. A topic is where 1+ consumers
> are "connected" and each receives a the message and each can service it as
> it sees fit. In pre-AMQP 1.0 terms what this seems to describe is a direct
> exchange. And a direct excahnge can have multiple consumers listening to a
> queue on that exchange. (Remember that fanout is just a generalization of
> topic in that all consumers get all fanout messages - there are no
> sub-topics etc.)

> In AMQP 1.0 the addressing doesn't care or know about exchanges but it can
> support this queue type behavior on an address or topic type behavior on an
> address.

> I know this isn't about AMQP specifically but therefore this is even more
> important. Topics are pub/sub with multiple consumer/services responding to
> a single message. Queues are next consumer up gets the next message.

> (BTW I've seen this kind of confusion also in early versions of MCollective
> in Puppet.)

> It might be better to change some of the references to "topic" to "address".
> This would solve the problem. i.e. a use case where one of many servers
> listening on an address services a message/request. And later all of servers
> listening on an address service a message/request. Addressing also solves
> the one-to-one as the address is specific to the server (and the others
> don't have to receive and reject the message).

> Please feel free to respond and critique my comments/suggestions.

> Best,
> William

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