[rabbitmq-discuss] Binding queues on a topic exchange

Mon Jul 2 20:25:34 BST 2007

Ben,

"Ben Hood" <0x6e6562 at gmail.com> writes:

> I was wondering the difference is when a topic listener:
>
> a) Declares a well known queue name within an exchange to consume
> from. It is assumed that a routing key has already been bound to this
> queue (e.g. by a producer or an admin).

Let me rephrase that slightly: You have declared a topic exchange and a
queue bound to that exchange with a particular pattern. Your consumers
all consume messages from that queue. Correct?

> b) Declares a privately named queue and binds a routing key to that
> queue that the consumer is interested in listening to.

Let me rephrase that slightly: You have declared a topic exchange (as
before). Each consumer creates a private queue that it binds to the
topic exchange with a particular pattern. Correct?

> My assumption would be that in scenario (a), each consumer gets
> notified using the same shared data whereas in (b) a copy of each
> message is enqueued in the buffer of the consumer's private queue,
> thus making this queue not really a pub/sub, but more really P2P.
>
> Is this assumption correct?

For a topic exchange, a message gets delivered to all queues that match
the message's routing key with their binding pattern.

When you have multiple consumers on one queue, every message gets
delivered to *one* of the consumers.

So, taking your above cases, in the first example every message with a
matching routing key will be delivered to *one* of the consumers, and in
the second case every message with a given routing key is delivered to
*all* queues that match the routing key with their pattern, and then in
turn to the consumer(s) hanging off those queues (one per queue in this
example).

Regards,

Matthias