[rabbitmq-discuss] Exchanges, Routing, and AMQP

Tue Oct 21 01:50:28 BST 2008

Edwin,

On Mon, Oct 20, 2008 at 11:17 PM, Edwin Fine
<rabbitmq-discuss_efine at usa.net> wrote:
> Sorry if this is answered elsewhere (I did look), but how (in a nutshell)
> would I implement my own custom exchange type?

1. The abridged version:

If you look into at functions rabbit_exchange:route/2 and
lookup_qpids:/1 in the same module on the 18776 branch, you might be
able to follow how the generalized routing mechanism works. Basically
you read the routing table for bindings to queue names that match your
key, resolve their corresponding process ids and the rest is handled
for you.

2. The realistic version:

Somehow you're going to have to make a routing decision, and this
usually has something to do with bindings, although you could
theoretically create some kind of foo exchange that just picks some
queue names out of a hat. But if you need to do stateful routing,
you're going to have to retrieve the state from somewhere.

You might also want to make sure that you are not breaking any other
functionality that you currently rely on when you do this.

This also wouldn't necessarily been spec compliant per se, although
section 3.1.3.6 of the 0-9 spec mentions the possibility of custom
exchanges. So you would have to be comfortable with that.

And, unless it was a really worthwhile addition, we probably wouldn't
maintain it in the main Rabbit tree.

Other than that, feel free to do so. We have talked about plugable
queues before (although Matthias has rightly pointed out that this is
much easier said than done) so why should exchanges be any different?

But despite all of these practical considerations, there is a lot of
merit to adding custom exchanges (which Kirk may interpret as custom
routing algorithms :-), because the exchange is point in the AMQP
pipeline with the greatest amount of leverage effect when it comes
down to deciding which messages go where.

HTH,

Ben