[rabbitmq-discuss] Question about architecture

Sun May 15 19:43:36 BST 2011

Michael,

apologies for the delay in getting back to you...

Michael Beam wrote:
> I'm new to RabbitMQ and AMQP, and I'd like to see what people with
> more experience have to say about exchange & queue design.  I'm
> experimenting with using RabbitMQ to replace a homegrown thrift-based
> messaging solution that is used to relay change notifications from 10
> or so CouchDB databases to several hundred running instances of our
> application.

Interesting. Distribution of DB change notifications seems to be an 
increasingly popular RabbitMQ application.

> My first thought was to create an exchange for each observed database
> observed, each exchange named after the database (i.e.,
> couchdb_changes.db1 ... couchdb_changes.db10).  A routing key would
> be set for messages based on the value of some field in the database
> document. Each client application session would then create and bind
> a queue to each of the exchanges.
> 
> My second thought was that I could create a single exchange named
> "couchdb_changes", and then add the database name to the routing key.
> When binding queues to the exchange the application would also
> incorporate the database name into the routing key.
> 
> So, are there advantages or disadvantages between the two approaches
> here, or are they largely functionally equivalent.  I prefer the feel
> an exchange per observed database; just curious if there are
> established patterns that I should know about to better inform design
> decisions.

I can think of two main criteria to guide your decision here:

1) Is the set of DB names fairly static, or does it change quite often? 
In the one-exchange-per-db model the list of exchanges need to be kept 
in sync with the list of databases, which might be tricky depending on 
how/when the list changes.

2) What kind of subscriptions do you want to support, i.e. how do 
consumers narrow the event stream down to what they want to receive? A 
single topic exchange would give you the most flexibility here, but a 
set of direct exchanges will perform better in most cases.

Regards,

Matthias.