[rabbitmq-discuss] exchanges and message processing

Joe Williams joe at joetify.com
Wed May 6 17:21:24 BST 2009

Thanks Matthias, that makes sense to me. Message order isn't an issue in 
my case so I think it might work well.


Matthias Radestock wrote:
> Joe,
> Joe Williams wrote:
>> This is in relation to a thread I started on the ruby-amqp list:
>> http://groups.google.com/group/ruby-amqp/browse_thread/thread/6296567fa545ca8d?hl=en 
>> My question is regarding exchanges, queues and dealing with messages 
>> that I would like to get processed only once. I am sending messages 
>> containing json and would like to use a field from the json as the 
>> routing key to alleviate needing to do numerous case statements in my 
>> code. It seems that fanout and direct exchanges would not work since 
>> they have possibility of having messages getting processed more than 
>> once by different consumers. The responder on the ruby-amqp list 
>> mentioned a shared queue that is in the 091 spec. But this seems to 
>> put me back at using a long case statement.
>> In an ideal situation I would have multiple consumers (written 
>> without epic case statements) and messages that get consumed once and 
>> only once.
>> Any suggestions on how to go about doing something like this?
> The role of exchanges is to route messages to queues. They *all* have 
> the possibility of resulting in messages getting routed to multiple 
> queues and hence processed by multiple consumers - that's what they 
> are there for. Whether that actually happens depends on the bindings 
> that have been configured.
> For your use case, the following setup should work:
> - one direct exchange
> - one queue for each of the case branches
> - one binding per queue to the direct exchange, with the binding key 
> set to the case branch discriminator
> - one consumer per queue, handling the messages for one particular 
> branch discriminator
> In this setup messages will get routed to exactly one consumer, based 
> on their routing key. You'd only get duplication if you set up 
> additional queues and consumers that bound to the direct exchange with 
> the same binding keys.
> One possible downside of this setup compared to a single queue is that 
> message order is not preserved across queues.
> Regards,
> Matthias.

Name: Joseph A. Williams
Email: joe at joetify.com
Blog: http://www.joeandmotorboat.com/

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