[rabbitmq-discuss] custom exchange

Thu May 31 10:56:31 BST 2012

On 31/05/12 07:34, Sujan Dutta wrote:
> Now my requirement is that - Add a custom header to the every messages
> which is delivered through the custom exchange.

I'm afraid there isn't a general API for that.

The problem is that exchanges are just for routing - in a complex 
exchange-to-exchange scenario where a message gets routed like:

  /--> X2 --\
  |          v
  X1         X4 --> Q
  |          ^
  \--> X3 --/

only one copy of the message ends up in Q - so did it pass through X2 or 
X3? Which one has the right to modify it?

Having said that, if you know that you *won't* be using such complex 
routing, you could do this by:

route(X, Delivery) ->
     case custom_header_exists(Delivery) of
         true  -> route_as_normal(Delivery);
         false -> rabbit_basic:publish(X, add_custom_header(Delivery)),
                  []
     end.

i.e. if the header needs to be added, don't route it at all, but instead 
republish it to yourself with the header added.

Cheers, Simon

-- 
Simon MacMullen
RabbitMQ, VMware