[rabbitmq-discuss] RPC across Federation

[Josh West]

So, I seem to have encountered a limitation with Federation.  This is 
with regards to how bindings of exchanges/queues are propagated 
asynchronously, thus leading to the possibility of a race condition.

If you take the example outlined in Tutorial Six ( 
http://www.rabbitmq.com/tutorials/tutorial-six-python.html), and modify 
the code so that exchanges are used, you can get RPC requests and 
responses to traverse federated links.  In this example, the rpc_client 
creates a temporary callback queue and attaches the address of that 
queue to the message as a reply_to property.  Once the rpc_server 
receives the message and calculates the Fibonacci number, it sends the 
response back via a new message to the reply_to address...

Ok. This is all simple basic stuff to this point.

The problem lies in when the rpc_client creates it's temporary callback 
queue.  There is a race condition in which the rpc_client may move on by 
then sending the fib(n) message, the rpc_server receives + calculates 
the Fibonacci number, and then sends a response to the rpc_client's 
callback queue specified in props.reply_to -- all before the Federation 
plugin has propagated the binding of the callback queue to the exchange 
and other federated exchanges.  Thus, the response message gets 
lost/dropped.

Has anybody else experienced an issue like this?  Does anybody have 
suggestions as to how to properly provide RPC functionality across 
federated exchanges?

Thanks!

-- 
Josh West
One.com -http://www.one.com

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