Redis pub/sub (which I assume you are talking about as an alternative) is fine, but you have to be listening when the message is sent otherwise you miss it.
Another problem is working out if where you sent it there are any listeners, which RabbitMQ deals with return queues. This is important for 404's when a client is trying to talk to a service that does not exist. In Redis you would just post to a pub/sub queue wait till nothing happens and timeout, rather than immediately knowing through a return queue.
Actually Redis returns the number of consumers that were subscribed during your publish and received the message, so you can detect that no service is available and return your 404 status. The real limitation for RPC over Redis pub/sub to me is that you can only broadcast your messages to all consumers, and not have them load-balanced between the subscribed applications. So you end-up following a discover/unicast call pattern where Redis doesn't bring much to the table for the actual RPC call. Maybe that's an opportunity for a cool addition to redis...
Cheers, I didn't know that about Redis. I actually really like Redis and it simplicity, it would be cool to get more options for passing messages over Redis.
Another problem is working out if where you sent it there are any listeners, which RabbitMQ deals with return queues. This is important for 404's when a client is trying to talk to a service that does not exist. In Redis you would just post to a pub/sub queue wait till nothing happens and timeout, rather than immediately knowing through a return queue.