Our requirement is very simple. Send messages to users subscribed to a topic. We need our messaging system to be able to support millions of topics and maybe millions of subscribers to any given topic in near real time. Our application is built with Java.
We almost decided on RabbitMQ because of the community support, documentation and features (possibly it will deliver everything we need). But I am very inclined towards using Redis because it looks promising and lightweight. Honestly I have limited understanding about Redis as a messaging system, but looking at a growing number of companies using it as a queuing(with Ruby Resque), I want to know if there is an offering like Resque in Java and what are the advantages or disadvantages of using Redis as a MQ over RabbitMQ.
RabbitMQ supports clustering and now has active/active High Availably queues allowing for greater scale out and availability options then possible with Redis out of the box.
RabbitMQ gives you a greater amount of control on everything from the users/permissions of exchanges/queues, to the durability of a specific exchange or queue (disk vs memory), to the guarantees of delivery (transactions, Publisher Confirms).
It also allows for more flexibility and options on your topologies (fanout, topic, direct) and routing to multiple queues, RPC with private queues and reply-to, etc.