I recently pushed a Rabbitbal update to Github - http://github.com/somic/rabbitbal.
The biggest enhancement (IMHO) is ability to increase or decrease the number of backend consumers based on any HTTP request headers. In “table” routing mode (see rabbitbal.yml), you can now specify array of tests against which incoming request headers will be matched. This will cause your request to be published with a matching key (note :key). Your backend consumers use the same YAML file and can bind to all or only some queues, thus giving you flexibility in adjusting the capacity. Old functionality is available by using “topic” routing mode.
Note that I still use topic-based exchange, because I wanted to support a use case where you want to aggregate all incoming requests into separate queues (routing key would be something like “request.#”) for bot detection, access log aggregation, etc. In other words, each request ultimately must end up in a single queue where it will be picked up by backend servers, while at the same time it can also be duplicated into other queues for other purposes.