circuit

https://github.com/maxmedia/circuit

Circuit is a rack application middleware that enables dynamic request mapping. Modeled after [Rack::Builder](https://github.com/rack/rack/blob/master/lib/rack/builder.rb), Circuit provides for a tree of url-mappings that is walked at request time. If you are interested in dynamically loading middleware and functionality into requests, circuit is a viable solution to do that in a maintainable way.