Web frameworks typically build on top of a lower level of the stack that handles dealing with HTTP. This lower level often contains little else, and this design drives the simple architectures of most frameworks, where all code happens in the context of some web request. Immutant expands on this lower level, providing many more primitives like message queues, daemons, distributed caching, and scheduled jobs.
Immutant is based on JBoss AS7, which is a Java Enterprise Edition application server with a long history. With this richer underpinning, Immutant enables much more sophisticated architectures and applications. While Rails apps must resort to external services like memcached and RabbitMQ, Immutant ...