Rabble is far from perfect, but it does allow isolating cpu heavy (and File system) actors into their own threads, whereas async I/O bound actors all share a thread. Actors can communicate across the network and their internal state can be retrieved at run time. The design is also a bit different in that it explicitly is designed for testing actors, so they all return messages instead of "sending" them directly. That way you can intercept them with a test scheduler, drop and re-order them.