Orleans is a Microsoft Research project and managed (.NET) software framework for building client + cloud applications. As outlined in the recently released paper on the topic:
Orleans defines an actor-like model of isolated grains that communicate through asynchronous messages and manage asynchronous computations with promises. The isolated state and constrained execution model of grains allows the Orleans runtime to persist, migrate, replicate, and reconcile grain state without programmer intervention. Orleans also provides lightweight, optimistic, distributed transactions that provide predictable consistency and failure handling for distributed operations across multiple grains.
More Info: Orleans Paper