The actor model has been a model of choice for building reliable distributed systems. On one hand, it ensures that message-processing is serialized within each actor, preserving the familiar sequential programming model. On the other hand, programs written in the actor model are location-transparent. The model is sufficiently low-level to express arbitrary message protocols. Composing these protocols is the key to high-level abstractions. Unfortunately, it is difficult to reuse or compose message protocols with actors.

Reactive isolates, proposed in this paper, simplify protocol composition with first-class typed channels and event streams. We compare reactive isolates and the actor model on concrete programs. We identify obstacles for composition in the classic actor model, and show how to overcome them. We then show how to build reusable, composable distributed computing components in the new model.