Compositional Semantics of an Actor-Based Language Using Constraint Automata

Rebeca is an actor-based language which has been successfully applied to model concurrent and distributed systems. The semantics of Rebeca in labeled transition system is not compositional. In this paper, we investigate the possibility of mapping Rebeca models into a coordination language, Reo, and present a natural mapping that provides a compositional semantics of Rebeca. To this end, we consider reactive objects in Rebeca as components in Reo, and specify their behavior using constraint automata as black-box components within Reo circuits. Modeling coordination and communication among reactive objects as Reo circuits, and the behavior of reactive objects as constraint automata, provides a compositional semantics for Rebeca. Although the result is a compositional model, its visual representation in Reo shows very well that it still reflects the tight coupling inherent in the communication mechanism of object-based paradigms, whereby the real control and coordination is built into the code of the reactive objects themselves. We describe an alternative design that overcomes this deficiency. This illustrates the differences between objects and components, and the challenges in moving from object-based to component-based designs.