Stepwise refinement of communicating systems

The action system formalism [6] is a state-based approach to distributed computing. In this paper, it is shown how the action system formalism may be used to describe systems that communicate with their environment through synchronised value-passing. Definitions and rules are presented for refining and decomposing such action systems into distributed implementations in which internal communication is also based on synchronised value-passing. An important feature of the composition rule is that parallel components of a distributed system may be refined independently of the rest of the system. Specification and refinement is similar to the refinement calculus approach [4, 26, 28]. The theoretical basis for communication and distribution is Hoareʹs CSP [16]. Use of the refinement and decomposition rules is illustrated by the design of an unordered buffer, and then of a distributed message-passing system.