Stepwise Refinement of Communicating Systems

The action system formalism 4] 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. Deenitions and rules are presented for reening 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 reened independently of the rest of the system. Speciication and reenement is similar to the reenement calculus approach 2, 22, 24]. The theoretical basis for communication and distribution is Hoare's CSP 11]. Use of the reenement and decomposition rules is illustrated by the design of an unordered buuer, and then of a distributed message-passing system.