On Proving Communication Closedness of Distributed Layers

The notion of communication closed layer has been introduced as a way to define structured composition of distributed sy~tems. An interesting question is how to verify the closedness of a layer. We formulate a proof rule proving closed ness of a distributed layer. The rule is developed as an extension of the Apt. Francez and de Roever proof system for CSP. The extension is proved to be sound and relatively complete. 1. In trod ucti on A recent paper of Elrad and Francez [EF82] introduces a novel methodology of analysing distributed programs: decomposition into communication closed layers. According to this methodology, parts of processes are grouped together into layers so that there is interaction only between commands which are in the same layer. Intuitively, an execution of such decomposed programs is equivalent to synchronizing all the processes in the distributed program at layer boundaries. This approach *This research was carried out during the first author's stays at the Department of Computer Science of the Technion in May 1985 and May 1986. lDepartment of Mathematics and Computing Science, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, the Netherlands. Electronic mail: mcvax!eutrc3!wsinrobg.UUCP. 2Supported in part by the Foundation for Computer Science Research in the Netherlands (SION) with financial aid from the Netherlands Organization for Pure Scientific Research (ZWO). 3The author is working in and supported by ESPRIT project 937: "Debugging and Specification of Ada Real-Time Embedded Systems (DESCARTES)". 4Laboratory for Computer Science, MIT, Cambridge MA 02139, USA and Department of Computer Science, Technion, Haifa 32000, Israel. Electronic mail: [email protected]. sSupported in part by an HTI Postdoctoral Fellowship.