Communication Abstraction and Process Refinement

Concurrent systems are collections of data, processes, and communication channels. Top-down, hierarchical design of concurrent systems needs powerful abstraction facilities provided by the implementation language. While most languages provide some structuring mechanisms for data and process abstraction, none seems to provide any equivalent mechanisms for communication structuring. Communication channels are to communicate data and, therefore, all data structuring mechanisms provided by a programming language must be available to structure channels as well. In order to preserve behaviour through successive levels of design refinement, these means of communication structuring must preserve the abstraction of atomic transfers of values of arbitrary types. Int r o duct ion Most concurrent programming languages [5, 6, 7, 11 support the abstraction of concurrent systems as collections of data, processes, and communication channels. However, while they provide some structuring mechanisms for data and process abstraction, none seems to provide any equivalent mechanisms for communication structuring. Interprocess communication is almost universally viewed as a synchronised atomic exchange of values between two concurrently active processes. This affects the whole design process and intervenes with the freedom and ease in the refinement of the process structure. The design transformation steps may be non-trivial in some cases and, therefore, difficult to arrive at and verify. In addition, the implementation may be less efficient, both in storage and speed, because of unnecessary data copying and context creation for process spawning. The data structuring mechanisms supported by the contemporary programming languages provide a uniform view on data and data types. Structured data types may consist of components of arbitrary types, including themselves, and values of such types are treated as wholes and may be passed as parameters, returned as results of functions, and assigned to variables. The same applies to processes [5, 71. No distinction of kind need be made between systems with and without substructure and, indeed, a system which at one level of abstraction may be considered to consist of a process and the environment in which it evolves, may be considered as a single system at a higher level of abstraction. A process which for one purpose is taken to be atomic