A Constructive Approach to the Design of Distributed Systems

The underlying model of distributed systems is that of loosely coupled components r running in parallel and communicating by message passing. Description, construction and evolution of these systems is facilitated by separating the system structure, as a set of components and their interconnections, from the functional description of individual component behaviour. Furthermore, component reuse and structuring flexibility is enhanced if components are context independent ie. selfcontained with a well defined interface for component interaction. The Conic environment for distributed programming supports this model. In particular, Conic provides a separate configuration language for the description, construction and evolution of distributed systems. The Conic environment has demonstrated a working environment which supports system distribution, reconfiguration and extension. We had initially supposed that Conic might pose difficult challenges for us as software designers. For example, what design techniques should we employ to develop a system that exploits the Conic facilities? In fact we have experienced quite the opposite. The principles of explicit system structure and context independent components that underlie Conic have lead us naturally to a design approach which differs from that of both current industrial practice and current research. Our approach is termed "constructive" since it emphasises the satisfaction of system requirements by composition of components. In this paper we describe the approach and illustrate its use by application to an example, a model airport shuttle system which has been implemented in Conic.