A Practical Approach to Incremental Specification

The object-oriented specification of concurrent and distributed systems has tended to emphasise the aspect of substitutability at the expense of code reuse. A variety of constraints has been imposed in order to guarantee substitutability in one form or another. This paper argues that the incremental development of software specifications needs to consider substitutability in the context of code reuse. Further, the common approach of starting with an abstract specification and then progressively refining it (in some general way) means that many existing substitutability constraints are too strong. In the context of Coloured Petri Nets, we advocate the use of three specific forms of refinement — type refinement, subnet refinement, and node refinement. These have weaker demands for substitutability, namely that every (complete) refined behaviour has a corresponding abstract behaviour, but not necessarily vice versa. An examination of case studies in the literature suggests that this approach is applicable in practice.