Towards a derivational style of distributed system design ? An example

Abstract

Rederivation of the train signalling protocol developed in the 19th century is used to illustrate the application of several formal techniques to assist in the derivation of an implementation of a simple distributed system from its specification. It was this train signalling protocol that gave rise to the now-familiar concept ofsemaphore. However, a study of the train system shows that there is much more to its protocol than simply the instantiation of the semaphore concept. The derivation makes use of several formal techniques including weakest precondition calculation, finite differencing, generalization, and weakening of invariants. Informal reasoning is used to treat concerns of availability of information, and availability of control. Opportunities for inserting additional robustness into the derived implementation are identified during the process. The objective is to demonstrate the degree to which an implementation can bederived, rather than merely verified after some unknown invention process.