Specifying parallel and distributed systems in Object-Z: the lift case study

There has been an increasing emphasis on formality in software system specification in the last few years. A number of standards bodies are recommending the use of formal notations for specifying software systems. Parallel and distributed systems have their own complex features such as: the concurrent interactions between various system components; the reactive nature of the systems; various message passing schemes between system components. Object-Z is an extension to the Z language specifically to facilitate specification in an object-oriented style. Because parallel and distributed systems are typically complex systems, the extra structuring afforded by the various Object-Z modelling constructs (i.e. the class, object containment constructs, and various composite operation expressions) enables the various hierarchical relationships and the communication between system components to be succinctly specified. Object-Z history invariants allow system temporal properties to be specified as well. The use of Object-Z in the specification of parallel and distributed systems is demonstrated by presenting a case study based on a multi-lift system. To enhance the understandability of the formal model, OMT notation is used to grasp the static structure of the system, and a finite state machine diagram is used to highlight the system behaviour.