CSP with Hierarchical State

Abstract

The process algebra CSP is designed for specifying interactions between concurrent systems. In CSP, and related languages, concurrent processes synchronise on common events, while the internal operations of the individual processes are treated abstractly. In some contexts, however, such as when modelling systems of systems, it is desirable to model both interprocess communications as well as the internal operations of individual processes. At the implementation level, shared state is often the method of communication between processes, and tests and updates of local state are used to implement internal operations. In this paper we propose an extension of the CSP language which maintains CSP’s core elegance in specifying process synchronisation, while also allowing state-based behaviour. State is treated hierarchically, allowing (nested) declarations of local and shared variables. The state can be accessed and modified using a refinement calculus-style specification command, which may be optionally paired with event synchronisation. The semantics of the extended language, preserves the original CSP rules. The approach we present is novel in that state is part of the process, rather than a meta-level construct appearing only in the rules.