Context constraints for compositional reachability analysis

Behaviour analysis of complex distributed systems has led to the search for enhanced reachability analysis techniques which support modularity and which control the state explosion problem. While modularity has been achieved, state explosion is still a problem. Indeed, this problem may even be exacerbated as a locally minimised subsystem may contain many states and transitions forbidden by its environment or context. Context constraints, specified as interface processes, are restrictions imposed by the environment on subsystem behaviour. Recent research has suggested that the state explosion problem can be effectively controlled if context constraints are incorporated in compositional reachability analysis (CRA). Although theoretically very promising, the approach has rarely been used in practice because it generally requires a more complex computational model and does not contain a mechanism to derive context constraints automatically. This paper presents a technique to automate the approach while using a similar computational model to that of CRA. Context constraints are derived automatically, based on a set of sufficient conditions for these constraints to be transparently included when building reachability graphs. As a result, the global reachability graph generated using the derived constraints is shown to be observationally equivalent to that generated by CRA without the inclusion of context constraints. In addition, constraints can also be specified explicitly by users, based on their application knowledge. Erroneous constraints which contravene transparency can be identified together with an indication of the error sources. User-specified constraints can be combined with those generated automatically. The technique is illustrated using a clients/server system