Causal Closure for MSC Languages

Message sequence charts (MSCs) are commonly used to specify interactions between agents in communicating systems. Their visual nature makes them attractive for describing scenarios, but also leads to ambiguities that can result in incomplete or inconsistent descriptions. One such problem is that of implied scenarios—a set of MSCs may imply new MSCs which are “locally consistent” with the given set. If local consistency is defined in terms of local projections of actions along each process, it is undecidable whether a set of MSCs is closed with respect to implied scenarios, even for regular MSC languages [3]. We introduce a new and natural notion of local consistency called causal closure, based on the causal view of a process—all the information it collects, directly or indirectly, through its actions. Our main result is that checking whether a set of MSCs is closed with respect to implied scenarios modulo causal closure is decidable for regular MSC languages.