Inherent Causal Orderings of Partial Order Scenarios

Scenario based requirements specifications are the industry norm for defining communication protocols. Basic scenarios captured as UML sequence diagrams, Message Sequence Charts (MSCs) or Live Sequence Charts (LSC) have partial order semantics that characterize system traces by restricting the possible order of events within those traces. The semantic partial order of the scenario specification is called the causal ordering. Semantic inconsistencies often occur in partial order scenarios between the specified causal ordering and the order that events can occur in practice. Such inconsistencies are known as race conditions. The paper proves that there is a unique race free partial order that is a minimal weakening of the causal ordering. In other words, there is a canonical generalization of the requirements that corrects all race conditions. Hence any race free generalization of the original scenario is in fact a generalization of the canonical scenario. The paper also proves the dual result, there is a unique race free partial order that is a minimal strengthening of the causal order. I.e. there is a canonical refinement of the requirements that corrects all race conditions.