Assert and negate revisited: modal semantics for UML sequence diagrams

Abstract

Live Sequence Charts (LSC) extend Message Sequence Charts (MSC), mainly by distinguishing possible from necessary behavior. They thus enable the specification of rich multi-modal scenario-based properties, such as mandatory, possible and forbidden scenarios. The sequence diagrams of UML 2.0 enrich those of previous versions of UML by two new operators, assert and negate, for specifying required and forbidden behaviors, which appear to have been inspired by LSC. The UML 2.0 semantics of sequence diagrams, however, being based on pairs of valid and invalid sets of traces, is inadequate, and prevents the new operators from being used effectively. We propose an extension of, and a different semantics for this UML language - Modal UML Sequence Diagrams (MUSD) - based on the universal/existential modal semantics of LSC. MUSD can be formally defined as a UML profile, thus paving the way to apply formal verification, synthesis, and scenario-based execution techniques from LSC to the mainstream UML standard.