Resolving the asymmetry of on-exit versus on-entry in executable models of behaviour

Abstract

For the UML, state charts are by far the most used modelling tools, both to communicate behaviour and to produce executable models. We investigate the inherent asymmetry of On-Entry and On-Exit Actions in UML Statecharts. We show first that the apparently simple and symmetric rules for handling the sequencing of On-Entry and On-Exit actions are hard to fully comprehend and apply effectively by software developers. Second, defining a semantics that results in executable models for applications such as reactive-systems and real-time systems is very delicate. Third, formal verification can be hampered because the semantics results in a combinatorial explosion of states. We evaluate the understandability of the semantics by taking out experiments with various tasks comprising sample UML Statechart and logic-labelled finite state machines (LLFSMs). Several experiments with software developers enable us to dissect how issues of understandability of state diagrams relate to nesting or event-driven vs logic-labelled. Since logic-labelled finite state machines achieve model composition through a subsumption architecture (suspend/restart/resume) we propose a specific alternative semantics for logic-labelled finite state machines that is suitable for robotic and embedded systems.