On Formalizing UML State Machines Using ASMs

UML (Unified Modeling Language [1]) is a standard notation based on a set of diagrams to describe the structure and the behavior of software systems. In [2] the authors claim that “UML is more than just a graphical language. Rather, behind every part of its graphical notation there is a specification that provides a textual statement of the syntax and semantics of that building block”. However, the UML semantics document [1] only gives an unambiguous textual definition of the syntax for the UML notations and leaves the behavioral content of various UML constructs largely open. The necessity to develop the UML as a precise (i.e. well defined) modeling language is widely felt [11,10,24] and the pUML (precise UML) group has been created to achieve this goal [23]. With this paper we want to survey previous work [3–5] done to provide a precise definition of some basic concepts of UML which are related to the dynamics of systems. Technical details are left out for lack of space but they are duly referenced. Our goal here is to discuss a framework for the rigorous description and analysis of different possible logically consistent interpretations of the intuitions which underly UML concepts: how (a) it faithfully reflects the original intuitions as far as possible; (b) it makes the so-called semantic variations points explicit; (c) clarifies some dark but semantically relevant points in the UML documents. This includes the event deferring and completion mechanism, the meaning of atomic and durative actions, concurrent internal activities and conflict situations which may arise through the concurrent behavior of active objects. We develop this framework for one of the principal diagram types which are used in UML for the description of system behavior, namely statecharts or UML state machines.