Modeling Events in Object-Process Methodology and in Statecharts

Complex systems are often reactive, i.e., they continuously respond to external and internal stimuli (events) and may have time constraints. When modeling such systems, the designer should be able to determine the system's behavior, as well as its flow of control. One common way for expressing control flows is via Event-Condition-Action (ECA) rules [1]. These rules specify for each action (process) its triggering event and its guarding condition. The action is executed when the triggering event occurs, if and only if the guarding condition is fulfilled at that time. In this paper, we specify how two modeling approaches, Statecharts and Object-Process Methodology (OPM), model the ECA paradigm and compare the expressive power of the respective models. We examine the types of supported events, how these event types are integrated into complete system specifications, and what are the potential implications on the code derived from each one of the specifications. Statecharts [3] is an extension of the conventional formalism of state machines and diagrams. It can be used either as a stand-alone behavioral description or as part of a more general analysis and design method, such as UML [5]. Statecharts is based on states, which specify a situation in which a system (or an object) exists, and transitions, which enable the system to move from one state to another. A transition has the form of an ECA rule, i.e., syntactically formulated as "event [condition] | action". OPM [2] is an integrated modeling method that unifies the system function, structure, and behavior within one frame of reference. The building blocks of OPM are objects, processes, states, and structural and behavioral links. Each OPM specification consists of a set of graphical representations, called Object-Process Diagrams (OPDs), and a corresponding natural language description, called Object-Process Language (OPL) script. The translation from an OPD set to the corresponding OPL script and vice versa is done automatically, so the designer can interchangeably work on the graphical or textual version of the specification. OPM/T [4], which is an extension of OPM for specifying reactive and real-time systems, has applied the ECA rules to OPM by defining triggering events, guarding conditions, temporal constraints, and timing exceptions. In this paper, OPM refers also to its OPM/T extension. Table 1 provides a Statecharts model and an OPM model (both an OPD and an OPL sentence) for each one of the common event types. Comparing the models suggested for state entrance, …