Using Integrated Formal and Informal Modeling Techniques To Analyze Software Requirements: A Petri-Net/SART Case Study

Many well-known graphical modeling techniques, for example, the Structured Analysis/Real Time (SA/RT) method developed by Hatley and Pirbhai, provide mechanisms that allow one to 1) abstract over details best left to later stages of development, 2) model an application along diierent views, and 3) modularize problems and solutions. It has also been stated that the (apparently) simple and graphical nature of the modeling constructs facilitates the creation of concise and understandable models. Our experiences indicate that the lack of rm semantic foundations for the modeling notations used in these methods can make understanding more apparent than real. In particular, the lack of adequate semantic foundations makes rigorous analysis of semantic properties captured by the models diicult. In this paper we illustrate how formal speciication techniques can be used to rigorously analyze models created by informal structured methods through a case study. In the case study, a Hatley/Pirbhai SA/RT model of an application is analyzed by generating, modifying, and exercising an extended Petri-Net model of the SA/RT model. The Petri-Net model can be viewed as a prototype of the behavior captured in the informal model, and is used primarily to dynamically validate the requirements expressed in the SA/RT model. The generation of a rst-cut extended Petri-Net model from a SA/RT model is supported by a set of transformation rules, and can be automated. Exercising the Petri-Net revealed problems with the informal SA/RT model related to incomplete, ambiguous, and inconsistent requirements.