Behavior modeling has proved to be successful in helping uncover design flaws of concurrent and distributed systems. Nevertheless, it has not had a widespread impact on practitioners because model construction remains a difficult task and because the benefits of behavior analysis appear at the end of the model construction effort. In contrast, scenario-based specifications have a wide acceptance in industry and are well suited for developing first approximations of intended behavior; however, they are still maturing with respect to rigorous semantics and analysis tools.This article proposes a process for elaborating system behavior that exploits the potential benefits of behavior modeling and scenario-based specifications yet ameliorates their shortcomings. The concept that drives the elaboration process is that of implied scenarios . Implied scenarios identify gaps in scenario-based specifications that arise from specifying the global behavior of a system that will be implemented component-wise. They are the result of a mismatch between the behavioral and architectural aspects of scenario-based specifications. Due to the partial nature of scenario-based specifications, implied scenarios need to be validated as desired or undesired behavior. The scenario specifications are then updated accordingly with new positive or negative scenarios. By iteratively detecting and validating implied scenarios, it is possible to incrementally elaborate the behavior described both in the scenario-based specification and models. The proposed elaboration process starts with a message sequence chart (MSC) specification that includes basic, high-level and negative MSCs. Implied scenario detection is performed by synthesis and automated analysis of behavior models. The final outcome consists of four artifacts: (1) an MSC specification that has been evolved from its original form to cover important aspects of the concurrent nature of the system that were under-specified or absent in the original specification, (2) a behavior model that captures the component structure of the system that, combined with (3) a constraint model and (4) a property model that provides the basis for modeling and reasoning about system design.
Scenario-based specifications such as Message Sequence Charts (MSCs) are useful as part of a requirements specification. A scenario is a partial story, describing how system components, the environment, and users work concurrently and interact in order to provide system level functionality. Scenarios need to be combined to provide a more complete description of system behavior. Consequently, scenario synthesis is central to the effective use of scenario descriptions. How should a set of scenarios be interpreted? How do they relate to one another? What is the underlying semantics? What assumptions are made when synthesizing behavior models from multiple scenarios? In this paper, we present an approach to scenario synthesis based on a clear sound semantics, which can support and integrate many of the existing approaches to scenario synthesis. The contributions of the paper are threefold. We first define an MSC language with sound abstract semantics in terms of labeled transition systems and parallel composition. The language integrates existing approaches based on scenario composition by using high-level MSCs (hMSCs) and those based on state identification by introducing explicit component state labeling. This combination allows stakeholders to break up scenario specifications into manageable parts and reuse scenarios using hMCSs; it also allows them to introduce additional domainspecific information and general assumptions explicitly into the scenario specification using state labels. Second, we provide a sound synthesis algorithm which translates scenarios into a behavioral specification in the form of Finite Sequential Processes. This specification can be analyzed with the Labeled Transition System Analyzer using model checking and animation. Finally, we demonstrate how many of the assumptions embedded in existing synthesis approaches can be made explicit and modeled in our approach. Thus, we provide the basis for a common approach to scenario-based specification, synthesis, and analysis.
Constructing comprehensive operational models of intended system behaviour is a complex and costly task. Consequently, practitioners have adopted techniques that support incremental elaboration of partial behaviour descriptions. A noteworthy example is the wide adoption of scenario-based notations such as message sequence charts. Scenario-based specifications are partial descriptions that can be incrementally elaborated to cover the system behaviour that is of interest. However, how should partial behavioural models described by different stakeholders with different viewpoints covering different aspects of behaviour be composed? How should partial models of component instances of the same type be put together?In this paper, we propose model merging as a general solution to these questions. We formally define model merging based on observational refinement and show that merging consistent models is a process that should result in a minimal common refinement. Because minimal common refinements are not guaranteed to be unique, we argue that the modeller should participate in the process of elaborating such a model. We also discuss the role of the least common refinement and the greatest lower bound of all minimal common refinements in this elaboration process. In addition, we provide algorithms for i) checking consistency between two models; ii) constructing their least common refinement if one exists; iii) supporting the construction of a minimal common refinement if there is no least common refinement.
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