Executable Protocol Models as a Requirements Engineering Tool

Functional prototypes and simulations are a well recognised and valued tool for building a shared understanding of requirements between users and developers. However, the develop-ment of such artefacts does not sit well with traditional modelling processes which tend to focus on structure rather than behaviour. Consequently building prototypes and simulations is a diversion form the mainstream development process, sometimes even competing with it. We propose that the resolution in this conflict lies in promoting the role of executable behavioural models, so that artefacts supporting behavioural simulation are a by-product of the mainstream modelling process. We discuss why conventional modelling techniques are not suited to this, and we describe an innovative behavioural modelling technique, Protocol Modelling, that is well adapted to direct execution. Using Protocol Modelling, a behavioural entity (business object or process) is modelled in terms of its event protocol: the conditions under which it accepts or refuses events. Such models capture the behavioural integrity rules at the level of business events; and can be composed using the semantics of Hoare’s CSP, allowing concise and incremental representation. Direct execution of the model is achieved using a tool that simulates a normal user interface, so that non-technical stakeholders can review and explore behaviour while requirements are being solidified.