Negative scenarios for implied scenario elicitation

Abstract

Scenario-based specifications such as Message Sequence Charts (MSCs) are popular for requirement elicitation and specification. MSCs describe two distinct aspects of a system: on the one hand they provide examples of intended system behaviour and on the other they outline the system architecture. A mismatch between architecture and behaviour may give rise to implied scenarios. Implied scenarios occur because a component's local view of the system state is insufficient to enforce specified system behaviour. An implied scenario indicates a gap in the MSC specification that needs to be clarified. It may simply mean that an acceptable scenario has been overlooked and should be added to the scenario specification. Alternatively, it may represent an unacceptable behaviour which should be documented and avoided in the final implementation. Thus implied scenarios can be used to iteratively drive requirements elicitation. However, in order to do so, tools for coping with rejected implied scenarios are needed. The contributions of this paper are twofold. Firstly, we define a language for describing negative scenarios. Secondly, we complement existing implied scenario detection methods with techniques for accommodating negative scenarios.