A co-operative scenario based approach to acquisition and validation of system requirements: How exceptions can help!

Scenarios, in most situations, are descriptions of required interactions be tween a desired system and its environment, which detail normative system b ehaviour. Our studies of current scenario use in requirements engineering h ave revealed that there is considerable interest in the use of scenarios fo r acquisition, elaboration and validation of system requirements. However, scenarios have seldom been used to study inappropriate or exceptional syste m behaviour. To account for non-normative or undesired system behaviour, it is vital to predict('what can go wrong') and explore the existence or occu rrence of 'exceptions' in a scenario when the system(1) might be prevented from delivering the required service. Identification of exceptions and incl usion of additional requirements to prevent their occurrence or mitigate th eir effects yield robust and fault-tolerant design solutions. In this article, we present a prototype software tool called CREWS-SAVRE fo r systematic scenario generation and use. We describe the innovative featur es of the tool and demonstrate them with an example of tool's use. Further, we have identified three kinds of exceptions: generic, permutation and pro blem exceptions, and have derived complex taxonomies of problem exceptions. We have populated SAVRE with the taxonomies of generic, permutation and pr oblem exceptions. The exceptions can be chosen by the requirements engineer to include them in the generated scenarios to explore the correctness acid completeness of requirements. In addition, the taxonomies of problem excep tions can also serve as checklists and help a requirements engineer to pred ict nonnormative system behaviour in a scenario. (C) 1999 Elsevier Science B.V. All rights reserved.