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

Scenario based requirements analysis is an inquiry based collaborative process which enables requirements engineers and other stakeholders to acquire, elaborate and validate system requirements. A scenario, in most situations, describes the normative or expected system behaviour during the interactions between the proposed system and its environment. To account for non-normative or undesired system behaviour, it is vital to predict and explore the existence or occurrence of ‘exceptions’ in a scenario. Identification of exceptions and inclusion of additional requirements to prevent their occurrence or mitigate their effects yields robust and fault-tolerant design solutions. In this paper, we outline the architecture of a toolkit for semi-automatic generation of scenarios. The toolkit is co-operative in the sense that it aids a requirements engineer in systematic generation and use of scenarios. The toolkit provides domain knowledge during requirements acquisition and validation of normative system behaviour. It also provides systematic guidance to the requirements engineer to scope the contents of a scenario. Furthermore, we have identified three kinds of exceptions: generic, permutation and problem exceptions, and have derived complex taxonomies of problem exceptions. We propose to populate the toolkit with lists of meaningful and relevant ‘what-if’ questions corresponding to the taxonomies of generic, permutation and problem exceptions. The exceptions can be chosen by the requirements engineer to include them in the generated scenarios to explore the correctness and completeness of requirements. In addition, the taxonomies of problem exceptions can also serve as checklists and help a requirements engineer to predict non-normative system behaviour in a scenario.