Addressing Degraded Service Outcomes and Exceptional Modes of Operation in Behavioural Models

A dependable software system should attempt to at least partially satisfy user goals if full service provision is impossible due to an exceptional situation. In addition, a dependable system should evaluate the effects of the exceptional situation on future service provision and adjust the set of services it promises to deliver accordingly. In this paper we show how to express degraded service outcomes and exceptional modes of operation in behavioural models, i.e. use cases, activity diagrams and state charts. We also outline how to integrate the task of discovering and defining degraded outcomes and exceptional modes of operation into a requirements engineering process by presenting the relevant parts of our dependability-focused requirements engineering process DREP.