Integrating Exception Handling in Goal Models

Missing requirements are known to be among the major sources of software failure. Incompleteness often results from poor anticipation of what could go wrong with an over-ideal system. Obstacle analysis is a model-based, goal-anchored form of risk analysis aimed at identifying, assessing and resolving exceptional conditions that may obstruct the behavioral goals of the target system. The obstacle resolution step is obviously crucial as it should result in more adequate and more complete requirements. In contrast with obstacle identification and assessment, however, this step has little support beyond a palette of resolution operators encoding tactics for producing isolated countermeasures to single risks. In particular, there is no single clue to date as to where and how such countermeasures should be integrated within a more robust goal model. To address this problem, the paper describes a systematic technique for integrating obstacle resolutions as countermeasure goals into goal models. The technique is shown to guarantee progress towards a complete goal model; it preserves the correctness of refinements in the overall model; and keeps the original, ideal model visible to avoid cluttering the latter with a combinatorial blow-up of exceptional cases. To allow for this, the goal specification language is slightly extended in order to capture exceptions to goals seperately and distinguish normal situations from exceptional ones. The proposed technique is evaluated on a non-trivial ambulance dispatching system