Idealized fault-tolerant components in requirements engineering
SERENE'11 Proceedings of the Third international conference on Software engineering for resilient systems
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Errors made during requirements development of dependable systems are very expensive to fix later in the lifecycle, and can result in minor to catastrophic failures. Discovering and documenting potential abnormal situations and irregular user behavior that can interrupt normal system interaction is of tremendous importance in the context of dependable systems development. Exceptions that are not identified during requirements elicitation might eventually lead to an incomplete system specification during analysis, and ultimately to an implementation that lacks certain functionality, or even behaves in an unreliable way. This thesis introduces a model-driven requirement development process that systematically guides the developer to consider reliability and safety concerns of dependable reactive systems. Our Dependability-Oriented Requirements Engineering Process (DREP) is comprised of five phases: requirements elicitation, requirements specification, dependability assessment, dependability-based refinement, and requirements summary. Our approach extends use case-based elicitation with ideas from the exception handling world. It leads the developer to explore exceptional situations arising in the environment that change user goals and service-related exceptional situations that threaten to fail user goals. The process requires the developer to specify means that detect such situations, to define the recovery measures that attempt to continue to provide service in a dependable manner, to offer degraded services where applicable or to ensure system and user safety. The need for exceptional modes of operation is also taken into account. The notions of dependability are integrated in activity diagrams and communication diagrams, which are used as specification models. We also propose a probabilistic extension of statecharts to formally model the interaction requirements defined in the use cases. The formalism, DA-Charts, allows a modeller to use success and failure probabilities to model the unreliable behaviour of the environment. Subsequently, the model can be probabilistically analyzed to determine achievable reliability and safety of the system interactions. A visual modelling environment for DA-Charts supporting automatic probability analysis has been implemented. We show that the proposed methodology leads to notable improvement in system dependability by applying our work to three case studies: the Elevator Control System, the 407 Express Toll Route System, and the Crisis Management System.