Using Scenarios to Identify and Trade-off Dependability Objectives in Design

Whilst there is no consensus on the exact definition of dependability, many agree that it is a composite term consisting of many different non-orthogonal attributes. These can be in conflict or in harmony. This characteristic of dependability inevitably results in compromises and trade-offs being made during the design process. During the evolution of a dependable system, it is important that stakeholders justify any trade-offs made. These decisions must be made with due consideration of the system’s operational role and context. Scenarios can be used to capture, in descriptive form, the different aspects and context of the system’s operation. Stakeholders can use scenarios to describe their views of intended system operation. Elaborating on the scenarios, stakeholders can elicit specific and more detailed dependability objectives (e.g. relating to safety, performance and availability). By providing a clear context, scenarios allow system stakeholders to reason about the acceptable bounds for each dependability objective. In the design of the system it can be hard to satisfy the dependability objectives of all scenarios. An understanding of the importance of each scenario (as elicited from the stakeholders), together with the bounds associated with scenario dependability objectives, can be used to support the process of trading-off one objective in favour of another during design. In this paper we describe this scenario-based approach to the identification and balancing of dependability objectives in design.