Using non-functional requirements to systematically support change

Non-functional requirements (or quality requirements, NFRs) such as confidentiality, performance and timeliness are often crucial to a software system. Our NFR-framework treats NFRs as goals to be achieved during the process of system development. Throughout the process, goals are decomposed, design tradeoffs are analysed, design decisions are rationalised, and goal achievement is evaluated. This paper shows how a historical record of the treatment of NFRs during the development process can also serve to systematically support evolution of the software system. We treat changes in terms: of (i) adding or modifying NFRs, or changing their importance, and (ii) changes in design decisions or design rationale. This incremental approach is illustrated by a study of changes in banking policies at Barclays Bank.