Distinguishing Soft-Goals and Quality Requirements in Software Requirements Modeling

Requirements engineering plays an essential role in software development. Requirements are prescriptive statements that express situations to be enforced by a system in terms of its effects on its environment. There have been many discussions of functional versus non-functional requirements, of hard-goals versus soft-goals, of non-functional and quality requirements versus soft-goals. Quality requirements have often been treated as special cases of non-functional requirements or of soft-goals, without a clearly convincing distinction. In this paper, we formulate a somewhat unusual definition and metamodel for “quality requirements” and we explore some of its consequences. Quality requirements are not defined as a special kind of soft-goals, but as constraints on goals. We adapt the usual techniques of goal refinement to our definition and we argue that thus distinguishing soft-goals and quality requirements contributes to clarifying the system development process and the management of quality by the resulting software products.