Introduction to Non-functional Requirements

Abstract

One of the most easily understood tasks during any systems design endeavor is to define the systems functional requirements. The functional requirements are a direct extension of the stakeholder’s purpose for the systems and the goals and objectives that satisfy them. Less easily understood are a systems non-functional requirements, or the constraints under which the entire system must operate. Identification of non-functional requirements should happen early in the conceptual design stage of the systems life cycle, for the same reason that functional requirements are defined up-front—that is, costs sky-rocket when new requirements are added late in a systems design sequence. Approaches for addressing non-functional requirements are rarely addressed in texts on systems design. In order to provide a logical and repeatable technique for addressing over 200 existing non-functional requirements, they must be reduced parsimoniously to a manageable number. Over 200 non-functional requirements are reduced, using results reported in eight models from the extant literature. The 27 resultant non-functional requirements have been organized in a taxonomy that categorizes the 27 major non-functional requirements within four distinct categories. Utilization of this taxonomy provides a framework for addressing non-functional requirements during the early system design stages.