Functional and Nonfunctional Requirements

This chapter defines the term “requirement,” both formally and informally. There are two kinds of requirements, functional and nonfunctional. Functional requirements are decisions the stakeholders make about policies and processes they want automated. Nonfunctional requirements are decisions stakeholders make constraining automation technology. The decisions developers make—in light of stakeholder decisions, the requirements—are the design and the code. It is important to understand that nonfunctional requirements are as much requirements as functional requirements. Perfect technology is just a mental exercise that helps separate one from the other. Functional requirements are always and only about policies and processes to be automated: policies that software needs to enforce and processes that software needs to carry out. The chapter tabulates the quality criteria applied to all requirements-related work to identify and remove requirements defects as quickly and as cheaply as possible.