Functional requirement

In software engineering, a functional requirement defines a function of a software system or its component. A function is described as a set of inputs, the behavior, and outputs (see also software).

Functional requirements may be calculations, technical details, data manipulation and processing and other specific functionality that define what a system is supposed to accomplish. Behavioral requirements describing all the cases where the system uses the functional requirements are captured in use cases. Functional requirements are supported by non-functional requirements (also known as quality requirements), which impose constraints on the design or implementation (such as performance requirements, security, or reliability). How a system implements functional requirements is detailed in the system design.

As defined in requirements engineering, functional requirements specify particular results of a system. This should be contrasted with non-functional requirements which specify overall characteristics such as cost and reliability. Functional requirements drive the application architecture of a system, while non-functional requirements drive the technical architecture of a system.

In some cases a requirements analyst generates use cases after gathering and validating a set of functional requirements. Each use case illustrates behavioral scenarios through one or more functional requirements. Often, though, an analyst will begin by eliciting a set of use cases, from which the analyst can derive the functional requirements that must be implemented to allow a user to perform each use case.

Process

A typical functional requirement will contain a unique name and number, a brief summary, and a rationale. This information is used to help the reader understand why the requirement is needed, and to track the requirement through the development of the system.

The crux of the requirement is the description of the required behavior, which must be clear and readable. The described behavior may come from organizational or business rules, or it may be discovered through elicitation sessions with users, stakeholders, and other experts within the organization. Many requirements may be uncovered during the use case development. When this happens, the requirements analyst may create a placeholder requirement with a name and summary, and research the details later, to be filled in when they are better known.

Books

• Wiegers, Karl E. (2003). Software Requirements 2: Practical techniques for gathering and managing requirements throughout the product development cycle (2nd ed.). Redmond: Microsoft Press. ISBN 978-0735618794. http://processimpact.com/pubs.shtml#reqs. 
• Stellman, Andrew; Greene, Jennifer (2005). Applied Software Project Management (1st ed.). Cambridge, MA: O'Reilly Media. ISBN 978-0596009489. http://www.stellman-greene.com/. 
• Sommerville, Ian (2006). Software Engineering (8th ed.). ISBN 978-0321313799. http://www.cs.st-andrews.ac.uk/~ifs/Books/SE8/index.html.