Requirement

In engineering, a requirement is a singular documented physical and functional need that a particular product or service must be or perform. It is most commonly used in a formal sense in systems engineering, software engineering, or enterprise engineering. It is a statement that identifies a necessary attribute, capability, characteristic, or quality of a system for it to have value and utility to a user.

In the classical engineering approach, sets of requirements are used as inputs into the design stages of product development. Requirements are also an important input into the verification process, since tests should trace back to specific requirements. Requirements show what elements and functions are necessary for the particular project. This is reflected in the waterfall model of the software life-cycle. However, when iterative methods of software development or agile methods are used, the system requirements are incrementally developed in parallel with design and implementation


what is requirement

Is a statement about an intended product that specifies what
it should do or how it should perform.
should be as specific, unambiguous and clear as possible.

Types of requirements

1Functional requirements
2non-funcional requirements

Types of product requirementsRequirements are typically placed into these categories:

Architectural requirements describe what must be done by identifying the necessary system architecture of a system,
Functional requirements describe the functionality that the system is to execute; for example, formatting some text or modulating a signal. They are sometimes known as capabilities.
Non-functional requirements describe characteristics of the system that the user cannot affect or (immediately) perceive. Nonfunctional requirements are sometimes known as quality requirements or ilities.
Constraint requirements impose limits upon the design alternatives or project/process operations. No matter how the problem is solved the constraint requirements must be adhered to.
Non-functional requirements can be further classified according to whether they are usability requirements, look and feel requirements, humanity requirements, performance requirements, maintainability requirements, operational requirements, safety requirements, reliability requirements, or one of many other types of requirements.

In software engineering this categorization is useful because only functional requirements can be directly implemented in software. The non-functional requirements are controlled by other aspects of the system. For example, in a computer system reliability is related to hardware failure rates, and performance is controlled by CPU and memory. Non-functional requirements can in some cases be decomposed into functional requirements for software. For example, a system level non-functional safety requirement can be decomposed into one or more functional requirements. See FURPS. In addition, a non-functional requirement may be converted into a process requirement when the requirement is not easily measurable. For example, a system level maintainability requirement may be decomposed into restrictions on software constructs or limits on lines or code.