In systems engineering and requirements engineering, a non-functional requirement (NFR) is a requirement that specifies criteria that can be used to judge the operation of a system, rather than specific behaviors. They are contrasted with functional requirements that define specific behavior or functions. The plan for implementing functional requirements is detailed in the system design. The plan for implementing non-functional requirements is detailed in the system architecture, because they are usually architecturally significant requirements.
Broadly, functional requirements define what a system is supposed to do and non-functional requirements define how a system is supposed to be. Functional requirements are usually in the form of "system shall do <requirement>", an individual action or part of the system, perhaps explicitly in the sense of a mathematical function, a black box description input, output, process and control functional model or IPO Model. In contrast, non-functional requirements are in the form of "system shall be <requirement>", an overall property of the system as a whole or of a particular aspect and not a specific function. The system's overall properties commonly mark the difference between whether the development project has succeeded or failed.
Non-functional requirements are often called "quality attributes" of a system however there is a distinction between the two. Non-functional requirements are the criteria for evaluating how a software system should perform and a software system must have certain quality attributes in order to meet non-functional requirements. So when we say a system should be "secure", "highly-available", "portable", "scalable" and so on, we are talking about its quality attributes. Other terms for non-functional requirements are "qualities", "quality goals", "quality of service requirements", "constraints", "non-behavioral requirements", or "technical requirements". Informally these are sometimes called the "ilities", from attributes like stability and portability. Qualities—that is non-functional requirements—can be divided into two main categories:
A system may be required to present the user with a display of the number of records in a database. This is a functional requirement. How current this number needs to be, is a non-functional requirement. If the number needs to be updated in real time, the system architects must ensure that the system is capable of displaying the record count within an acceptably short interval of the number of records changing.
Sufficient network bandwidth may be a non-functional requirement of a system. Other examples include:
- Auditability and control
- Availability (see service level agreement)
- Capacity, current and forecast
- Configuration management
- Cost, initial and Life-cycle cost
- Data integrity
- Data retention
- Dependency on other parties
- Development environment
- Disaster recovery
- Efficiency (resource consumption for given load)
- Effectiveness (resulting performance in relation to effort)
- Emotional factors (like fun or absorbing or has "Wow! Factor")
- Environmental protection
- Extensibility (adding features, and carry-forward of customizations at next major version upgrade)
- Failure management
- Fault tolerance (e.g. Operational System Monitoring, Measuring, and Management)
- Flexibility (e.g. to deal with future changes in requirements)
- Integrability ability to integrate components
- Internationalization and localization
- Legal and licensing issues or patent-infringement-avoidability
- Maintainability (e.g. mean time to repair – MTTR)
- Network topology
- Open source
- Performance / response time (performance engineering)
- Platform compatibility
- Privacy (compliance to privacy laws)
- Quality (e.g. faults discovered, faults delivered, fault removal efficacy)
- Reliability (e.g. mean time between/to failures – MTBF/MTTF)
- Resource constraints (processor speed, memory, disk space, network bandwidth, etc.)
- Response time
- Safety or factor of safety
- Scalability (horizontal, vertical)
- Security (cyber and physical)
- Software, tools, standards etc. Compatibility
- Usability (human factors) by target user community
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