08-16-2017, 10:45 PM
In systems engineering and requirements engineering, a non-functional requirement 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 of 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. Other terms for non-functional requirements are "qualities", "quality goals", "quality of service requirements", "constraints" and "non-behavioral 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:
Execution qualities, such as security and usability, which are observable at run time.
Evolution qualities, such as testability, maintainability, extensibility and scalability, which are embodied in the static structure of the software system.
Examples
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 up-to-date [update] 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 updating the [displayed] 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:
Accessibility
Audit and control
Availability (see service level agreement)
Backup
Capacity, current and forecast
Certification
Compliance
Configuration management
Dependency on other parties
Deployment
Documentation
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
Escrow
Exploitability
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)
Legal and licensing issues or patent-infringement-avoidability
Interoperability
Maintainability
Modifiability
Network topology
Open source
Operability
Performance / response time (performance engineering)
Platform compatibility
Price
Privacy
Portability
Quality (e.g. faults discovered, faults delivered, fault removal efficacy)
Recovery / recoverability (e.g. mean time to recovery - MTTR)
Reliability (e.g. mean time between failures - MTBF, or availability)
Reporting
Resilience
Resource constraints (processor speed, memory, disk space, network bandwidth, etc.)
Response time
Reusability
Robustness
Safety or Factor of safety
Scalability (horizontal, vertical)
Security
Software, tools, standards etc. Compatibility
Stability
Supportability
Testability
Transparency
Usability by target user community
