Nonfunktionale

Nonfunktionale, often written as Nichtfunktionale Anforderungen in German, refers to the aspects of a system that describe how it operates rather than what it does. In software engineering, nonfunctional (NFR) requirements specify quality attributes and constraints that influence the system's design, implementation, and operation.

Typical categories include performance (response time, throughput), reliability (mean time between failures), availability (uptime), scalability (ability to handle growth), security (confidentiality, integrity, authentication), usability (ease of use), maintainability (ease of updates and debugging), portability (ability to run in different environments), interoperability (compatibility with other systems), and compliance (regulatory and standards adherence).

NFRs are assessed with metrics and tests, such as load and performance testing, reliability testing, security testing, and usability studies. Quality models like ISO/IEC 25010 provide a framework of product quality attributes and subattributes to guide specification and evaluation. In practice, NFRs are often treated as constraints or acceptance criteria that must be satisfied under defined conditions.

Elicitation and management of nonfunctional requirements require stakeholder involvement and trade-off analysis, since improving one attribute may degrade another (for example, security vs. usability, or performance vs. cost). Clear documentation, early architectural decisions, and ongoing verification help ensure that nonfunctional goals are met throughout the project lifecycle. The term nonfunktionale is commonly used in German-language contexts to refer to these considerations.