NFR

Non-functional requirements (NFRs) are a class of requirements in software engineering that specify how a system operates rather than the specific behaviors it must perform. They describe quality attributes and constraints that the system must satisfy, shaping its overall performance, reliability, security, usability, and other aspects of its operation.

NFRs differ from functional requirements, which define the concrete services or functions the system must provide. While a functional requirement might state that a user can generate a report, an NFR would constrain how long it should take to generate the report, how often the system remains available, or how secure the data in the report must be. NFRs are typically measurable through criteria and metrics that enable verification during testing and deployment.

Common categories of NFRs include performance (such as response time and throughput), reliability and availability (uptime and MTBF), security (protection against threats and data privacy), usability and accessibility (ease of use and accommodation for diverse users), maintainability and extensibility ( ease of modification and growth), portability (ability to run on different platforms), and interoperability (compatibility with other systems). Quality models, such as ISO/IEC 25010, provide structured descriptions of these characteristics and their subcategories to guide specification and assessment.

NFRs often require trade-offs; for example, increasing security may impact usability or performance, and architectural decisions can influence multiple attributes. They are typically addressed early in requirements engineering and validated through non-functional testing, performance testing, security testing, and reliability testing, with ongoing monitoring in operation. Clear, measurable NFRs improve design decisions, testing, and overall system quality.