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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 ...
A software requirements specification (SRS) is a description of a software system to be developed.It is modeled after the business requirements specification.The software requirements specification lays out functional and non-functional requirements, and it may include a set of use cases that describe user interactions that the software must provide to the user for perfect interaction.
Within the application boundary, non-functional requirements are associated with relevant categories and their sub-categories. Using a standardized set of basic criteria, each of the sub-categories is then sized according to its type and complexity; the size of such a requirement is the sum of the sizes of its sub-categories.
Within systems engineering, quality attributes are realized non-functional requirements used to evaluate the performance of a system. These are sometimes named architecture characteristics, or "ilities" after the suffix many of the words share. They are usually architecturally significant requirements that require architects' attention. [1]
For example, a non-functional requirement to be free from backdoors may be satisfied by replacing it with a process requirement to use pair programming. Other non-functional requirements will trace to other system components and be verified at that level. For example, system reliability is often verified by analysis at the system level.
Hence, BRD documents are complemented with a systems reference document (SRD) OR Technical Design Document (TDD) that details the design, technology performance and infrastructure expectations, including any technology requirements (non-functional) pertaining to quality of service, such as performance, maintainability, adaptability, reliability ...
ISO/IEC 9126 distinguishes between a defect and a nonconformity, a defect being "The nonfulfilment of intended usage requirements", whereas a nonconformity is "The nonfulfilment of specified requirements". A similar distinction is made between validation and verification, known as V&V in the testing trade.
Requirements Triage or prioritization of requirements is another activity which often follows analysis. [4] This relates to Agile software development in the planning phase, e.g. by Planning poker , however it might not be the same depending on the context and nature of the project and requirements or product/service that is being built.