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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 ...
Generally, functional requirements are expressed in the form "system must do <requirement>," while non-functional requirements take the form "system shall be <requirement>." [3] The plan for implementing functional requirements is detailed in the system design, whereas non-functional requirements are detailed in the system architecture. [4] [5]
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.
A functional requirement in a functional specification might state as follows: When the user clicks the OK button, the dialog is closed and the focus is returned to the main window in the state it was in before this dialog was displayed. Such a requirement describes an interaction between an external agent (the user) and the software system ...
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]
When talking about architecture, the terms non-functional requirements or quality attributes are often used. [2] However recent empirical studies show that, for a software system, not all non-functional requirements affect its architecture, and functional requirements can also affect its architecture.
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.
ISVV goes beyond "traditional" verification and validation techniques, applied by development teams. While the latter aims to ensure that the software performs well against the nominal requirements, ISVV is focused on non-functional requirements such as robustness and reliability, and on conditions that can lead the software to fail.