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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 ...
The term elicitation is used in books and research to raise the fact that good requirements cannot just be collected from the customer, as would be indicated by the name requirements gathering. Requirements elicitation is non-trivial because you can never be sure you get all requirements from the user and customer by just asking them what the ...
Functional requirements are supported by non-functional requirements (also known as "quality requirements"), which impose constraints on the design or implementation (such as performance requirements, security, or reliability). Generally, functional requirements are expressed in the form "system must do <requirement>," while non-functional ...
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 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.
Non-functional requirements can to some extent be described in use case style using misuse cases to define negative goals; but the (positive) goals thus discovered are often functional. For example, if theft is a threat to security , then fitting locks is a mitigation; but that a door can be locked is a functional requirement.