non-functional requirements

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What are functional and non functional requirements and why both matter

In software engineering (and Systems Engineering), a functional requirement defines a function of a system or its component. A function is described as a set of inputs, the behavior, and outputs (see also software). Functional requirements may be calculations, technical details, data manipulation and processing and other specific functionality that define what a system is supposed to accomplish. Behavioral requirements describing all the cases where the system uses the functional requirements are captured in use cases. A non-functional requirement is a requirement that specifies criteria that can be used to judge the operation of a system, rather than specific behaviors. This should be contrasted with functional requirements that define specific behavior or functions. The plan for implementing functional requirements is detailed in the system design. The plan for implementing non-functional requirements is detailed in the system architecture. So, with simpler words, the non-functional requirements, in addition to the obvious features and functions that you will provide in your system, are other requirements that don’t actually DO anything, but are important characteristics nevertheless. For example, attributes such as performance, security, usability, compatibility. aren’t a “feature” of the system, but are a required characteristic. Most of the time you can’t write a specific line of code to implement them, rather they are “emergent” properties that arise from the entire solution. The specification needs to describe any such attributes…


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