Non-Functional Requirements Analysis

Functional requirements relate what a solution DOES.

They describe components, behaviors, entities, actions, inputs, and outputs. They contain the details of the design the user sees and the mechanisms that generate results.

Non-Functional requirements relate to what a system IS.

They describe qualities in terms of “-ilities,” e.g., reliability, modularity, flexibility, robustness, maintainability, scalability, usability, and so on.

I include descriptions of how the system is intended to be maintained and governed within non-functional requirements, but I suppose that’s a philosophical point.

All requirements should include the criteria by which functional and non-functional elements will be judged to be acceptable.

Requirements represent the To-Be State in abstract terms. The design represents the To-Be State in more concrete terms.

Many sources enumerate possible non-functional requirements, but the Wikipedia page provides a pretty inclusive list.

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