2. Defining Nonfunctional Requirements

"The Internet was done so well that most people think of it as a natural resource like the Pacific Ocean, rather than something that was man-made." — Alan Kay

Functional requirements = what screens, what buttons, what each operation does. Nonfunctional requirements = fast, reliable, secure, legally compliant, maintainable. Usually unwritten because they seem obvious — and just as important: an app that is unbearably slow or unreliable might as well not exist.

This chapter covers four of them:

1. Performance — defining and measuring it
2. Reliability — continuing to work correctly even when things go wrong
3. Scalability — efficiently adding capacity as load grows
4. Maintainability — keeping it workable long-term

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