When building software, we define two types of requirements:
- Functional: What the system should do (e.g., login, payment).
- Non-functional: How well it should do it (e.g., speed, security, scalability).
Without this distinction, you risk neglecting important aspects of system quality that don’t show up in features alone. You can implement a feature that works but still fails the user if it’s slow, insecure, or unreliable. Performance is considered non-functional because it affects user experience but doesn’t define what the system does.
It is similar to the frontend-backend layers but not exactly. Frontend-backend is about where things happen. Functional vs. non-functional is about what vs. how well. But there's overlap — non-functional requirements often live in the backend, yet both layers can have them.
In the beginning of a project, non-functional requirements are often forgotten because
- They’re abstract and harder to measure
- Not visible in UI
But ignoring them leads to rework, failure at scale, and unhappy users. Functional requirements get you a working, minimum viable product. Non-functional requirements make it usable, scalable, and successful.