Functional vs non-functional requirements in Software Engineering - When to Use Which

The Big Idea

What if your app works but feels slow or crashes often? Understanding these requirements can prevent that.

The Scenario

Imagine building a house by only telling the builder what rooms you want, but not how strong the walls should be or how safe the wiring must be.

The Problem

Without clear details on both what the system should do and how well it should perform, projects get delayed, cost more, and often fail to meet user needs.

The Solution

Separating requirements into functional (what the system does) and non-functional (how well it does it) helps teams build software that works correctly and is reliable, fast, and secure.

Before vs After

✗ Before

User must login
System should be fast

✓ After

Functional: User can login
Non-functional: Login response under 2 seconds

What It Enables

This distinction enables building software that not only works but also delights users with quality and performance.

Real Life Example

When ordering online, functional requirements ensure you can add items to a cart, while non-functional requirements ensure the website loads quickly and keeps your data safe.

Key Takeaways

Functional requirements define what a system must do.

Non-functional requirements define how well the system performs those functions.

Both are essential for successful software development.