Functional vs Non-Functional Requirements in System Design

Introduction

When designing any software system, understanding the requirements is the very first and most important step. Requirements define what the system should do and how it should behave. These requirements fall into two broad categories: functional and non-functional.

What Are Functional Requirements?

Functional requirements describe what the system should do. They are specific features or behaviors that the system must support to fulfill user needs. Think of them as the 'functions' the system must perform.

Examples of Functional Requirements

Example 1: E-commerce Application

Suppose you are designing an e-commerce website. Functional requirements might include:

• Users should be able to search for products.
• Users should be able to add items to a cart.
• The system should allow checkout with payment gateway integration.
• Admin should be able to add, edit, or delete product listings.

Example 2: Messaging App

For a basic messaging application like WhatsApp, functional requirements may be:

• Send and receive text messages
• Create and join group chats
• Send multimedia (images, videos, documents)
• View delivery and read receipts

Ask Yourself

Question: If I were to remove this requirement, would the main functionality of the app break?

Answer: If yes, then it's likely a functional requirement. For instance, if users can’t send messages in a messaging app, the core functionality breaks. Hence, it’s a functional requirement.

What Are Non-Functional Requirements?

Non-functional requirements describe how the system should behave. They focus on performance, usability, reliability, security, scalability, and more. These are often called quality attributes of a system.

Examples of Non-Functional Requirements

Example 1: Online Banking Platform

Let’s say you are designing an online banking application. Some non-functional requirements might include:

• The system should be available 99.99% of the time.
• All transactions should be processed within 2 seconds.
• Passwords must be stored in an encrypted format.
• System should handle 100,000 concurrent users during peak hours.

Example 2: Video Streaming Platform

For a service like YouTube or Netflix, non-functional requirements might include:

• Video should start playing within 1 second of clicking 'Play'.
• System should scale automatically with increasing traffic.
• System should have high availability with minimal downtime.
• Interface should be user-friendly and responsive across devices.

Another Intuition Check

Question: If the requirement is missing, does the core functionality still work, but users may complain?

Answer: Yes? Then it's likely a non-functional requirement. For example, if a video takes 20 seconds to load, it still plays, but users will be unhappy. It’s a performance issue — a non-functional aspect.

Comparison Table: Functional vs Non-Functional Requirements

Aspect	Functional Requirement	Non-Functional Requirement
Definition	Specifies what the system should do	Specifies how the system should behave
Examples	Login, Search, Payment	Performance, Security, Scalability
Impact	Directly impacts system’s functionality	Impacts user satisfaction and system quality
Measurable?	Usually yes (pass/fail tests)	Yes, often via benchmarks or SLAs

Why Understanding This Difference Matters

Both types of requirements are critical for building a successful system. Missing out on functional requirements means the system won’t do what users need. Ignoring non-functional requirements leads to a system that might work, but performs poorly, isn’t secure, or doesn’t scale.

Quick Quiz

1. Requirement: The login page must load within 1 second.

Answer: Non-functional (This relates to performance)

2. Requirement: Users must be able to reset their password.

Answer: Functional (This is a core action a user must be able to perform)

Key Takeaways

• Functional requirements define what the system does; non-functional define how it behaves.
• Both are essential to a complete and robust system design.
• Always gather both types of requirements during system analysis and planning.