Requirements gathering (functional and non-functional) in HLD - System Design Guide

Problem Statement

Without clear requirements, systems often fail to meet user needs or perform poorly under real conditions. This leads to costly redesigns, missed deadlines, and unhappy users because the system either lacks essential features or cannot handle expected loads.

Solution

Requirements gathering collects detailed user needs and system constraints before design begins. It separates what the system must do (functional) from how well it must perform or behave (non-functional), ensuring the design addresses both capabilities and quality attributes.

Architecture

Stakeholders & Users

→Requirements Team

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Functional Requirements

Functional Requirements

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Non-Functional Requirements

Non-Functional Requirements

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System Design & Planning

System Design & Planning

This diagram shows how input from stakeholders flows to a requirements team, which separates functional and non-functional requirements before passing them to system design.

Trade-offs

✓ Pros

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Ensures the system meets actual user needs by clarifying expected features.

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Identifies quality and performance goals early, reducing costly fixes later.

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Improves communication between stakeholders and developers through clear documentation.

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Helps prioritize features and constraints to focus design efforts effectively.

✗ Cons

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Can be time-consuming and delay project start if done too extensively.

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Miscommunication or incomplete input can still lead to missing or incorrect requirements.

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Balancing conflicting stakeholder needs requires negotiation and compromises.

Always use for projects with multiple stakeholders or complex features, especially when quality attributes like scalability, security, or availability are critical.

For very small or throwaway projects where formal documentation overhead outweighs benefits.

Real World Examples

Amazon

Amazon gathers detailed functional and non-functional requirements to ensure their e-commerce platform supports millions of users with high availability and fast response times.

Netflix

Netflix collects non-functional requirements like streaming latency and fault tolerance to deliver smooth video playback worldwide.

Uber

Uber defines functional requirements for ride matching and non-functional ones for real-time location accuracy and system scalability.

Alternatives

Agile User Stories

Focuses on small, incremental requirements captured as user stories rather than comprehensive upfront documentation.

Use when: When rapid iteration and evolving requirements are expected.

Prototyping

Uses early mockups or prototypes to gather feedback instead of detailed written requirements.

Use when: When user needs are unclear or hard to express verbally.

Summary

Clear requirements prevent costly redesigns by defining what the system must do and how well it must perform.

Separating functional and non-functional requirements helps address both features and quality attributes effectively.

Engaging stakeholders early and documenting needs improves communication and design accuracy.