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

↓

Functional Requirements

Functional Requirements

↓

Non-Functional Requirements

Non-Functional Requirements

↓

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.

Practice

1. Which of the following best describes a functional requirement in system design?

easy

A. A usability guideline for user interface design

B. A quality that defines how fast the system should respond

C. A security measure to protect user data

D. A feature that defines what the system should do

Solution

1. Step 1: Understand functional requirements

   Functional requirements specify the actions or features the system must perform, such as user login or data processing.

2. Step 2: Differentiate from non-functional requirements

   Non-functional requirements describe qualities like speed, security, or usability, not specific features.

3. Final Answer:

   A feature that defines what the system should do -> Option D

4. Quick Check:

   Functional = What system does [OK]

Hint: Functional = what system does; Non-functional = how well [OK]

Common Mistakes:

• Confusing functional with non-functional requirements
• Thinking security is a functional requirement
• Mixing usability with system features

2. Which of the following is a correct example of a non-functional requirement?

easy

A. The system shall respond to user requests within 2 seconds

B. The system shall provide a search feature

C. The system shall store user profiles

D. The system shall allow users to reset their password

Solution

1. Step 1: Identify non-functional requirements

   Non-functional requirements describe system qualities like performance, reliability, or security.

2. Step 2: Match options to definitions

   The system shall respond to user requests within 2 seconds specifies response time, a performance metric, which is non-functional. Others describe features (functional).

3. Final Answer:

   The system shall respond to user requests within 2 seconds -> Option A

4. Quick Check:

   Non-functional = system qualities [OK]

Hint: Look for qualities like speed or security for non-functional [OK]

Common Mistakes:

• Choosing features as non-functional requirements
• Ignoring time or performance constraints
• Confusing functional and non-functional examples

3. Given these requirements:

• The system must allow users to upload files.
• Uploads must complete within 5 seconds.
• The system must support 1000 concurrent uploads.

Which of these is a functional requirement?

medium

A. Uploads must complete within 5 seconds

B. The system must support 1000 concurrent uploads

C. The system must allow users to upload files

D. The system must ensure uploaded files are available 99.9% of the time

Solution

1. Step 1: Identify functional requirements

   Functional requirements describe what the system does, such as allowing file uploads.

2. Step 2: Analyze each option

   A is performance (non-functional). B is capacity/scalability (non-functional). D is availability (non-functional).

3. Final Answer:

   The system must allow users to upload files -> Option C

4. Quick Check:

   Functional = system features [OK]

Hint: Features = functional; qualities like speed = non-functional [OK]

Common Mistakes:

• Selecting performance, capacity, or availability as functional
• Confusing capacity constraints as functional
• Ignoring direct feature statements

4. A system requirement states: "The system shall handle 1000 requests per second." Which type of requirement is this and what is wrong if it is listed as functional?

medium

A. Functional; it describes a feature, so it's correct

B. Non-functional; it describes performance, so listing as functional is incorrect

C. Functional; it describes user behavior, so it's correct

D. Non-functional; it describes security, so listing as functional is incorrect

Solution

1. Step 1: Identify requirement type

   Handling 1000 requests per second is about performance, a non-functional quality.

2. Step 2: Explain why listing as functional is wrong

   Functional requirements describe features, not performance metrics. Mislabeling causes confusion in design and testing.

3. Final Answer:

   Non-functional; it describes performance, so listing as functional is incorrect -> Option B

4. Quick Check:

   Performance = non-functional [OK]

Hint: Performance metrics are non-functional, not features [OK]

Common Mistakes:

• Calling performance a functional requirement
• Confusing user behavior with system qualities
• Ignoring the impact of mislabeling requirements

5. You are gathering requirements for a new messaging app. Which combination below correctly pairs functional and non-functional requirements?

hard

A. Users can send messages; messages must be delivered within 1 second

B. Messages must be encrypted; users can create profiles

C. The app must support dark mode; users can block contacts

D. The app must be available 24/7; Messages must be delivered with 99.99% reliability

Solution

1. Step 1: Identify functional requirements in options

   Functional requirements describe features like sending messages or creating profiles.

2. Step 2: Identify non-functional requirements in options

   Non-functional requirements describe qualities like speed, availability, or security.

3. Step 3: Analyze each option pair

   Users can send messages; messages must be delivered within 1 second pairs sending messages (functional) with delivery speed (non-functional). Others mix two functional or two non-functional requirements incorrectly.

4. Final Answer:

   Users can send messages; messages must be delivered within 1 second -> Option A

5. Quick Check:

   Functional + Non-functional = Users can send messages; messages must be delivered within 1 second [OK]

Hint: Functional = features; Non-functional = qualities like speed or availability [OK]

Common Mistakes:

• Mixing two functional or two non-functional requirements
• Confusing UI features with system qualities
• Ignoring delivery time as non-functional