Requirements gathering (functional and non-functional) in HLD - Scalability & System Analysis

Scalability Analysis - Requirements gathering (functional and non-functional)

Growth Table: Requirements Gathering Impact

Users	Functional Requirements	Non-Functional Requirements	Impact on Design
100 users	Basic features, simple workflows	Minimal performance, basic security	Simple architecture, monolithic app possible
10,000 users	More features, user roles, error handling	Improved performance, availability, security	Modular design, caching, load balancing
1,000,000 users	Complex workflows, integrations, multi-tenancy	High availability, scalability, strict security	Microservices, distributed systems, CDN
100,000,000 users	Global features, personalization, compliance	Extreme scalability, disaster recovery, compliance	Multi-region deployment, sharding, automation

First Bottleneck: Incomplete or unclear requirements

When requirements are not well gathered, the system design may miss critical features or constraints. This leads to rework, poor scalability, and user dissatisfaction. At small scale, this causes delays. At large scale, it causes costly redesigns and failures.

Scaling Solutions: Improving Requirements Gathering

• Stakeholder Interviews: Engage all users and stakeholders early to capture needs.
• Use Cases & User Stories: Define clear scenarios to understand functional needs.
• Non-Functional Workshops: Discuss performance, security, and compliance early.
• Prototyping: Build simple models to validate assumptions.
• Iterative Refinement: Continuously update requirements as feedback arrives.
• Documentation & Traceability: Keep clear records to track changes and decisions.

Back-of-Envelope Cost Analysis

Good requirements reduce costly redesigns. For example, a missed security requirement can cause expensive fixes later. Early investment in gathering requirements saves time and money during scaling.

• Time spent interviewing: ~10-20 hours per major stakeholder group
• Prototyping cost: Low compared to rework
• Documentation effort: Ongoing but small compared to development
• Cost of missed requirements: Can be 10x or more in rework and downtime

Interview Tip: Structuring Scalability Discussion on Requirements

Start by explaining the difference between functional and non-functional requirements. Then discuss how incomplete requirements cause bottlenecks. Next, describe how improving requirements gathering scales the system design process. Finally, give examples of techniques and their impact on cost and time.

Self Check Question

Your project has 1000 users and basic requirements. Now you expect 10x growth and new security needs. What is your first action?

Answer: Revisit and expand the requirements gathering process to include new functional and non-functional needs, especially security. This ensures the design can scale properly without costly rework.

Key Result

Clear and complete requirements gathering is the foundation for scalable system design; missing or unclear requirements become the first bottleneck as user scale grows.

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