Requirements and use cases in LLD - Scalability & System Analysis

Scalability Analysis - Requirements and use cases

Growth Table: Requirements and Use Cases

Users	Requirements Complexity	Use Case Diversity	System Impact
100 users	Basic functional requirements, simple workflows	Few use cases, mostly core features	Minimal load, easy to manage
10,000 users	More detailed requirements, edge cases appear	Multiple use cases, some variations in user behavior	Moderate load, need for performance optimization
1,000,000 users	Complex requirements, scalability and reliability critical	Wide range of use cases, including rare and concurrent scenarios	High load, requires distributed systems and fault tolerance
100,000,000 users	Extensive requirements, global compliance, multi-tenancy	Very diverse use cases, internationalization, personalization	Massive load, advanced scaling, monitoring, and automation

First Bottleneck

At small scale, unclear or incomplete requirements cause delays and rework.

As users grow, the first technical bottleneck is often the system's inability to handle diverse use cases efficiently.

Without clear use cases, the system design may not support necessary workflows, causing performance and reliability issues.

Scaling Solutions

• Clarify and prioritize requirements: Regularly update requirements to reflect user needs and system goals.
• Use modular design: Break system into components to handle different use cases independently.
• Implement flexible workflows: Support variations in use cases without major redesign.
• Automate testing: Ensure all use cases are covered and system behaves as expected.
• Plan for scalability: Design with growth in mind, including load balancing and caching.

Back-of-Envelope Cost Analysis

At 100 users, requirements gathering and use case definition cost dominate.

At 10,000 users, development and testing costs increase due to more use cases.

At 1,000,000 users, infrastructure costs rise sharply to support complex workflows and high availability.

At 100,000,000 users, operational costs for monitoring, compliance, and continuous improvement become significant.

Interview Tip

Start by clearly defining requirements and use cases.

Explain how these influence system design and scalability.

Discuss how evolving requirements impact bottlenecks and solutions.

Use examples to show understanding of real-world complexity.

Self Check

Your system requirements are simple and support 1000 QPS. Traffic grows 10x. What do you do first?

Answer: Review and update requirements and use cases to ensure the system can handle increased load and complexity. Then plan scaling strategies accordingly.

Key Result

Clear, evolving requirements and well-defined use cases are essential to scale systems effectively from small to massive user bases.

Practice

1. What is the main purpose of requirements in system design?

easy

A. To write the system's source code

B. To describe how users interact with the system

C. To design the user interface layout

D. To specify what the system must do

Solution

1. Step 1: Understand the definition of requirements

   Requirements define the functions and features the system must provide.

2. Step 2: Differentiate from use cases

   Use cases describe user interactions, not the system's core functions.

3. Final Answer:

   To specify what the system must do -> Option D

4. Quick Check:

   Requirements = system functions [OK]

Hint: Requirements = what system must do, not how users act [OK]

Common Mistakes:

• Confusing requirements with use cases
• Thinking requirements include UI design
• Assuming requirements are code instructions

2. Which of the following best describes a use case in system design?

easy

A. A scenario showing user interaction with the system

B. A code module for user authentication

C. A diagram of system architecture

D. A detailed list of system features

Solution

1. Step 1: Define use case

   A use case is a story or scenario describing how a user uses the system.

2. Step 2: Eliminate unrelated options

   System features list, architecture diagrams, and code modules are not use cases.

3. Final Answer:

   A scenario showing user interaction with the system -> Option A

4. Quick Check:

   Use case = user scenario [OK]

Hint: Use case = user story or interaction example [OK]

Common Mistakes:

• Mixing use cases with system features
• Confusing use cases with technical diagrams
• Thinking use cases are code components

3. Given a system that allows users to upload photos and comment, which of the following is a valid use case?

medium

A. System stores photos in a database

B. User uploads a photo and receives confirmation message

C. Database schema includes photo metadata fields

D. Server runs a nightly backup process

Solution

1. Step 1: Identify user actions in options

   User uploads a photo and receives confirmation message describes a user action and system response, fitting a use case.

2. Step 2: Recognize system internals vs user interaction

   Options A, C, and D describe internal system details, not user interactions.

3. Final Answer:

   User uploads a photo and receives confirmation message -> Option B

4. Quick Check:

   User action + system response = use case [OK]

Hint: Use case = user action plus system reaction [OK]

Common Mistakes:

• Choosing system internal processes as use cases
• Ignoring the user perspective
• Confusing data storage details with use cases

4. A system design document lists: "Users can reset passwords via email." Which of the following is a problem in this use case description?

medium

A. It includes too much technical detail about email servers

B. It lacks the steps describing how users reset passwords

C. It incorrectly describes a system requirement instead of a use case

D. It is missing the database schema for password storage

Solution

1. Step 1: Analyze the use case description

   The statement is a high-level feature but lacks detailed user steps.

2. Step 2: Understand use case completeness

   Use cases should describe user actions step-by-step, not just state features.

3. Final Answer:

   It incorrectly describes a system requirement instead of a use case -> Option C

4. Quick Check:

   Use case needs detailed user steps [OK]

Hint: Use cases must show user steps, not just features [OK]

Common Mistakes:

• Confusing requirements with use cases
• Expecting technical details in use cases
• Ignoring the need for step-by-step user actions

5. You are designing a ride-sharing app. Which of the following best combines requirements and use cases for the feature "Request a ride"?

hard

A. Requirement: The system must match riders with drivers within 2 minutes. Use case: Rider opens app, enters destination, and confirms ride request.

B. Requirement: The app must have a blue color theme. Use case: Driver updates profile picture.

C. Requirement: The system stores all ride data for 1 year. Use case: Admin views system logs.

D. Requirement: The app sends promotional emails weekly. Use case: Rider rates driver after ride.

Solution

1. Step 1: Identify relevant requirement for ride request

   Matching riders with drivers quickly is a key system requirement for this feature.

2. Step 2: Identify matching use case

   The use case describes the rider's interaction to request a ride step-by-step.

3. Step 3: Verify other options

   Other options mix unrelated requirements or use cases not tied to "Request a ride" feature.

4. Final Answer:

   Requirement: The system must match riders with drivers within 2 minutes. Use case: Rider opens app, enters destination, and confirms ride request. -> Option A

5. Quick Check:

   Requirement + user steps = correct combination [OK]

Hint: Match system needs with user steps for correct design [OK]

Common Mistakes:

• Mixing unrelated requirements and use cases
• Ignoring user interaction details
• Choosing cosmetic or admin features instead of core ones