Why Requirements and use cases in LLD? - Purpose & Use Cases

The Big Idea

What if you built a system nobody wanted because you skipped this crucial step?

The Scenario

Imagine building a complex system like an online store without writing down what it should do or who will use it. You just start coding based on guesses.

The Problem

This guesswork leads to confusion, missed features, and lots of rework. Developers waste time fixing things that don't meet real needs, and users get frustrated.

The Solution

By clearly defining requirements and use cases upfront, everyone understands what the system must do and how users will interact with it. This guides design and development smoothly.

Before vs After

✗ Before

Start coding without clear goals; fix bugs as they appear.

✓ After

Write requirements and use cases first; build features that fit real needs.

What It Enables

Clear requirements and use cases enable building systems that truly solve problems and delight users.

Real Life Example

Before launching a ride-sharing app, defining use cases like booking rides, driver matching, and payment ensures the app works well for both riders and drivers.

Key Takeaways

Requirements clarify what the system must do.

Use cases describe how users interact with the system.

Together, they prevent costly mistakes and guide effective design.

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