What LLD covers vs HLD - Design Approaches Compared

Design: Understanding Low-Level Design (LLD) vs High-Level Design (HLD)

Focus on conceptual differences and coverage of HLD and LLD. Do not cover detailed coding or specific technology implementations.

Functional Requirements

FR1: Explain the purpose of HLD and LLD in system design

FR2: Describe the main components and focus areas of HLD

FR3: Describe the main components and focus areas of LLD

FR4: Highlight differences between HLD and LLD with examples

FR5: Provide clarity on when and why each design level is used

Non-Functional Requirements

NFR1: Use simple language without jargon

NFR2: Keep explanations beginner-friendly

NFR3: Use relatable real-life analogies

NFR4: Avoid overly technical or implementation-specific details

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

System architecture overview

Module/component breakdown

Data flow and control flow

Class diagrams and sequence diagrams

Interface definitions and APIs

Design Patterns

Top-down design approach

Modular design

Separation of concerns

Abstraction layers

Reference Architecture

HLD and LLD Relationship Diagram:

+-------------------+       +-------------------+
|   High-Level      |       |   Low-Level       |
|   Design (HLD)    | ----> |   Design (LLD)    |
+-------------------+       +-------------------+
| - System overview  |       | - Detailed logic  |
| - Major modules    |       | - Class details   |
| - Data flow       |       | - Algorithms      |
| - Interfaces      |       | - Data structures |
+-------------------+       +-------------------+

Components

High-Level Design (HLD)

Conceptual Design

Provides an overview of the system architecture, major components, and their interactions.

Low-Level Design (LLD)

Detailed Design

Focuses on detailed internal logic of components, including classes, methods, and data structures.

Request Flow

1. Start with requirements gathering.

2. Create HLD to outline system modules and their relationships.

3. Use HLD as a blueprint for LLD.

4. Develop LLD to specify detailed design for each module.

5. LLD guides developers during coding and implementation.

Database Schema

Not applicable as this is a conceptual design comparison.

Scaling Discussion

Bottlenecks

Confusion between HLD and LLD roles can cause design gaps.

Skipping LLD may lead to unclear implementation details.

Over-detailing HLD can delay design progress.

Solutions

Clearly define scope and purpose of HLD and LLD in project documentation.

Use HLD for stakeholder communication and LLD for developer guidance.

Maintain balance: keep HLD abstract and LLD detailed but manageable.

Interview Tips

Time: Spend 10 minutes explaining HLD, 15 minutes on LLD, and 5 minutes on differences and examples.

HLD is like a city map showing neighborhoods; LLD is like detailed building blueprints.

HLD focuses on what components exist and how they connect.

LLD focuses on how each component works internally.

Both are essential and complement each other in system design.

Clear communication of design levels helps smooth development.

Practice

1. Which of the following best describes what High-Level Design (HLD) focuses on?

easy

A. Database schema and indexing details

B. Detailed class structures and method implementations

C. Writing code for individual modules

D. The overall system architecture and main components

Solution

1. Step 1: Understand HLD purpose

   HLD is about the big picture, showing main parts and how they connect.

2. Step 2: Compare options

   Only The overall system architecture and main components describes overall architecture and main components, which matches HLD.

3. Final Answer:

   The overall system architecture and main components -> Option D

4. Quick Check:

   HLD = overall system view [OK]

Hint: HLD = big picture, LLD = details [OK]

Common Mistakes:

• Confusing HLD with detailed coding
• Thinking HLD includes method-level details
• Mixing database design with HLD focus

2. Which of these is typically included in Low-Level Design (LLD)?

easy

A. System component interactions

B. User interface wireframes

C. Class diagrams with methods and attributes

D. Network topology diagrams

Solution

1. Step 1: Identify LLD details

   LLD focuses on detailed design like classes, methods, and data structures.

2. Step 2: Match options to LLD

   Class diagrams with methods and attributes describes class diagrams with methods and attributes, which fits LLD.

3. Final Answer:

   Class diagrams with methods and attributes -> Option C

4. Quick Check:

   LLD = detailed class design [OK]

Hint: LLD = classes and methods details [OK]

Common Mistakes:

• Choosing high-level diagrams for LLD
• Confusing UI design with LLD
• Mixing network diagrams with LLD

3. Given a system design process, which step correctly uses HLD and LLD?

medium

A. Write detailed code first, then create HLD diagrams

B. Create HLD diagrams to plan architecture, then LLD for coding details

C. Skip HLD and directly write LLD documents

D. Use LLD to explain user requirements

Solution

1. Step 1: Understand design flow

   Good practice is to start with HLD for overall planning, then LLD for detailed design.

2. Step 2: Evaluate options

   Create HLD diagrams to plan architecture, then LLD for coding details matches this flow: HLD first, then LLD for coding guidance.

3. Final Answer:

   Create HLD diagrams to plan architecture, then LLD for coding details -> Option B

4. Quick Check:

   Design flow = HLD then LLD [OK]

Hint: Plan big picture first (HLD), then details (LLD) [OK]

Common Mistakes:

• Starting with code before design
• Skipping HLD and losing overview
• Using LLD for user requirements

4. A developer wrote detailed class diagrams but missed showing system modules and their interactions. What design level is missing?

medium

A. High-Level Design (HLD)

B. Low-Level Design (LLD)

C. Both HLD and LLD

D. Neither HLD nor LLD

Solution

1. Step 1: Analyze given design

   Detailed class diagrams are part of LLD, but system modules and interactions belong to HLD.

2. Step 2: Identify missing design level

   Since modules and their interactions are missing, HLD is incomplete.

3. Final Answer:

   High-Level Design (HLD) -> Option A

4. Quick Check:

   Missing modules = missing HLD [OK]

Hint: Modules and interactions belong to HLD [OK]

Common Mistakes:

• Confusing class diagrams with HLD
• Thinking LLD covers system modules
• Assuming both designs are missing

5. You are designing a new feature for a large system. Which approach correctly uses HLD and LLD to ensure scalability and maintainability?

hard

A. Design the system modules and their interactions first, then define classes and methods

B. Focus only on database schema and skip other designs

C. Write code immediately and document design later

D. Start with detailed class design, then create overall system diagrams

Solution

1. Step 1: Understand scalable design approach

   Good design starts with HLD to define modules and interactions, then LLD for detailed classes.

2. Step 2: Evaluate options for best practice

   Design the system modules and their interactions first, then define classes and methods follows this approach, ensuring clear structure and maintainability.

3. Final Answer:

   Design the system modules and their interactions first, then define classes and methods -> Option A

4. Quick Check:

   HLD first, then LLD for scalable design [OK]

Hint: Big picture first, then details for scalable design [OK]

Common Mistakes:

• Starting with code before design
• Ignoring system modules and interactions
• Focusing only on database schema