HLD vs LLD distinction - Design Approaches Compared

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

Focus on conceptual distinction between HLD and LLD; exclude detailed coding or implementation specifics

Functional Requirements

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

FR2: Describe the main differences between HLD and LLD

FR3: Show examples of components included in HLD and LLD

FR4: Clarify when to use HLD and when to use LLD

Non-Functional Requirements

NFR1: Use simple language without technical jargon

NFR2: Provide clear, relatable examples

NFR3: Keep explanations concise and easy to understand

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

Key Components

System architecture overview

Module breakdown

Data flow diagrams

Class diagrams

Interface definitions

Design Patterns

Top-down design approach

Modular design

Abstraction layers

Reference Architecture

  +---------------------+        +---------------------+
  |   High-Level Design  | -----> |   Low-Level Design   |
  +---------------------+        +---------------------+
          |                                |
          | System overview                | Detailed module design
          | Components and interfaces      | Data structures and algorithms
          | Technology choices             | Code-level details
          v                                v
  +---------------------+        +---------------------+
  |  System Architecture |        |  Detailed Design Docs |
  +---------------------+        +---------------------+

Components

High-Level Design (HLD)

Conceptual diagrams and documents

Provide an overview of the system architecture, main components, and their interactions

Low-Level Design (LLD)

Detailed diagrams and specifications

Describe internal logic, data structures, and detailed module design for implementation

Request Flow

1. Start with requirements gathering

2. Create HLD to outline system structure and major components

3. Review HLD with stakeholders for feedback

4. Develop LLD to specify detailed design of each component

5. Use LLD as a guide for coding and testing

Database Schema

Not applicable as this is a conceptual design distinction

Scaling Discussion

Bottlenecks

Confusion between HLD and LLD can cause design delays

Too much detail in HLD can overwhelm stakeholders

Too little detail in LLD can cause implementation errors

Solutions

Clearly separate HLD and LLD phases with defined goals

Use simple diagrams and summaries in HLD for clarity

Ensure LLD includes all necessary technical details for developers

Interview Tips

Time: Spend 10 minutes explaining HLD, 10 minutes on LLD, and 5 minutes on their relationship and examples

HLD focuses on what the system does and its architecture

LLD focuses on how each part works internally

HLD is for stakeholders and architects; LLD is for developers

Both are essential for successful system design and implementation

Practice

1. Which of the following best describes the purpose of a High-Level Design (HLD)?

easy

A. It provides detailed code-level logic for each module.

B. It shows the overall system architecture and main components.

C. It lists all the database table schemas.

D. It contains the exact API request and response formats.

Solution

1. Step 1: Understand HLD focus

   HLD focuses on the big picture, showing main parts and their relationships.

2. Step 2: Compare options

   The option that shows the overall system architecture and main components describes HLD.

3. Final Answer:

   It shows the overall system architecture and main components. -> Option B

4. Quick Check:

   HLD = Big Picture [OK]

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

Common Mistakes:

• Confusing HLD with detailed design
• Thinking HLD includes code logic
• Mixing database schema details with HLD

2. Which statement correctly differentiates Low-Level Design (LLD) from High-Level Design (HLD)?

easy

A. LLD is used for planning; HLD is used for coding.

B. LLD shows system modules and their interactions; HLD shows code details.

C. LLD explains detailed logic and data structures; HLD shows system components.

D. LLD focuses on user interface design; HLD focuses on database design.

Solution

1. Step 1: Identify LLD and HLD roles

   LLD explains detailed logic and data structures, while HLD shows system components and their relationships.

2. Step 2: Evaluate options

   The option stating 'LLD explains detailed logic and data structures; HLD shows system components.' correctly differentiates them, matching their definitions.

3. Final Answer:

   LLD explains detailed logic and data structures; HLD shows system components. -> Option C

4. Quick Check:

   LLD = Details, HLD = Components [OK]

Hint: LLD = details, HLD = overview [OK]

Common Mistakes:

• Reversing roles of HLD and LLD
• Confusing planning and coding phases
• Mixing UI and database focus incorrectly

3. Given a system design process, which output is expected from the Low-Level Design (LLD) phase?

medium

A. A diagram showing major system components and their interactions.

B. A high-level flowchart of system data flow.

C. A list of user requirements and business goals.

D. Detailed class diagrams and sequence diagrams for modules.

Solution

1. Step 1: Recall LLD deliverables

   LLD includes detailed design artifacts like class diagrams and sequence diagrams showing module internals.

2. Step 2: Match options to LLD

   Detailed class diagrams and sequence diagrams for modules match the detailed diagrams expected in LLD; others describe higher-level or requirement outputs.

3. Final Answer:

   Detailed class diagrams and sequence diagrams for modules. -> Option D

4. Quick Check:

   LLD = Detailed diagrams [OK]

Hint: LLD = detailed diagrams like class and sequence [OK]

Common Mistakes:

• Choosing high-level diagrams for LLD
• Confusing requirements with design outputs
• Selecting flowcharts meant for HLD

4. A team mistakenly uses High-Level Design (HLD) diagrams to write detailed code logic. What is the main issue with this approach?

medium

A. HLD lacks the detailed information needed for coding.

B. HLD contains too much detail, causing confusion.

C. HLD diagrams are only for database design.

D. HLD is only used after coding is complete.

Solution

1. Step 1: Understand HLD purpose

   HLD provides a broad overview, not detailed logic needed for coding.

2. Step 2: Identify problem

   Using HLD for coding misses necessary details, causing incomplete or incorrect code.

3. Final Answer:

   HLD lacks the detailed information needed for coding. -> Option A

4. Quick Check:

   HLD = overview, not detailed code [OK]

Hint: HLD is overview, not detailed code [OK]

Common Mistakes:

• Thinking HLD has too much detail
• Believing HLD is only for database
• Assuming HLD is post-coding

5. You are designing a new online shopping system. Which design phase should you use to decide the main system components and their interactions, and which phase to define the detailed logic of the payment module?

hard

A. Use HLD for main components and LLD for payment module details.

B. Use LLD for main components and HLD for payment module details.

C. Use HLD for both main components and payment module details.

D. Use LLD for both main components and payment module details.

Solution

1. Step 1: Identify phase for main components

   Main system components and their interactions are defined in High-Level Design (HLD).

2. Step 2: Identify phase for detailed module logic

   Detailed logic, like payment module internals, is defined in Low-Level Design (LLD).

3. Final Answer:

   Use HLD for main components and LLD for payment module details. -> Option A

4. Quick Check:

   HLD = components, LLD = details [OK]

Hint: HLD = components, LLD = detailed logic [OK]

Common Mistakes:

• Swapping HLD and LLD roles
• Using one phase for all design
• Ignoring detailed design for modules