HLD vs LLD distinction - Architecture Patterns Compared

System Overview - HLD vs LLD distinction

This system explains the difference between High-Level Design (HLD) and Low-Level Design (LLD) in software architecture. It highlights how HLD focuses on the big picture and overall system structure, while LLD dives into detailed components and their interactions.

Architecture Diagram

HLD Layer

↓

LLD Layer

↓

Implementation Details

Components

User

actor

Person learning about system design concepts

HLD Layer

design_layer

Shows overall system architecture and main components

LLD Layer

design_layer

Details internal design of components and their interactions

Implementation Details

code_level

Actual code and configuration based on LLD

Request Flow - 3 Hops

User → HLD Layer

HLD Layer → LLD Layer

LLD Layer → Implementation Details

Failure Scenario

Component Fails:HLD Layer

Impact:Without a clear high-level design, the system lacks overall structure, causing confusion and poor planning.

Mitigation:Create or revisit the HLD to ensure clear system boundaries and component roles before proceeding.

Architecture Quiz - 3 Questions

Which design layer focuses on the overall system structure and main components?

A Low-Level Design (LLD)

B High-Level Design (HLD)

C Implementation Details

D Testing Phase

Design Principle

This architecture highlights the separation of concerns between high-level and low-level design. HLD provides a broad system view to guide planning, while LLD offers detailed blueprints for implementation. Clear distinction helps teams work efficiently and reduces errors.

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