What LLD covers vs HLD - Architecture Trade-offs

Problem Statement

When designing software, teams often get confused about what details to include at different stages. Without clear separation, designs become either too vague to implement or too detailed to manage, causing delays and errors.

Solution

High-Level Design (HLD) focuses on the big picture, showing system components and their interactions. Low-Level Design (LLD) zooms in to detail how each component works internally, including classes, methods, and data structures. This separation helps teams plan broadly first, then implement precisely.

Architecture

┌───────────────┐          ┌───────────────┐
│   High-Level  │          │   Low-Level   │
│    Design     │          │    Design     │
│               │          │               │
│ - System      │          │ - Classes     │
│   Components  │          │ - Methods     │
│ - Interactions│─────────▶│ - Data Structures│
└───────────────┘          └───────────────┘

This diagram shows HLD defining system components and their interactions, which then guide the detailed LLD of classes and methods.

Trade-offs

✓ Pros

→

Separates concerns: HLD manages overall architecture, LLD handles implementation details.

→

Improves communication: stakeholders see big picture, developers get precise instructions.

→

Facilitates parallel work: teams can work on different modules with clear boundaries.

✗ Cons

→

Requires extra effort to create two design levels.

→

If not synchronized, LLD may drift from HLD causing integration issues.

→

May slow down initial development due to detailed planning.

Use when building medium to large systems with multiple teams or complex components needing clear structure.

Avoid for very small projects or prototypes where detailed design overhead outweighs benefits.

Real World Examples

Amazon

Amazon uses HLD to define services like order processing and inventory, then LLD to specify class structures and APIs within each service.

Uber

Uber applies HLD to map out rider-driver matching system components, and LLD to design algorithms and data models for each component.

LinkedIn

LinkedIn creates HLD diagrams for social graph services, then LLD documents the internal data handling and caching mechanisms.

Alternatives

Unified Design

Combines high-level and low-level details in a single design document without clear separation.

Use when: Choose when project size is small and team is tight-knit, reducing overhead.

Agile Emergent Design

Starts with minimal design and evolves details during development rather than upfront HLD and LLD.

Use when: Choose when requirements are unclear or rapidly changing.

Summary

High-Level Design shows the overall system structure and component interactions.

Low-Level Design details the internal workings of each component like classes and methods.

Separating HLD and LLD improves clarity, communication, and development efficiency.

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