What is LLD Interview?

What is Low-Level Design (LLD) Interview?

Low-Level Design (LLD) Interview is a technical interview where you’re asked to design a software system at a detailed, implementation-focused level. Unlike High-Level Design (HLD) which focuses on system architecture, LLD dives deep into classes, interfaces, relationships, and code structure.

Simple Analogy

Think of building a house:

• High-Level Design (HLD) = Architectural blueprint showing rooms, floors, and overall structure
• Low-Level Design (LLD) = Detailed blueprints showing electrical wiring, plumbing, exact measurements, and material specifications

LLD interviews focus on the “how” of implementation, not just the “what” of the system!

Visual: LLD vs HLD

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Why Do Companies Conduct LLD Interviews?

Companies conduct LLD interviews to assess several critical skills:

1. Object-Oriented Design Skills

Can you design clean, maintainable classes? Do you understand OOP principles?

What Interviewers Look For

• Can you identify the right classes and their responsibilities?
• Do you apply SOLID principles correctly?
• Can you use design patterns appropriately?
• Is your code clean and maintainable?

2. Problem-Solving Approach

How do you break down complex problems? Can you think systematically?

Example Scenario:

“Design a Parking Lot System”

Good Approach:

1. Identify actors (Driver, Admin, System)
2. Identify entities (ParkingLot, ParkingSpot, Vehicle, Ticket)
3. Assign responsibilities
4. Design relationships
5. Define APIs

Poor Approach:

• Jumping straight to code
• Not thinking about edge cases
• Missing important entities

3. Code Quality & Maintainability

Can you write code that’s easy to understand, test, and extend?

graph TD
    A[LLD Interview] --> B[Code Quality]
    A --> C[Maintainability]
    A --> D[Extensibility]
    
    B --> B1[Clean Code]
    B --> B2[Proper Naming]
    B --> B3[Comments]
    
    C --> C1[Easy to Understand]
    C --> C2[Easy to Modify]
    C --> C3[Well Organized]
    
    D --> D1[Open/Closed Principle]
    D --> D2[Easy to Add Features]
    D --> D3[Design Patterns]

4. Communication Skills

Can you explain your design decisions? Can you collaborate effectively?

Interview Tip

LLD interviews are collaborative! Interviewers want to see:

• How you communicate your thoughts
• How you handle feedback
• How you think through problems out loud
• How you justify design decisions

5. Real-World Experience

Do you understand how to build production-ready systems?

What They Assess:

• Handling edge cases
• Error handling
• Scalability considerations
• Trade-offs and decisions

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The Significance of LLD Interviews

Why LLD Skills Matter

LLD interviews are significant because they test skills that directly impact your day-to-day work as a software engineer:

1. Foundation of Good Software

Good LLD skills lead to:

• Maintainable code - Easy to understand and modify
• Scalable systems - Can grow without major refactoring
• Testable code - Easy to write unit tests
• Collaborative code - Easy for team members to work with

2. Career Growth

Strong LLD skills are essential for:

• Senior Engineer roles - Expected to design systems
• Tech Lead positions - Guide team’s code quality
• Architecture roles - Design scalable systems
• Code reviews - Provide valuable feedback

3. Problem-Solving Mindset

LLD interviews teach you:

• Systematic thinking - Break problems into smaller parts
• Design thinking - Think before coding
• Trade-off analysis - Understand pros and cons
• Best practices - Learn industry standards

Long-Term Impact

Mastering LLD doesn’t just help you pass interviews - it makes you a better engineer! The skills you develop will help you write better code throughout your career.

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

Understanding the difference between HLD and LLD is crucial. Let’s break it down:

Visual Comparison

Detailed Comparison

High-Level Design (HLD)

Focus: System architecture and overall structure

What You Design:

• System components and services
• Database schema (tables, relationships)
• API endpoints and contracts
• Infrastructure (servers, load balancers)
• Data flow between components
• Scalability and performance strategies

Example: Design a URL Shortener (HLD)

Key Questions:

• What services do we need?
• How do components communicate?
• What databases should we use?
• How do we handle scale?

Low-Level Design (LLD)

Focus: Code structure and implementation details

What You Design:

• Classes and their responsibilities
• Interfaces and abstract classes
• Methods and their signatures
• Class relationships (inheritance, composition)
• Design patterns to use
• Data structures and algorithms

Example: Design a URL Shortener (LLD)

Key Questions:

• What classes do we need?
• What are their responsibilities?
• How do classes interact?
• What design patterns fit?

Side-by-Side Comparison

Aspect	High-Level Design (HLD)	Low-Level Design (LLD)
Focus	System architecture	Code structure
Scope	Entire system	Individual components
Level	Macro (big picture)	Micro (implementation)
Components	Services, databases, APIs	Classes, interfaces, methods
Questions	”What to build?"	"How to implement?”
Output	Architecture diagrams	Class diagrams, code structure
Skills	System design, scalability	OOP, design patterns, clean code
Example	”Design a URL shortener system"	"Design URLShortener class with methods”

Real-World Example: E-Commerce System

Let’s see how HLD and LLD differ in a real scenario:

High-Level Design (HLD)

HLD Focus:

• What services exist?
• How do they communicate?
• What databases are needed?
• How is the system scaled?

Low-Level Design (LLD)

LLD Focus:

• What classes exist?
• What are their methods?
• How do classes relate?
• What design patterns to use?

Key Takeaway

HLD = “What services and components do we need?”
LLD = “How do we implement each component with classes and code?”

Both are important, but LLD interviews focus on the latter!

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When Do Companies Conduct LLD Interviews?

LLD interviews are typically conducted for:

1. Software Engineer Roles (SDE-1, SDE-2)

Focus: Can you write clean, maintainable code?

What They Test:

• OOP concepts
• Basic design patterns
• Code structure
• Problem-solving approach

2. Senior Software Engineer Roles (SDE-3)

Focus: Can you design robust, scalable components?

What They Test:

• Advanced design patterns
• SOLID principles
• System design within components
• Trade-off analysis

3. Tech Lead / Staff Engineer Roles

Focus: Can you guide team’s code quality?

What They Test:

• Architecture within components
• Code review skills
• Best practices
• Mentoring approach

Interview Format

LLD interviews are usually:

• 45-60 minutes long
• Collaborative - interviewer discusses with you
• Code-focused - you write code or pseudo-code
• Design-focused - you draw diagrams and explain

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What Makes a Good LLD Interview?

Characteristics of a Strong LLD Interview Performance

graph TD
    A[Strong LLD Interview] --> B[Clear Communication]
    A --> C[Systematic Approach]
    A --> D[Clean Design]
    A --> E[Handles Edge Cases]
    
    B --> B1[Explains Thought Process]
    B --> B2[Asks Clarifying Questions]
    B --> B3[Justifies Decisions]
    
    C --> C1[Identifies Actors]
    C --> C2[Identifies Entities]
    C --> C3[Assigns Responsibilities]
    
    D --> D1[SOLID Principles]
    D --> D2[Design Patterns]
    D --> D3[Clean Code]
    
    E --> E1[Error Handling]
    E --> E2[Null Checks]
    E --> E3[Boundary Conditions]

Key Success Factors

1. Clear Communication

   • Explain your thought process
   • Ask clarifying questions
   • Justify design decisions

2. Systematic Approach

   • Don’t jump to code immediately
   • Break down the problem
   • Think about structure first

3. Clean Design

   • Follow SOLID principles
   • Use appropriate design patterns
   • Write maintainable code

4. Handle Edge Cases

   • Think about error scenarios
   • Consider null/empty inputs
   • Handle boundary conditions

Interview Success Formula

Clear Thinking + Clean Design + Good Communication = Strong LLD Interview Performance

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Common LLD Interview Topics

Typical Problem Categories

1. System Design Components

   • Parking Lot System
   • Elevator System
   • ATM System
   • Vending Machine

2. Data Structures

   • Design a HashMap
   • Design a LRU Cache
   • Design a Trie

3. Real-World Applications

   • Design a Library Management System
   • Design a Restaurant Management System
   • Design a Movie Ticket Booking System

4. API Design

   • Design REST APIs for a system
   • Design class interfaces
   • Design method signatures

Preparation Tip

Practice problems from each category! Most LLD interviews follow similar patterns regardless of the specific problem.

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Summary: Understanding LLD Interviews

Key Takeaways

• LLD Interviews focus on code structure, classes, and implementation details
• Companies conduct them to assess OOP skills, problem-solving, and code quality
• Significance - These skills directly impact your day-to-day work
• Difference from HLD - HLD is “what to build”, LLD is “how to implement”
• Success factors - Clear communication, systematic approach, clean design

Visual Summary

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Next Steps

Now that you understand what LLD interviews are, let’s learn how to approach them:

Next: Steps in LLD Interview →

This next guide will walk you through the systematic approach to solving LLD problems, including identifying actors, entities, assigning responsibilities, and more!

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Remember

LLD interviews are not just about writing code - they’re about demonstrating your ability to think systematically, design clean solutions, and communicate effectively. Master these skills, and you’ll excel in any LLD interview!