📋 Executive Summary

Document: Low-Level Design (LLD)
Type: Technical Documentation
Reading Time: ~20 min
Last Updated: December 2025

📊 Quick Stats

🎯 Main Topics Covered

1. LLD Fundamentals — What is LLD, when it’s used, interview expectations
2. Class Design — Entities, attributes, methods, relationships
3. SOLID Principles — Applying in LLD interviews
4. Design Patterns — Factory, Singleton, Observer, Strategy, State
5. UML Diagrams — Class diagrams, sequence diagrams
6. Object Relationships — Association, aggregation, composition
7. Interface Design — Public APIs, contracts, abstractions
8. Common Systems — Parking lot, elevator, chess, ATM, library
9. Code Organization — Package structure, modularity
10. Testing Considerations — Testability, mocking, dependency injection

💡 What You’ll Learn

• Convert problem statements into class diagrams and code
• Apply SOLID principles to design maintainable systems
• Recognize which design patterns solve specific problems
• Design extensible systems (easy to add new features)
• Handle requirements like concurrency and edge cases
• Communicate design decisions with UML diagrams
• Balance between over-engineering and under-engineering
• Write clean, production-ready object-oriented code
• Excel in LLD rounds of tech interviews (Amazon, Microsoft, etc.)

📚 Prerequisites

• Strong OOP fundamentals (classes, inheritance, polymorphism)
• Experience with object-oriented languages (Java/C++/Python)
• Understanding of SOLID principles
• Familiarity with basic design patterns
• Ability to read UML class diagrams

👥 Target Audience

✅ Interview Candidates — Preparing for LLD rounds (SDE-2/SDE-3)
✅ Backend Engineers — Designing clean, maintainable codebases
✅ Junior Developers — Learning professional coding practices
✅ Code Reviewers — Evaluating design quality
✅ Tech Leads — Setting team design standards

🎓 Learning Path

Beginner → Practice 3-5 simple systems (parking lot, library)
Intermediate → Add complexity (chess, elevator, ride-sharing)
Advanced → Design with concurrency, scalability constraints

🔑 LLD Interview Template

Standard Approach:

1. Clarify requirements (features, constraints)
2. Identify core entities/actors
3. Define relationships and responsibilities
4. Draw class diagram with key methods
5. Code critical classes
6. Discuss extensibility and trade-offs

🎯 Top LLD Problems

1. Parking Lot System (most common!)
2. Elevator/Lift System
3. Library Management System
4. ATM Machine
5. Chess Game
6. Ride-Sharing (Uber/Lyft)
7. Hotel Booking System
8. File System Design
9. Notification Service
10. Rate Limiter

Low-Level Design

Intro, core concepts, and practical examples.