Low-Level Design (LLD) — Complete Preparation Guide

📌 Table of Contents

• What is Low-Level Design?
• Why LLD Is Important
• How to Prepare for LLD Interviews
• LLD Concepts You Must Know
• Common LLD Interview Problems
• Free Resources (Websites + Docs)
• Best YouTube Channels
• Practice Platforms
• Preparation Roadmap

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🧩 What is Low-Level Design?

Low-Level Design (LLD) focuses on detailed class structures, interactions, and internal component architecture.

While HLD tells what the system contains,
LLD tells how each component works internally.

LLD includes:

• Class diagrams
• Sequence diagrams
• Interfaces
• Inheritance & composition
• Object modeling
• Applying design patterns

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⭐ Why LLD Is Important

• Shows your ability to convert requirements → code-ready structure
• Ensures scalable and maintainable software
• Highly tested in companies like Microsoft, Amazon, Uber, Swiggy, Atlassian

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🏗 How to Prepare for LLD Interviews

1. Master OOP fundamentals
2. Understand SOLID & design principles
3. Learn 8–12 core design patterns
4. Practice real LLD problems
5. Draw UML class diagrams
6. Study GitHub repositories
7. Watch LLD breakdowns on YouTube

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📚 LLD Concepts You Must Know

1. Core OOP

• Abstraction
• Encapsulation
• Inheritance
• Polymorphism

2. Design Principles

• SOLID
• DRY
• KISS
• YAGNI
• Composition > Inheritance

3. Design Patterns

Most commonly asked:

• Singleton
• Factory + Abstract Factory
• Strategy
• Observer
• Decorator
• Adapter
• Proxy
• Builder

4. UML

• Class diagrams
• Sequence diagrams
• Activity diagrams

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📝 Common LLD Interview Problems

• Parking Lot System
• Elevator System
• Restaurant / Food Delivery App
• Hotel Management System
• Library Management
• Splitwise
• Uber / Ola Ride Booking
• Snake and Ladder
• Chess / Tic Tac Toe
• Notification Service
• Logging Framework
• Rate Limiter (Token Bucket / Leaky Bucket)

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🌐 Free Resources (Websites & Documentation)

Websites

• System Design Primer (GitHub):
  https://github.com/donnemartin/system-design-primer
• Refactoring Guru (Design Patterns):
  https://refactoring.guru/design-patterns
• UML Documentation (Microsoft):
  https://learn.microsoft.com/en-us/visualstudio/modeling/uml-diagrams
• GFG LLD Section:
  https://www.geeksforgeeks.org/low-level-design/
• SourceMaking Patterns:
  https://sourcemaking.com/design_patterns

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🎥 Best YouTube Channels for LLD

• Sudhanshu LLD/HLD
  https://www.youtube.com/@sudhanshuchannel
• Gaurav Sen
  https://www.youtube.com/@gkcs
• System Design Interview
  https://www.youtube.com/@systemdesigntutorial
• Tech Dummies Narendra L
  https://www.youtube.com/@TechDummiesNarendraL
• CodeKarle
  https://www.youtube.com/@CodeKarle

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💻 Practice Platforms

• InterviewReady (LLD Problems)
  https://interviewready.io
• Grokking Low-Level Design (Paid)
  https://www.designgurus.io
• LeetCode OOD Questions
  https://leetcode.com/problemset/all/?topicSlugs=object-oriented

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🗺 Preparation Roadmap

Week 1 — OOP + Design Principles

• Master OOP
• Learn SOLID
• Draw UML diagrams

Week 2 — Design Patterns

• Study & implement 8–12 patterns

Week 3 — Full LLD Case Studies

• Parking Lot
• Elevator
• Library
• Ride Booking

Week 4 — Mock Interviews

• Solve 1 problem/day
• Explain thought process
• Focus on extensibility

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