🎯 Low Level Design (LLD) - Complete Implementation Collection

A comprehensive collection of 25+ Low Level Design implementations demonstrating advanced design patterns, clean architecture, SOLID principles, and enterprise-level software engineering practices.

📋 Table of Contents

Overview
What is Low Level Design?
Projects by Difficulty
Design Patterns Covered
Architecture Principles
Getting Started
Project Structure
Learning Path
Contributing

🎯 Overview

This repository contains complete, production-ready implementations of popular Low Level Design problems commonly asked in software engineering interviews.

Each project demonstrates:

✅ Clean Architecture - Separation of concerns, layered architecture
✅ Design Patterns - Creational, Structural, and Behavioral patterns
✅ SOLID Principles - Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion
✅ Thread Safety - Concurrent operations with proper locking
✅ Type Safety - Complete type hints throughout
✅ Comprehensive Documentation - Detailed README files for each project
✅ Working Demos - Executable demo scripts showcasing all features

🎓 What is Low Level Design?

Low Level Design (LLD) focuses on the detailed design of individual components, classes, and their interactions. It bridges the gap between High Level Design (system architecture) and actual code implementation.

Key Aspects of LLD:

Class Design - Defining classes, their attributes, and methods
Design Patterns - Applying appropriate patterns to solve problems
SOLID Principles - Writing maintainable, extensible code
Data Structures - Choosing the right data structures for efficiency
API Design - Designing clean, intuitive interfaces
Error Handling - Robust exception handling and validation
Thread Safety - Handling concurrent operations correctly

📊 Projects by Difficulty

🟢 Easy Level Projects

Perfect for beginners to understand basic OOP concepts and simple design patterns.

Project	Description	Key Patterns	README
Parking Lot	Design a parking lot system with multiple parking spots, vehicle types, and payment	Strategy, Factory	README
Coffee Vending Machine	Design a vending machine with inventory, payment, and dispensing	State, Observer, Factory	README
Logging Framework	Design a logging system with multiple log levels and handlers	Chain of Responsibility, Strategy	README
Traffic Signal System	Design a traffic signal control system with multiple intersections	State, Strategy, Observer	README
Tic Tac Toe	Design a Tic Tac Toe game with player management and win detection	State, Observer	README
Task Management System	Design a task management system with states, priorities, and assignments	State, Observer, Strategy, Builder, Repository	README

🟡 Medium Level Projects

Intermediate projects covering multiple design patterns and complex business logic.

Project	Description	Key Patterns	README
ATM System	Design an ATM with card authentication, transaction processing, and cash dispensing	State, Command, Adapter, Factory	README
Library Management	Design a library system with book lending, reservations, and fine calculation	Strategy, Factory, Observer, Repository	README
Restaurant Management	Design a restaurant system with table management, orders, and billing	State, Command, Decorator, Observer	README
Elevator System	Design an elevator system with floor requests, direction control, and scheduling	State, Strategy, Observer, Repository	README
Pub-Sub System	Design a publish-subscribe messaging system with topics and subscribers	Strategy, Factory, Observer, Repository	README
Social Networking	Design a social network with friend requests, posts, and news feed	Observer, Strategy, State, Repository	README
Online Auction	Design an auction system with bidding, time-based closing, and notifications	State, Command, Mediator, Chain of Responsibility, Decorator	README
Airline Management	Design an airline system with flight booking, seat selection, and pricing	State, Strategy, Observer, Decorator, Repository	README
Online Stock Exchange	Design a stock trading system with order matching and portfolio management	Observer, Strategy	README

🔴 Hard Level Projects

Advanced projects with complex business logic, multiple subsystems, and sophisticated design patterns.

Project	Description	Key Patterns	README
Movie Ticket Booking	Design a movie ticket booking system with seat selection, payment, and show management	Strategy, Observer, Repository	README
Splitwise	Design an expense splitting system with debt simplification and balance tracking	Strategy, Observer, Builder, Facade	README
Snake and Ladder	Design a board game with dice rolling, player movement, and win conditions	State, Observer	README
Online Shopping	Design an e-commerce system with cart, checkout, payment, and inventory	Strategy, Observer, Factory	README
Ride Sharing (Uber)	Design a ride-sharing system with driver matching, pricing, and trip management	Strategy, Observer, Decorator	README
CricInfo	Design a cricket information system with match tracking, score updates, and statistics	Builder, Observer, State	README
Spotify	Design a music streaming service with playlists, recommendations, and playback	Strategy, Observer	README
UPI Payment System	Design a UPI payment system with multiple banks, transaction processing, and security	Abstract Factory, Adapter, Chain of Responsibility, Command, Decorator, Proxy	README

🎨 Design Patterns Covered

Creational Patterns

✅ Singleton - Single instance management (TaskManager, Repositories)
✅ Factory - Object creation (Payment factories, Vehicle factories)
✅ Abstract Factory - Families of related objects (UPI payment processors)
✅ Builder - Complex object construction (Task, Expense, Match)

Structural Patterns

✅ Adapter - Interface adaptation (Payment adapters, Bank adapters)
✅ Decorator - Dynamic behavior extension (Price decorators, Feature decorators)
✅ Proxy - Access control and lazy loading (Payment proxies)
✅ Facade - Simplified interface to complex subsystems (Service facades)

Behavioral Patterns

✅ Observer - Event-driven notifications (Task updates, Expense notifications)
✅ Strategy - Algorithm selection (Search strategies, Split strategies, Pricing strategies)
✅ State - State-based behavior (Task states, Payment states, Game states)
✅ Command - Encapsulate requests (ATM commands, Restaurant orders)
✅ Chain of Responsibility - Request processing chain (Payment processing, Logging)
✅ Mediator - Centralized communication (Auction mediator)
✅ Repository - Data access abstraction (Task repository, User repository)

🏗️ Architecture Principles

SOLID Principles

All projects follow SOLID principles:

Single Responsibility - Each class has one reason to change
Open/Closed - Open for extension, closed for modification
Liskov Substitution - Subtypes must be substitutable for their base types
Interface Segregation - Clients shouldn't depend on interfaces they don't use
Dependency Inversion - Depend on abstractions, not concretions

Clean Architecture

Projects follow layered architecture:

┌─────────────────────────────────┐
│      Presentation/Demo Layer    │
│      (demo.py, run.py)          │
└─────────────────────────────────┘
              │
┌─────────────────────────────────┐
│         Service Layer           │
│    (Business Logic)             │
└─────────────────────────────────┘
              │
┌─────────────────────────────────┐
│      Repository Layer           │
│    (Data Access)                │
└─────────────────────────────────┘
              │
┌─────────────────────────────────┐
│        Domain Layer             │
│    (Models, Entities)           │
└─────────────────────────────────┘
              │
┌─────────────────────────────────┐
│    Strategy/Pattern Layer       │
│    (Design Patterns)            │
└─────────────────────────────────┘

🚀 Getting Started

Prerequisites

Python 3.8+ - All projects are implemented in Python
Basic OOP Knowledge - Understanding of classes, inheritance, polymorphism
Design Patterns - Familiarity with common design patterns (optional, you'll learn as you go)

Quick Start

Clone the repository

git clone <repository-url>
cd low_level_design

Navigate to a project

cd taskManagementSystem  # or any other project

Run the demo

python3 demo.py  # or run.py depending on the project

Read the README Each project has a comprehensive README explaining:

Architecture and design decisions
Design patterns used
How to run and use the system
Code examples

Project Structure

Each project follows a consistent structure:

projectName/
├── app/
│   ├── models/          # Domain entities
│   ├── services/        # Business logic
│   ├── repositories/     # Data access
│   ├── strategies/      # Strategy pattern implementations
│   ├── states/          # State pattern implementations
│   ├── observers/       # Observer pattern implementations
│   ├── builders/        # Builder pattern implementations
│   ├── factories/       # Factory pattern implementations
│   ├── commands/        # Command pattern implementations
│   ├── decorators/      # Decorator pattern implementations
│   └── ...
├── demo.py              # Demo script
├── run.py               # Alternative entry point
└── README.md            # Project documentation

📚 Learning Path (For LLD)

1.1 Start with Easy Projects

Parking Lot
Coffee Vending Machine
Tic Tac Toe
Task Management System

1.2 Learn Basic Patterns

Singleton
Factory
Strategy
Observer

1.3 Understand SOLID Principles

Read code comments
See how principles are applied
Try modifying code to see why design matters

2.1. Tackle Medium Projects

ATM System
Library Management
Elevator System
Pub-Sub System

2.2. Explore Advanced Patterns

State Pattern
Command Pattern
Chain of Responsibility
Repository Pattern

2.3. Study Architecture

Clean Architecture layers
Dependency Injection
Service Layer patterns

3.1. Master Hard Projects

Movie Ticket Booking
Splitwise
UPI Payment System
Ride Sharing Service

3.2. Deep Dive into Complex Patterns

Abstract Factory
Mediator
Multiple pattern combinations

3.3. Optimize and Extend

Add new features
Optimize performance
Add thread safety
Implement persistence

🎯 Key Learning Outcomes

After going through these projects, you will:

✅ Master Design Patterns - Understand when and how to apply 15+ design patterns
✅ Write Clean Code - Follow SOLID principles and clean architecture
✅ Design Scalable Systems - Build systems that can grow and evolve
✅ Handle Concurrency - Implement thread-safe operations
✅ Think Like a Senior Engineer - Make design decisions considering trade-offs
✅ Ace Interviews - Be prepared for LLD interviews at top tech companies

📖 How to Use This Repository

1. Study Approach

Start with easy projects
Read the README first
Run the demo to see it in action
Study the code structure
Understand design decisions

2. Practice Approach

Try to implement a project yourself first
Compare with the solution
Understand the differences
Learn from the design choices

3. Interview Preparation

Pick a problem statement
Design the solution
Implement it
Compare with the solution here
Understand trade-offs

🔍 Project Highlights

Most Comprehensive Projects

UPI Payment System - 10+ design patterns, complex payment processing
Task Management System - Complete state management, search strategies, observer pattern
Splitwise - Advanced debt simplification algorithm, multiple split strategies
Online Auction System - Complex bidding logic, time-based events, mediator pattern

Best for Learning Specific Patterns

Observer Pattern: Task Management, Splitwise, Social Networking
State Pattern: ATM, Elevator, Task Management, Traffic Signal
Strategy Pattern: Parking Lot, Pub-Sub, Search systems
Builder Pattern: Task Management, Splitwise, CricInfo
Command Pattern: ATM, Restaurant Management, Online Auction
Chain of Responsibility: UPI Payment, Logging Framework

🤝 Contributing

Contributions are welcome! Here's how you can help:

Add New Projects - Implement new LLD problems
Improve Documentation - Enhance README files
Fix Bugs - Report and fix issues
Add Features - Extend existing projects
Code Reviews - Review and suggest improvements

Contribution Guidelines

Follow the existing code structure
Add comprehensive documentation
Include working demo scripts
Follow SOLID principles
Add type hints
Write clean, readable code

🎓 Resources

Online Resources

Refactoring Guru - Excellent design pattern explanations
SOLID Principles

📊 Statistics

25+ Projects - Complete implementations
15+ Design Patterns - Creational, Structural, Behavioral
1000+ Files - Well-structured, documented code
SOLID Principles - Applied throughout all projects
Thread Safety - Concurrent operations handled correctly

🏆 Success Stories

This repository helps you:

✅ Write Better Code following industry best practices
✅ Understand Design Patterns through real-world implementations
✅ Build Scalable Systems with clean architecture
✅ Think Like a Senior Engineer making design decisions

📞 Support

For questions, issues, or contributions:

Create an issue on GitHub
Review individual project READMEs
Study the code and documentation

🌟 Star the Repository

If you find this repository helpful, please ⭐ star it to help others discover it!

Made with ❤️ to help developers master Low Level Design and ace their interviews!

Happy Learning! 🚀

Name		Name	Last commit message	Last commit date
Latest commit History 93 Commits
OnlineStockExchange		OnlineStockExchange
RideSharingService		RideSharingService
SnakeAndLadderGame		SnakeAndLadderGame
Spotify		Spotify
airlineManagementService		airlineManagementService
atm		atm
coffeeVendingMachine		coffeeVendingMachine
cricinfo		cricinfo
elevatorSystem		elevatorSystem
interview_questions		interview_questions
libraryManagementService		libraryManagementService
loggingFramework		loggingFramework
movieTicketBookingService		movieTicketBookingService
onlineAuctionSystem		onlineAuctionSystem
onlineShoppingService		onlineShoppingService
parkingLot		parkingLot
pubSubService		pubSubService
restrauntService		restrauntService
socialNetworkingService		socialNetworkingService
splitwise		splitwise
taskManagementSystem		taskManagementSystem
ticTacToe		ticTacToe
trafficSignalSystem		trafficSignalSystem
upi		upi
.gitignore		.gitignore
README.md		README.md

Folders and files

Latest commit

History

Repository files navigation

🎯 Low Level Design (LLD) - Complete Implementation Collection

📋 Table of Contents

🎯 Overview

🎓 What is Low Level Design?

Key Aspects of LLD:

📊 Projects by Difficulty

🟢 Easy Level Projects

🟡 Medium Level Projects

🔴 Hard Level Projects

🎨 Design Patterns Covered

Creational Patterns

Structural Patterns

Behavioral Patterns

🏗️ Architecture Principles

SOLID Principles

Clean Architecture

🚀 Getting Started

Prerequisites

Quick Start

Project Structure

📚 Learning Path (For LLD)

🎯 Key Learning Outcomes

📖 How to Use This Repository

1. Study Approach

2. Practice Approach

3. Interview Preparation

🔍 Project Highlights

Most Comprehensive Projects

Best for Learning Specific Patterns

🤝 Contributing

Contribution Guidelines

🎓 Resources

Recommended Reading

Online Resources

📊 Statistics

🏆 Success Stories

📞 Support

🌟 Star the Repository

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