LLD Hub ⚡
OOP → SOLID → Patterns → LLD. Build your design foundation from scratch.
Overall Progress
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0/15 foundations
0/21 LLD problems
Foundation Track
Start here — build the mental models before tackling full LLD systems
🧱OOP Foundations
Classes, encapsulation, abstraction, inheritance, polymorphism, and relationships
Encapsulation: Car – Engine – Wheel
Model a Car that owns an Engine and 4 Wheels. Practice proper encapsulation: fie…
Abstraction: User – Account – Transaction
Design a banking model where User holds multiple Accounts (Savings, Current, Fix…
Composition over Inheritance: Employee Roles
You are given a broken design where Manager extends Employee extends Person — a …
Polymorphism: Payment System
Design a payment system where Cash, CreditCard, and UPI all implement the same P…
Relationships: Order – Item – Payment
Model an e-commerce Order to understand the three types of object relationships:…
⚖️SOLID Principles
SRP, OCP, LSP, ISP, DIP — the 5 principles that make code maintainable
SRP: Fix the Overloaded Logger
You are given a Logger class that does too much: it formats log messages, filter…
OCP: Extend Discounts Without Modifying
A PriceCalculator class uses a long if-else chain to apply discounts: if user is…
LSP: The Square–Rectangle Problem
The classic LSP violation: Square extends Rectangle. Since a square's width and …
ISP: Break the Fat Worker Interface
A Worker interface has four methods: work(), eat(), sleep(), requestLeave(). Hum…
DIP: Decouple the Database Layer
UserService directly instantiates MySQLDatabase inside its constructor: `this.db…
🔮Design Patterns
Factory, Builder, Singleton, Observer, Decorator — patterns that appear in every interview
Factory Pattern: Notification Creator
A NotificationService creates different notification objects based on type: EMAI…
Builder Pattern: SQL Query Builder
Building a SQL query string by concatenating strings leads to bugs and unreadabl…
Singleton Pattern: Thread-Safe Config Manager
Design an AppConfig singleton that loads configuration from environment/file onc…
Observer Pattern: Stock Price Alerts
Design a StockMarket system where multiple observers (mobile app, email alert, d…
Decorator Pattern: Coffee Customisation
Design a coffee ordering system where a base Coffee can be decorated with add-on…
🌱BeginnerL1–L3
0/5 doneParking Lot System
Design a parking lot system that can handle multiple floors, different vehicle t…
Library Management System
Design a library system where members can search, borrow, and return books. Libr…
ATM Machine
Design an ATM system that handles card authentication, balance inquiry, cash wit…
Tic Tac Toe Game
Design a Tic Tac Toe game supporting 2 players (human or AI) on an N×N board.…
Logger / Logging Framework
Design a flexible logging framework that supports multiple log levels, formatter…
🔥IntermediateL4–L6
0/10 doneFood Delivery System (Swiggy/Zomato)
Design a food delivery platform where customers can browse restaurants, place or…
Chat Application (WhatsApp-like)
Design a messaging system supporting 1-on-1 chats, group chats, message status, …
Notification System
Design a notification service that can send alerts via multiple channels based o…
Ride Sharing System (Uber/Ola)
Design a ride sharing platform where riders request rides and drivers accept the…
LRU Cache System
Design an in-memory cache system with LRU eviction policy, TTL support, and thre…
Movie Ticket Booking (BookMyShow)
Design a movie ticket booking platform with seat selection, concurrent booking s…
Inventory Management System
Design an inventory system for an e-commerce warehouse with stock tracking, reor…
Hotel Booking System
Design a hotel room booking platform with availability search, pricing, and rese…
Expense Splitter (Splitwise-like)
Design an app that tracks shared expenses and calculates who owes whom in a grou…
URL Shortener (bit.ly)
Design a URL shortening service that generates short aliases, handles redirects,…
⚡AdvancedL7–L10
0/6 doneElevator System
Design an elevator control system for a building with multiple elevators and flo…
Payment Gateway
Design a payment processing system that handles transactions across multiple pay…
Distributed Job Scheduler
Design a job scheduling system that can queue, execute, and monitor background j…
Social Media Feed (Twitter/Instagram)
Design a social media platform with posts, follows, and a personalized news feed…
Rate Limiter
Design a rate limiting service that restricts request rates per user/IP using mu…
Event Ticketing System
Design a large-scale event ticketing platform (like Ticketmaster) with high conc…
Built for engineers who want to think, not memorize.