Cracking LLD & Code Design Interviews

(updated with version 1.1)

Note : I plan to keep improving and expanding this book. Existing buyers will receive all future updates for free.

Most engineers fail LLD interviews not because they can't code, but because they can't justify their choices.

Drawing a "Parking Lot" class diagram is easy. Explaining why you put the price calculation in the Ticket class instead of ParkingSpot is what gets you the Senior Engineer offer.

This isn't a pattern memorization guide. It's a framework for thinking.

What's Inside?

• The "Problem-First" Methodology: I don't bore you with verification of the definition of a Singleton. I show you a broken database connection pool and force you to fix it.
• The "holy Trinity" of LLD:
  • 15+ Design Patterns explained via real-world chaos (Payment gateways, Notification systems).
  • 7 Core Foundations: Thread Safety, Audit Trails, Legacy Integration (The stuff seniors know).
  • 10+ Full System Walkthroughs: Parking Lot, Movie Ticket Booking, Rate Limiter, Tic-Tac-Toe.

Why This Book?

Other resources usually teach you what. This teaches you why.

• Don't just build a Chess Game. Learn how to extend it for 3-player variants without rewriting a single class
• Don't just use a Factory Pattern. Learn why new is dangerous when you have 47 payment providers
• Don't just memorize SOLID. Understand when breaking Single Responsibility is the right choice.

A single mock interview costs $150. A failed onsite costs you $20,000+ in sign-on bonuses. This book costs less than a DoorDash order.

Table of Contents

Part I: Design Patterns Mastery

Creational Patterns

• 01 Factory: How do you create objects when the type depends on runtime input?
• 02 Builder: How do you build complex objects step-by-step?
• 03 Singleton: How do you ensure only one instance of a resource exists?

Structural Patterns

• 04 Decorator: How do you add behavior to objects without modifying their code?
• 05 Adapter: How do you integrate incompatible interfaces?
• 06 Proxy: How do you control access to an object without clients knowing?
• 07 Facade: How do you simplify a complex subsystem with a unified interface?

Behavioral Patterns

• 08 Strategy: How do you design a system where business rules change frequently?
• 09 Observer: How do you notify multiple systems when something changes?
• 10 Command: How do you implement undo/redo, task queues, or transaction logs?
• 11 Chain of Responsibility: How do you build middleware or approval workflows?
• 12 Iterator: How do you traverse collections without exposing internals?
• 13 State: How do you handle objects that behave differently based on state?
• 14 Template Method: How do you define an algorithm's structure with customizable steps?
• 15 Memento: How do you save and restore state without violating encapsulation?

Part II: Foundations & Design Thinking

• 16 Thread Safety: How do you design objects that can be safely shared across threads?
• 17 Audit Trail: How do you track every change and make any action reversible?
• 18 Legacy Integration: How do you integrate incompatible systems without polluting your code?
• 19 Multi-Client Support: How do you support multiple clients with different requirements?
• 20 Safe APIs: How do you design APIs that are hard to misuse?
• 21 Evolving Design: How do you design code when requirements are unclear and changing?

Part III: Classic System Design Problems

• 22 How would you design a Parking Lot system?
• 23 How would you design an Elevator system?
• 24 How would you design a Vending Machine?
• 25 How would you design Chess?
• 26 How would you design Snake Game?
• 27 How would you design a Hotel Management System?
• 28 How would you design a Logging Framework?
• 29 How would you design a Rate Limiter?
• 30 How would you design a Movie Ticket Booking System?
• 31 How would you design Tic-Tac-Toe?
• 32 How would you design an LRU Cache?
• 33 How would you design Stack Overflow?
• 34 How would you design a Social Network?
• 35 How would you design a File Storage System (Google Drive)?

Bonus Content