12. Command Pattern: Undo/Redo Engine & Task Queue Serialization • Low-Level Design (LLD) & Object-Oriented Machine Coding • Rishabh Ki Kaksha

Low-Level Design (LLD) & Object-Oriented Machine Coding › 12. Command Pattern: Undo/Redo Engine & Task Queue Serialization

45 mins

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Course Syllabus

Module 1: Object-Oriented Principles, UML & SOLID Architecture

01. SOLID Principles Deep Dive with Real-World Refactoring Examples 02. UML Class Diagrams & Sequence Diagrams for Big Tech Machine Coding Rounds 03. Composition over Inheritance & The Open-Closed Architectural Principle

Module 2: Creational Design Patterns in Production

04. Thread-Safe Singleton Pattern (Double-Checked Locking & Bill Pugh Implementation) 05. Factory Method vs Abstract Factory Pattern for Extensible Plug-and-Play Systems 06. Builder Pattern with Method Chaining & Immutable Data Object Guarantees

Module 3: Structural Design Patterns & Object Composition

07. Adapter & Facade Patterns: Wrapping Legacy APIs and Third-Party SDKs 08. Decorator Pattern: Dynamic Behavior Wrapping (Logging, Caching, Encryption) 09. Proxy Pattern (Virtual, Protection & Remote Proxies) with Caching Layers

Module 4: Behavioral Design Patterns & Event Decoupling

10. Strategy Pattern: Pluggable Pricing, Payment Gateways & Routing Algorithms 11. Observer / PubSub Pattern: Decoupled Event Emitters & State Sync 12. Command Pattern: Undo/Redo Engine & Task Queue Serialization 13. State Pattern: Order Lifecycle Management & Finite State Machines (FSM)

Module 5: Concurrency, Thread Pools & Lock-Free Design

14. Multi-Threaded Producer-Consumer Architecture with BlockingQueues 15. Thread Pool Executor Implementation & Task Scheduling Engine 16. Read-Write Locks, Mutexes & Optimistic Concurrency Control in Memory

Module 6: FAANG Machine Coding Live Case Studies

17. Machine Coding Case Study 1: Multi-Level Parking Lot System (Vehicles, Slots, Payments) 18. Machine Coding Case Study 2: High-Concurrency Elevator Management System 19. Machine Coding Case Study 3: Rate Limiter (Token Bucket, Leaky Bucket, Sliding Window) 20. Machine Coding Case Study 4: Splitwise / Expense Sharing App with Graph Simplification

Interactive Lesson Studio

# 12. Command Pattern: Undo/Redo Engine & Task Queue Serialization

> Low-Level Design (LLD) Notes by Rishabh - Topic 12

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### 1. Command Pattern: Encapsulating Requests as Objects 📜

Request ke action (`execute()`) aur reverse action (`undo()`) ko ek standalone object ke andar pack kar dena:

```typescript
interface ICommand {
    execute(): void;
    undo(): void;
}

class TextEditor {
    private text: string = "";
    public write(str: string) { this.text += str; }
    public erase(length: number) { this.text = this.text.slice(0, -length); }
    public getText() { return this.text; }
}

class WriteCommand implements ICommand {
    constructor(private editor: TextEditor, private textToAppend: string) {}

execute() { this.editor.write(this.textToAppend); }
    undo() { this.editor.erase(this.textToAppend.length); }
}

// Invoker with History Stack:
class CommandHistory {
    private undoStack: ICommand[] = [];
    private redoStack: ICommand[] = [];

executeCommand(cmd: ICommand) {
        cmd.execute();
        this.undoStack.push(cmd);
        this.redoStack = []; // Clear redo on new command
    }

undo() {
        const cmd = this.undoStack.pop();
        if (cmd) {
            cmd.undo();
            this.redoStack.push(cmd);
        }
    }

redo() {
        const cmd = this.redoStack.pop();
        if (cmd) {
            cmd.execute();
            this.undoStack.push(cmd);
        }
    }
}
```

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### 2. Master Comparison Matrix

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### 3. Interview Ke 3 Sawal Is Topic Se

1. **Memento Pattern aur Command Pattern me kya difference hai?** -> Command pattern state changes ko revert karne ke liye inverse operations execute karta hai; Memento pattern state ka snapshot capture karke direct restore karta hai.
2. **Transactional Sagas me Command Pattern kaise use hota hai?** -> Distributed transactions me Compensating Transactions (Undo commands) execute karne ke liye.
3. **Command objects ko serialize karke disk par store karne ka kya fayda hota hai?** -> Server restart hone par crash recovery aur audit log replay capability milti hai.

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**Next Topic:** 13. State Pattern: Order Lifecycle Management & Finite State Machines (FSM)

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