16. Read-Write Locks, Mutexes & Optimistic Concurrency Control in Memory • Low-Level Design (LLD) & Object-Oriented Machine Coding • Rishabh Ki Kaksha

Low-Level Design (LLD) & Object-Oriented Machine Coding › 16. Read-Write Locks, Mutexes & Optimistic Concurrency Control in Memory

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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

# 16. Read-Write Locks, Mutexes & Optimistic Concurrency Control in Memory

> Low-Level Design (LLD) Notes by Rishabh - Topic 16

---

### 1. Read-Heavy Concurrency (ReadWriteLock 📚)

Agar 99% threads sirf data read kar rahe hain aur 1% write:
*   Standard Mutex saare reads ko bhi block kar deta hai (Slow!).
*   **`ReadWriteLock`:** Multiple concurrent **Readers** ko parallel allow karta hai, lekin **Writer** ko exclusive lock deta hai!

```
[ Reader 1 ] [ Reader 2 ] [ Reader 3 ] ➔ (Parallel Read Allowed!)
                    ❌ [ Writer ] ➔ (Blocks all Readers & Writers!)
```

---

### 2. Optimistic Concurrency Control (CAS - Compare-And-Swap ⚡)

Bina locks ke high-performance concurrent updates karna using Version / Timestamp:

```typescript
class ConcurrentAccount {
    private balance: number = 1000;
    private version: number = 1;

public updateBalance(amount: number, expectedVersion: number): boolean {
        if (this.version !== expectedVersion) {
            return false; // 💥 Optimistic Lock Conflict! Retry needed.
        }
        this.balance += amount;
        this.version++;
        return true;
    }
}
```

---

### 3. Master Comparison Matrix

| Concurrency Mechanism | Read Performance | Write Performance | Contention Handling |
| :--- | :--- | :--- | :--- |
| **Mutex / Synchronized** | Serialized (Slow) | Serialized | Blocks threads |
| **Read-Write Lock** | ⚡ **Blazing Fast (Shared)** | Serialized (Exclusive) | Blocks on write |
| **Optimistic (CAS / Version)**| ⚡ **Zero-Lock Speed** | Fast on low contention | Retries on conflict |

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

1. **ReadWriteLock me Writer Starvation kaise hota hai?** -> Agar continuous naye readers aate rahein, toh writer thread ko kabhi exclusive lock nahi milega (Fair lock policy `ReentrantReadWriteLock(true)` fixes this).
2. **ABA Problem in Lock-Free Data Structures kya hoti hai?** -> Value $A \rightarrow B \rightarrow A$ change ho jaati hai jisse normal CAS ko lagta hai ki koi change nahi hua (`AtomicStampedReference` with version solves this).
3. **Deadlock ke 4 Coffman conditions kya hote hain?** -> Mutual Exclusion, Hold and Wait, No Preemption, Circular Wait.

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**Next Topic:** 17. Machine Coding Case Study 1: Multi-Level Parking Lot System (Vehicles, Slots, Payments)

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