15. Thread Pool Executor Implementation & Task Scheduling Engine • Low-Level Design (LLD) & Object-Oriented Machine Coding • Rishabh Ki Kaksha

Low-Level Design (LLD) & Object-Oriented Machine Coding › 15. Thread Pool Executor Implementation & Task Scheduling Engine

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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

# 15. Thread Pool Executor Implementation & Task Scheduling Engine

> Low-Level Design (LLD) Notes by Rishabh - Topic 15

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### 1. Why Thread Pools Are Mandatory ⚠️

Har task ke liye naya OS Thread banana expensive hai (~1MB stack + OS context switch overhead).  
**Thread Pool** fixed number of reusable worker threads maintain karta hai jo queue se tasks pick karte hain.

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### 2. ThreadPoolExecutor Parameters (Big Tech Deep Dive ⚙️)

```
[ Core Pool (Threads: 5) ] ➔ If full ➔ [ Work Queue (Capacity: 100) ] ➔ If full ➔ [ Max Pool (Threads: 20) ] ➔ If full ➔ [ Rejection Handler ]
```

1. **`corePoolSize`:** Minimum active worker threads.
2. **`maxPoolSize`:** Maximum threads allowed during traffic spikes.
3. **`keepAliveTime`:** Idle threads ko kill karne ka timeout.
4. **`workQueue`:** Pending tasks ka blocking buffer.
5. **`RejectedExecutionHandler`:** Queue overflow hone par action (`AbortPolicy`, `CallerRunsPolicy`, `DiscardOldestPolicy`).

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### 3. Master Comparison Matrix

| Rejection Policy | Behavior on Queue Overflow |
| :--- | :--- |
| **`AbortPolicy` (Default)** | Throws `RejectedExecutionException` |
| **`CallerRunsPolicy`** | Calling thread (e.g. Main thread) itself executes the task (Throttles input!) |
| **`DiscardPolicy`** | Silently drops the task |
| **`DiscardOldestPolicy`** | Drops the oldest unhandled task from queue and retries |

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

1. **CPU-bound vs I/O-bound tasks ke liye Thread Pool Size ka formula kya hota hai?** -> CPU-bound: `Number of Cores + 1`; I/O-bound: `Number of Cores * (1 + Wait Time / Compute Time)`.
2. **`Executors.newFixedThreadPool()` production me use kyu nahi karna chahiye?** -> Kyunki ye unbounded `LinkedBlockingQueue` use karta hai jo memory overflow (`OutOfMemoryError`) create kar sakti hai.
3. **Graceful shutdown me `shutdown()` aur `shutdownNow()` me kya difference hai?** -> `shutdown()` naye tasks accept karna band karta hai aur existing tasks complete hone deta hai; `shutdownNow()` running threads ko interrupt karta hai.

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**Next Topic:** 16. Read-Write Locks, Mutexes & Optimistic Concurrency Control in Memory

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