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🚦 Multithreading Guide with Design Patterns

Multithreading enables concurrent execution of tasks, improving performance and responsiveness, especially on multi-core processors.

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🔄 Basics of Multithreading

• Thread: A lightweight unit of execution within a process.
• Concurrency: Multiple tasks in progress (not necessarily simultaneously).
• Parallelism: Multiple tasks executing simultaneously (on multiple cores).
• Race condition: When threads access shared data without proper synchronization.
• Deadlock: When threads wait on each other indefinitely.
• Thread-safe: Code that functions correctly when accessed by multiple threads concurrently.

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🧰 Key Design Patterns for Multithreading

1. Thread Pool Pattern

Reuses a fixed set of threads for executing tasks instead of spawning a new thread per task. Reduces overhead and increases performance.

// Java
ExecutorService executor = Executors.newFixedThreadPool(5);
executor.submit(() -> { /* task */ });

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2. Producer-Consumer Pattern

A producer creates data and puts it into a buffer; a consumer takes the data. They synchronize via queues.

// Python
import queue, threading

q = queue.Queue()

def producer():
    q.put("data")

def consumer():
    data = q.get()
    print(data)

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3. Future/Promise Pattern

Allows a thread to compute a result in the future, which another thread can retrieve once available.

// Java
Future<String> future = executor.submit(() -> "result");
String result = future.get(); // blocks until ready

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4. Read-Write Lock Pattern

Allows multiple readers or one writer at a time, improving concurrency when reads are more frequent than writes.

// Java
ReadWriteLock lock = new ReentrantReadWriteLock();
lock.readLock().lock();
// reading...
lock.readLock().unlock();

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5. Fork-Join Pattern

Splits tasks into smaller subtasks that are processed in parallel and then joined for final result.

// Java
ForkJoinPool pool = new ForkJoinPool();
pool.invoke(new RecursiveTaskExample());

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6. Active Object Pattern

Encapsulates method execution and its data in a separate thread, decoupling the caller from the execution.

// Caller submits task, the active object runs it on its own thread

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7. Double-Checked Locking

Used for lazy initialization with better performance by minimizing synchronization.

// Java
if (instance == null) {
  synchronized(MyClass.class) {
    if (instance == null) {
      instance = new MyClass();
    }
  }
}

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🧪 Tips for Safe Multithreading

• Always use thread-safe data structures (e.g., ConcurrentHashMap)
• Avoid shared mutable state when possible
• Use immutability and message-passing where applicable
• Watch out for deadlocks and starvation
• Prefer higher-level concurrency abstractions (like ExecutorService, Channel, Coroutine, etc.)

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🔍 Recommended Tools and Libraries

Language	Library / Tool
Java	java.util.concurrent
Python	threading, concurrent.futures, asyncio
Go	Goroutines + Channels
Rust	Tokio, async/await
C#	Tasks, async/await

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