What is clustering in Node.js and why is it used?

Clustering in Node.js is a technique used to create multiple instances of a Node.js application that can run on different CPU cores. This is important because Node.js runs in a single thread by default, meaning it can only utilize one CPU core—even on multi-core systems.

🔧 Why Clustering Is Used

1. To improve performance and scalability on multi-core machines.

2. To handle more concurrent requests by distributing load.

3. To provide basic fault tolerance—if one worker crashes, others continue running.

4. To avoid blocking the event loop with heavy traffic.

🧠 How It Works

• Uses the built-in cluster module.

• The master process spawns worker processes (child processes).

• Each worker runs its own event loop and can handle requests independently.

• Workers can share server ports (e.g., http.createServer) via the master process.

🧪 Example

const cluster = require('cluster');
const http = require('http');
const os = require('os');

if (cluster.isMaster) {
  const numCPUs = os.cpus().length;
  console.log(`Master process PID: ${process.pid}`);
  
  // Fork workers
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  // Optional: listen for dying workers
  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died, starting a new one`);
    cluster.fork();
  });

} else {
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Handled by worker: ${process.pid}\n`);
  }).listen(3000);

  console.log(`Worker started: ${process.pid}`);
}

⚠️ Notes

• Workers do not share memory; they run in separate processes.

• Use inter-process communication (IPC) or tools like Redis for shared data/state.