Name: Scaling Node.js with the Cluster Module
Rating: 3.9 (4336 reviews)
Author: sarvabharan

The Cluster module allows Node.js to leverage multi-core systems, improving app performance. Let's explore how to use it effectively.

Why Cluster?

Utilize all CPU cores
Improve app responsiveness
Increase reliability through worker redundancy

Basic Usage

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died`);
  });
} else {
  // Workers can share any TCP connection
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('Hello World\n');
  }).listen(8000);

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

Load Balancing

Node.js handles load balancing automatically using a round-robin approach.

Inter-Process Communication (IPC)

if (cluster.isMaster) {
  const worker = cluster.fork();
  worker.send('Hi there');
} else {
  process.on('message', (msg) => {
    console.log('Message from master:', msg);
  });
}

Zero-Downtime Restarts

if (cluster.isMaster) {
  cluster.on('exit', (worker, code, signal) => {
    if (!worker.exitedAfterDisconnect) {
      console.log('Worker crashed. Starting a new worker');
      cluster.fork();
    }
  });

  process.on('SIGUSR2', () => {
    const workers = Object.values(cluster.workers);
    const restartWorker = (workerIndex) => {
      const worker = workers[workerIndex];
      if (!worker) return;

      worker.on('exit', () => {
        if (!worker.exitedAfterDisconnect) return;
        console.log(`Exited process ${worker.process.pid}`);
        cluster.fork().on('listening', () => {
          restartWorker(workerIndex + 1);
        });
      });

      worker.disconnect();
    };

    restartWorker(0);
  });
}

Best Practices

Use worker_threads for CPU-intensive tasks
Implement proper error handling in workers
Monitor worker health and restart if necessary
Use a process manager like PM2 for production

Pitfalls to Avoid

Sharing server handles explicitly (Node.js does this automatically)
Overusing IPC (can become a bottleneck)
Neglecting to handle worker crashes

Cluster module is powerful for horizontal scaling, but use judiciously. Always profile to ensure it's solving your specific performance needs.

Cheers🥂