Cluster Module in Node.js

What is Cluster Module?

The cluster module allows you to create child processes (workers) that share the same server port, enabling you to take advantage of multi-core systems.

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`);
    cluster.fork(); // Replace dead worker
  });
} else {
  // Workers share 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 automatically distributes incoming connections across workers using round-robin.

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

if (cluster.isMaster) {
  const numWorkers = os.cpus().length;
  
  for (let i = 0; i < numWorkers; i++) {
    cluster.fork();
  }
} else {
  const app = express();
  
  app.get('/', (req, res) => {
    res.send(`Handled by worker ${process.pid}`);
  });
  
  app.listen(3000);
}

Worker Communication

if (cluster.isMaster) {
  const worker = cluster.fork();
  
  // Send message to worker
  worker.send('Hello Worker');
  
  // Receive message from worker
  worker.on('message', (msg) => {
    console.log('Master received:', msg);
  });
} else {
  // Receive message from master
  process.on('message', (msg) => {
    console.log('Worker received:', msg);
    
    // Send message to master
    process.send('Hello Master');
  });
}

Graceful Shutdown

if (cluster.isMaster) {
  const workers = [];
  
  for (let i = 0; i < numCPUs; i++) {
    workers.push(cluster.fork());
  }
  
  process.on('SIGTERM', () => {
    workers.forEach(worker => {
      worker.send('shutdown');
      worker.disconnect();
    });
  });
} else {
  const server = http.createServer((req, res) => {
    res.end('OK');
  });
  
  process.on('message', (msg) => {
    if (msg === 'shutdown') {
      server.close(() => {
        process.exit(0);
      });
    }
  });
  
  server.listen(3000);
}

PM2 Alternative

# Install PM2
npm install -g pm2

# Start app with cluster mode
pm2 start app.js -i max

# Monitor
pm2 monit

# Reload without downtime
pm2 reload app

Interview Tips

• Explain cluster: Multi-core utilization
• Show master-worker: Process architecture
• Demonstrate load balancing: Automatic distribution
• Discuss communication: Message passing
• Mention PM2: Production cluster management

Summary

Cluster module creates worker processes to utilize multi-core CPUs. Master process manages workers, distributes load using round-robin. Workers share server ports. Use for CPU-intensive applications or high-traffic servers.