What is the Cluster module in Node.js? | Node.js Interview Questions

What You'll Learn

Why clustering is needed
How to implement clustering
Load balancing strategies

Why Clustering?

Node.js runs on a single thread. Cluster module allows you to create child processes (workers) that share the same server port, utilizing all CPU cores.

Basic Cluster Setup

code.jsJavaScript

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 each CPU
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

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

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

Using PM2 (Recommended)

$ terminalBash

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

# Or specify number of instances
pm2 start app.js -i 4

Load Balancing

Node.js uses round-robin approach by default (except Windows).

code.jsJavaScript

// Customize scheduling
cluster.schedulingPolicy = cluster.SCHED_RR; // Round-robin