Load Balancing with Nginx

Introduction

Node.js is single-threaded by nature. If you deploy an Express application on a multi-core server (like an 8-core machine), by default, it will only use a single core, leaving the other 7 cores completely idle!

To utilize all available CPU resources and handle high traffic, you must run multiple instances of your Express application and load balance the traffic across them.

1. The Native cluster Module

Node.js provides a built-in cluster module that allows you to spawn multiple child processes (workers) that share the same server port.

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

if (cluster.isMaster) {
  // Get the number of CPUs available
  const numCPUs = os.cpus().length;

  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. Restarting...`);
    cluster.fork(); // Automatically restart dead workers
  });
} else {
  // Workers can share any TCP connection
  const app = express();
  
  app.get('/', (req, res) => {
    res.send(`Handled by worker \${process.pid}`);
  });

  app.listen(3000, () => {
    console.log(`Worker \${process.pid} started`);
  });
}

2. Using PM2 for Load Balancing

Instead of writing cluster code manually, it is highly recommended to use PM2 in production. PM2 has a built-in cluster mode that does exactly what the code above does, but without modifying your application logic.

# Start your app across all available CPU cores
pm2 start app.js -i max

3. Reverse Proxies (Nginx)

If you have multiple physical servers (not just multiple cores on one server), you will need a Reverse Proxy like Nginx or HAProxy to distribute HTTP traffic across your fleet of servers. Nginx acts as the entry point and forwards requests to your internal Express instances using algorithms like Round Robin or Least Connections.

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