How to Scale Node.js Applications Using the Cluster Module

Tung NguyenSoftware Engineer at Autograb

Jan 11, 2026

In this article, I will walk you through the easiest way to scale up your Node.js application to serve more requests: using the Cluster module.

Node.js runs on a single thread, which means that by default it uses only one CPU core. When your application receives a large number of requests or executes long-running, CPU-intensive tasks, the event loop can become blocked. As a result, the entire application may appear unresponsive—even though the server still has multiple idle CPU cores available.

But what happens if your server has many CPU cores? How can you take advantage of all of them?

The Problem: The Single-Threaded Bottleneck

Let's look at a practical example. Below is a simple Node.js application with two endpoints:

/fib: Calculates the 50th Fibonacci number. This is a CPU-bound, blocking task.
/info: Returns basic server information. This is a lightweight task.

const express = require("express");
const os = require("os");

const app = express();
const PORT = 3000;

// Simple recursive Fibonacci (not optimized for large numbers)
// This simulates a heavy CPU task
function fib(n) {
  if (n <= 1) return n;
  return fib(n - 1) + fib(n - 2);
}

// Endpoint to return the 50th Fibonacci number
app.get("/fib", (req, res) => {
  const start = process.hrtime();
  const result = fib(50); // This blocks the event loop!
  const end = process.hrtime(start);
  const executionTime = (end[0] * 1000 + end[1] / 1000000).toFixed(2); // Convert to milliseconds

  res.json({
    fibonacciNumber: result,
    processingTime: `${executionTime}ms`,
    processId: process.pid,
  });
});

// Endpoint to return server info
app.get("/info", (req, res) => {
  res.json({
    name: os.hostname(),
    numberOfCpus: os.cpus().length,
    processId: process.pid,
  });
});

// Start the server
app.listen(PORT, () => {
  console.log(`Server is running on http://localhost:${PORT}`);
});