Cluster

TL;DR

The cluster module allows you to easily create child processes (workers) that run simultaneously and share the same server port. This is the built-in way to scale a single-threaded Node.js web server to utilize all available CPU cores.

Mental Model

How It Works

1. A Master process is started. Its only job is to manage the workers.
2. The Master uses cluster.fork() to spawn multiple Worker processes (usually one per CPU core).
3. All workers share the same TCP port (e.g., 3000).
4. When requests arrive, the Master process distributes them among the workers using a round-robin algorithm.

Example

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

const numCPUs = os.cpus().length;

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

  // Fork a worker for each CPU core
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  // Replace dead workers
  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died. Restarting...`);
    cluster.fork();
  });
} else {
  // Workers share the TCP connection in this block
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Hello from Worker ${process.pid}\n`);
  }).listen(8000);

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

Common Interview Questions

Do cluster workers share memory?

No. Each worker is a completely separate OS process with its own V8 instance and its own memory. If Worker A updates a local variable, Worker B will not see the change. You must use a database like Redis to share state (like sessions) between workers.

Should I use the Cluster module in production?

Often, no. In modern cloud infrastructure, it is usually better to deploy multiple single-threaded Node.js containers (via Docker/Kubernetes) and let the infrastructure load balancer (like AWS ALB or NGINX) handle the traffic distribution, rather than managing processes manually with the cluster module.