Node.js Cluster and Scaling

What is Cluster and Scaling in Node.js?

The cluster module forks the application into several processes that share one listening port, so a multi-core machine can use all its cores instead of one. In production this is usually delegated to PM2.

Cluster and Scaling example

import cluster from 'node:cluster';
import os from 'node:os';

if (cluster.isPrimary) {
  const cores = os.cpus().length;
  for (let i = 0; i < cores; i++) cluster.fork();

  cluster.on('exit', worker => {
    console.error(`Worker ${worker.process.pid} died — restarting`);
    cluster.fork();
  });
} else {
  startServer();   // each worker runs the app
}

The same thing with PM2

pm2 start src/index.js -i max --name api
pm2 logs api
pm2 reload api        # zero-downtime restart
pm2 save && pm2 startup

Key points to remember

• Workers share nothing — in-memory state and sessions must move to Redis or a database.
• PM2 or a container orchestrator handles this better than hand-written cluster code.
• It multiplies throughput; it does not make a single slow request faster.

Common mistakes with Cluster and Scaling

• Keeping sessions or caches in process memory, so behaviour depends on which worker answers.
• Running more workers than cores, which adds context switching for no gain.