Cluster — To take advantage of multi-core systems

A single instance of Node.js runs in a single thread. To take advantage of multi-core systems, the user will sometimes want to launch a cluster of Node.js processes to handle the load.

Here is the below example for creating the simple worker processes with cluster in node.

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

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });
} else {
  // Workers can share any TCP connection
  // In this case it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

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

Running Node.js will now share port 8000 between the workers:(if your system is 4 core system).

$ node server.js
Master 3596 is running
Worker 4324 started
Worker 4520 started
Worker 6056 started
Worker 5644 started

The cluster module supports the method that is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

For more information kindly refer https://nodejs.org/docs/latest-v9.x/api/cluster.html