Uses of cluster module and for multicore cpu

cluster.js

const cluster = require('cluster');
const http = require('http');
// number of cpu cores
const numberOfCPU = require('os').cpus().length;

if (cluster.isMaster) {

 console.log(`Master is running: ${process.pid}`);

 // Fork workers.
 for (let i = 0; i < numberOfCPU; i++) {
  cluster.fork();
 }

 cluster.on('exit', (worker, code) => {
  console.log(`worker ${worker.process.pid} died`);
  // to increase the availability of the application
  // we create new worker using fork() method if the worker is crushed.
  // so that we can increase our applications availability.
  if (code !== 0 && !worker.exitedAfterDisconnect) {
    console.log(`Worker ${worker.id} crashed. \nStarting a new worker...`);
    cluster.fork();
  }
 });

} else {

 // Create a HTTP server
 http.createServer((req, res) => {
  res.writeHead(200);
  res.end('hello world\n');
 }).listen(4000);

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