message event (Worker)

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

if (cluster.isMaster) {

  // Keep track of http requests
  var numReqs = 0;
  setInterval(() => {
    console.log('numReqs =', numReqs);
  }, 1000);

  // Count requests
  function messageHandler(msg) {
    if (msg.cmd && msg.cmd == 'notifyRequest') {
      numReqs += 1;
    }
  }

  // Start workers and listen for messages containing notifyRequest
  const numCPUs = require('os').cpus().length;
  for (var i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  Object.keys(cluster.workers).forEach((id) => {
    cluster.workers[id].on('message', messageHandler);
  });

} else {

  // Worker processes have a http server.
  http.Server((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');

    // notify master about the request
    process.send({ cmd: 'notifyRequest' });
  }).listen(8000);
}