Question

I have working (stock) script from node

var cluster = require('cluster');
var http = require('http');
var numReqs = 0;

if (cluster.isMaster) {
  // Fork workers.
  for (var i = 0; i < 2; i++) {
    var worker = cluster.fork();

    worker.on('message', function(msg) {
      if (msg.cmd && msg.cmd == 'notifyRequest') {
        numReqs++;
      }
    });
  }

  setInterval(function() {
    console.log("numReqs =", numReqs);
  }, 1000);
} else {
  // Worker processes have a http server.
  http.Server(function(req, res) {
    res.writeHead(200);
    res.end("hello world\n");
    // Send message to master process
    process.send({ cmd: 'notifyRequest' });
  }).listen(8000);
}

In the above script I can send data from worker to master process with ease. But how to send data from master to the worker/workers? With examples, if it possible.

Answer 1

Because cluster.fork is implemented on top of child_process.fork, you can send messages from a master to the worker by using worker.send({ msg: 'test' }), and from a worker to a master by process.send({ msg: 'test' });. You receive the messages like so: worker.on('message', callback) (from worker to master) and process.on('message', callback); (from master to worker).

Here’s my full example, you can test it by browsing http://localhost:8000/ Then the worker will send a message to the master and the master will reply:

var cluster = require('cluster');
var http = require('http');
var numReqs = 0;
var worker;

if (cluster.isMaster) {
  // Fork workers.
  for (var i = 0; i < 2; i++) {
    worker = cluster.fork();

    worker.on('message', function(msg) {
      // we only want to intercept messages that have a chat property
      if (msg.chat) {
        console.log('Worker to master: ', msg.chat);
        worker.send({ chat: 'Ok worker, Master got the message! Over and out!' });
      }
    });

  }
} else {
  process.on('message', function(msg) {
    // we only want to intercept messages that have a chat property
    if (msg.chat) {
      console.log('Master to worker: ', msg.chat);
    }
  });
  // Worker processes have a http server.
  http.Server(function(req, res) {
    res.writeHead(200);
    res.end("hello world\n");
    // Send message to master process
    process.send({ chat: 'Hey master, I got a new request!' });
  }).listen(8000);
}