Simple message passing between cluster master and workers in Node.js

node-cluster-messaging.js

var cluster = require('cluster');

if (cluster.isWorker) {

  console.log('Worker ' + process.pid + ' has started.');

  // Send message to master process.
  process.send({msgFromWorker: 'This is from worker ' + process.pid + '.'})

  // Receive messages from the master process.
  process.on('message', function(msg) {
    console.log('Worker ' + process.pid + ' received message from master.', msg);
  });

}


if (cluster.isMaster) {

  console.log('Master ' + process.pid + ' has started.');

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

    // Receive messages from this worker and handle them in the master process.
    worker.on('message', function(msg) {
      console.log('Master ' + process.pid + ' received message from worker ' + this.pid + '.', msg);
    });

    // Send a message from the master process to the worker.
    worker.send({msgFromMaster: 'This is from master ' + process.pid + ' to worker ' + worker.pid + '.'});
  }

  // Be notified when worker processes die.
  cluster.on('death', function(worker) {
    console.log('Worker ' + worker.pid + ' died.');
  });

}

Why would workers need to communicate between other workers? Shouldn't the database be the single point of truth that each worker gets any persisted data from?

For many reasons. For example in a big project you need some parallel computing for improving performance, the database wouldn't be a solution because it would end up being a bottleneck of performance. Take a look on this DigitalOcean case study: https://medium.com/better-programming/from-15-000-database-connections-to-under-100-dfd34062da4c

Additionally, persistent data is not always the case.

Just to NOTE: workers keep on sending some internal nodejs related data to the master, so keep a check on msg key

Great stuff.