How to send a message from the cluster master process to the worker process in Node.js?

Published October 31, 2020

This blog is related to a previous blog called How to run the Node.js server on every CPU core?. Go check that out first if you want to learn more about the Master and Worker process.

To send a message from a master process to the worker process, you can use the send() method on the worker process object.

Let's say we forked a worker process like this,

if (cluster.isMaster) {
  // This block of code is where the master process lives

  // fork a single worker
  const worker = cluster.fork();
} else {
  // This block of code is where worker process lives
}

Now let's use the send() method to send a message to the forked worker.

if (cluster.isMaster) {
  // This block of code is where the master process lives

  // fork a single worker
  const worker = cluster.fork();

  // send a messsage from master process
  // to the wroker process
  worker.send({ data: "Hi Worker!" });
} else {
  // This block of code is where worker process lives
}

Now to receive the message in the worker process, we have to listen for any messages from the master process.

This can be done using the message event on the process object like this,

if (cluster.isMaster) {
  // This block of code is where the master process lives

  // fork a single worker
  const worker = cluster.fork();

  // send a messsage from master process
  // to the wroker process
  worker.send({ data: "Hi Worker!" });
} else {
  // This block of code is where the worker process lives
  process.on("message", (msg) => {});
}

Inside the msg object, you can check for the data property where the message is located.

if (cluster.isMaster) {
  // This block of code is where the master process lives

  // fork a single worker
  const worker = cluster.fork();

  // send a messsage from master process
  // to the wroker process
  worker.send({ data: "Hi Worker!" });
} else {
  // This block of code is where the worker process lives
  process.on("message", (msg) => {
    // check for data property
    // on msg object
    if (msg.data) {
      console.log(`Message received to worker from master: ${msg.data}`);
    }
  });
}

We have successfully passed our message from the master process to the worker process 🔥.

Feel free to share if you found this useful 😃.


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