Node.js send message from worker on exit event

Question

I have a Node application creating a number of worker processes via fork() to perform some tasks on their own. As a byproduct, the workers produce a snapshot of actions taken in the form of object. Each worker has an event listener attached to the 'exit' event, at which time they send their snapshot back to the parent process via process.send() .

Here's an example of the set up:

// parent.js
const exec = require('child_process');

const worker = exec.fork('worker.js', [], {
  stdio: ['pipe', 'pipe', null, 'ipc']
});

worker.on('message', (snapshot) => {
  // Handle the snapshot sent from worker
});

// worker.js
process.on('exit', () => {
  process.send({ /* snapshot data */ })
});
/* Arbitrary task work */

Is this an acceptable pattern for parent/worker IPC? Specifically, will this reliably about the reliability of receiving messages in the parent process given the nature of the exit event and process.send() ?

Is the beforeExit event better suited to this pattern?

Answer 1

It is pretty clear in the nodejs doc that you should only do synchronous operations in the exit event. Here's an excerpt from the doc for the exit event :

Listener functions must only perform synchronous operations. The Node.js process will exit immediately after calling the 'exit' event listeners causing any additional work still queued in the event loop to be abandoned.

So, doing an async operation in the exit event is likely a crapshoot that will depend upon internal implementation and whether it needs the event loop to work.

---

You can do asynchronous things in the beforeExit event so that is where you should probably do it. Excerpt from beforeExit doc :

The 'beforeExit' event is emitted when Node.js empties its event loop and has no additional work to schedule. Normally, the Node.js process will exit when there is no work scheduled, but a listener registered on the 'beforeExit' event can make asynchronous calls, and thereby cause the Node.js process to continue.

The listener callback function is invoked with the value of process.exitCode passed as the only argument.

The 'beforeExit' event is not emitted for conditions causing explicit termination, such as calling process.exit() or uncaught exceptions.

The 'beforeExit' should not be used as an alternative to the 'exit' event unless the intention is to schedule additional work.