managing multiple node child processes - node.js

Question

How can I manage multiple concurrent child processes that have been forked?
In this example, start_child() can be invoked multiple times, and each invocation can run indefinitely. When forking an arbitrary number of child processes like this, how can I communicate with / address each individual child process? Let's say I have 3 forked child processes running, and they keep running indefinitely, but I want to kill (or send a message to) child process number 2. How would I do that?
If stop_child() is invoked, it kills all of the currently running child processes. How do I refactor this code so that I can call stop_child() on an individual child process?
let app = require('express')();
let server = require('http').Server(app);
let io = require('socket.io')(server);
let fork = require('child_process').fork;
server.listen(80);
app.get('/', function(request, response) {
response.sendFile(__dirname + '/index.html');
});
io.on('connection', function(socket) {
socket.on('start_child', function () {
start_child();
});
socket.on('stop_child', function () {
child.kill();
}
}
function start_child() {
child = fork('./child_script.js');
//conditional logic to either run
//start_child() again or let it end
}
UPDATE
I tried this, based on some of the comments. But if I launch 3 processes and then call, for example, child[0].kill(), I get an error: Cannot read property 'kill' of undefined. I'm guessing my problem is that I'm not correctly passing the i variable to the io.on() call:
let app = require('express')();
let server = require('http').Server(app);
let io = require('socket.io')(server);
let fork = require('child_process').fork;
let i = 0;
let child = [];
server.listen(80);
app.get('/', function(request, response) {
response.sendFile(__dirname + '/index.html');
});
io.on('connection', function(socket) {
socket.on('start_child', function (i) {
start_child(i++);
});
//this is hard coded just for testing
socket.on('stop_child', function () {
child[0].kill();
}
}
function start_child(i) {
child[i] = fork('./child_script.js');
//conditional logic to either run
//start_child() again or let it end
}
UPDATE #2
Okay, I figured out that I need to send the incrementing variable from the client side, passed through an object coming from the emit call. Now when I call child[0].kill() there is no error. The problem is that the child process is not killed:
server.js
let app = require('express')();
let server = require('http').Server(app);
let io = require('socket.io')(server);
let fork = require('child_process').fork;
server.listen(80);
app.get('/', function(request, response) {
response.sendFile(__dirname + '/index.html');
});
io.on('connection', function(socket) {
socket.on('start_child', function (count) {
let num = count.count;
start_child(num);
});
//this is hard coded just for testing
socket.on('stop_child', function (count) {
let num = count.count;
child[num].kill();
}
}
function start_child(num) {
child[num] = fork('./child_script.js');
//conditional logic to either run
//start_child() again or let it end
}
index.html
$(function () {
let socket = io();
let i = 0;
$('#start').on('click', function () {
socket.emit('start_child', {"count": i++});
});
$('#stop').on('click', function () {
//the count is hard coded here just for testing purposes
socket.emit('stop_child', {"count": 0});
});
});
FINAL UPDATE - WITH RESOLUTION
Resolution #2 (right above this) is actually the solution. The problem I was having after that (where the child.kill() call didn't seem to do anything) was caused by a piece of the code that I had left out (in the code comment: 'conditional logic to either run start_child() again or let it end').
This is what was in there:
if (condition) {
setTimeout(start_child, 5000);
} else {
console.log('this child process has ended');
}
And this is what I changed it to (basically, I just had to pass the incrementing variable to the start_child() function so that it would have the same place in the child array when it restarted):
if (condition) {
setTimeout(function() {
start_child(num)
}, 5000);
} else {
console.log('this child process has ended');
}

Answer 1

fork() returns a ChildProcess object which has methods and events on that object for interprocess communication with that process. So you have to save each ChildProcess object from when you call start_child() and then use the appropriate object in order to communicate with the other process.
You can see events and methods of the ChildProcess object here. There are also numerous code examples.

Answer 2

In order to send message to the child process, use the following example :
parent.js
**
const { fork } = require('child_process');
const forked = fork('child.js');
forked.on('message', (msg) => {
console.log('Message from child', msg);
});
forked.send({ hello: 'world' });
**
child.js
process.on('message', (msg) => {
console.log('Message from parent:', msg);
});
let counter = 0;
setInterval(() => {
process.send({ counter: counter++ });
}, 1000);
For further information refer this article :
https://medium.freecodecamp.com/node-js-child-processes-everything-you-need-to-know-e69498fe970a