How ports work when scaling server using nodejs pm2 - node.js

Question

I'm learning how to scale servers in a little sandbox I've setup. Here's the very simple code:
'use strict';
const express = require('express');
const bodyParser = require('body-parser');
const app = express();
const instanceId = parseInt(Math.random() * 1000);
//Allow all requests from all domains & localhost
app.all('/*', function(req, res, next) {
res.header("Access-Control-Allow-Origin", "*");
res.header("Access-Control-Allow-Headers", "X-Requested-With, Content-Type, Accept");
res.header("Access-Control-Allow-Methods", "POST, GET");
next();
});
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({extended: false}));
app.get('/', function(req, res) {
console.log(`[${new Date()}] ${req.method} ${req.originalUrl} from ${req.ip} at ${instanceId}`);
res.send(`received at ${Date.now()} from ${instanceId}`);
});
app.listen(6069);
Nothing crazy, just spits out the date and the instance the request was received at.
The pm2 docs for scaling a nodejs server advised me to run:
pm2 start server.js -i 5
which worked perfectly fine. Here's an example output when I stress tested it using npm module loadtest:
server-0 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 847
server-1 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 261
server-3 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 328
server-2 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 163
server-4 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 351
server-0 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 847
server-3 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 328
server-1 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 261
server-2 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 163
server-4 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 351
server-0 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 847
server-3 [Sun Aug 07 2016 00:13:53 GMT-0400 (EDT)] GET / from ::ffff:127.0.0.1 at 328
Here's my question. Why didn't node throw an error that port 6069 is in use? Multiple servers are attempting to use the port—yet there's no complaining. Why?

Answer 1

PM2 creates it's own "embedded load-balancer which uses Round-robin algorithm to better distribute load among the workers". So it basically wraps a load-balancer around your app and proxies the request to each node it creates.
When using Round-robin scheduling policy, the master accepts() all
incoming connections and sends the TCP handle for that particular
connection to the chosen worker (via IPC).