Run websocket (e.g socket.io) in pm2 cluster on multiple ports(for NGINX reverse proxy)

app.js

// run: pm2 start app.js -i max

var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);

app.get('/', function(req, res){
    res.send(process.env.NODE_APP_INSTANCE);
});

io.on('connection', function(socket){
  console.log('a user connected');
});

var p = 3000 + (parseInt(process.env.pm_id) || 0);
http.listen(p, function(){
  console.log('listening on *:3000');
});