Too many connections

[bureson]

Hello good people of Engine.io.

We have been using this library for a few years now, but recently we have started observing strange behavior. On our AWS setup of two EC2 instances running under Elastic Beanstalk behind Amazon Load Balancer we noticed that one of the two servers has a lot of open connections. We can see that almost at all times there are 2 thousand connections and occasionally (last week we are observing this once a day) we run up to 11 thousand connections which takes up to 100% CPU and therefore makes our backend on that server irresponsive.

We can't see any obvious reason of what might be causing this and we were curious if we could get a push in the right direction over here.

We are using Restify server and we measure the connection count as Object.keys(this.socketSet):

server.js

const restify = require('restify');
const { SocketServer } = require('./socketServer');
const socketEventListener = require('./socketEventListener'); // Note: not sure if I should elaborate this one

async function listening ({ SocketEventDispatcher }) { // Note: not sure if I should elaborate this one either
  const server = restify.createServer();
  ...
  const socketServer = new SocketServer(server, { transports: ['polling'] });
  server.socketEventDispatcher = new SocketEventDispatcher(socketServer);
  socketEventListener.listening({ server });
  ...
}

socketServer.js

const EventEmitter = require('events');
const socketEngine = require('engine.io');

class SocketServer extends EventEmitter {
  constructor (server) {
    super();
    this.socketSet = {};
    this.app = server.app;
    const socketEngineServer = socketEngine.Server();
    socketEngineServer.attach(server.server);
    monitor.debug({ service: 'socket', event: 'listen' });
    socketEngineServer.on('connection', this.onConnection.bind(this));
  }

  onConnection (socket) {
    this.socketSet[socket.id] = socket;
    socket.on('close', async (code, message) => {
      await this.onClose(socket, code, message);
    });
    socket.on('message', async (data) => {
      try {
        await this.receivingMessage(socket, message);
      } catch (error) {
        ...
      }
    });
  }

  async receivingMessage (socket, message) {
    ...
  }

  onClose (socket, code, message) {
    this.emit('close', { socketId: socket.id });
    delete this.socketSet[socket.id];
  }
}