Maximum concurrent Socket.IO connections

This question has been asked previously but not recently and not with a clear answer.

Using Socket.io, is there a maximum number of concurrent connections that one can maintain before you need to add another server?

Does anyone know of any active production environments that are using websockets (particularly socket.io) on a massive scale? I'd really like to know what sort of setup is best for maximum connections?

Because Websockets are built on top of TCP, my understanding is that unless ports are shared between connections you are going to be bound by the 64K port limit. But I've also seen reports of 512K connections using Gretty. So I don't know.

This article may help you along the way: http://drewww.github.io/socket.io-benchmarking/

I wondered the same question, so I ended up writing a small test (using XHR-polling) to see when the connections started to fail (or fall behind). I found (in my case) that the sockets started acting up at around 1400-1800 concurrent connections.

This is a short gist I made, similar to the test I used: https://gist.github.com/jmyrland/5535279

I tried to use socket.io on AWS, I can at most keep around 600 connections stable.

And I found out it is because socket.io used long polling first and upgraded to websocket later.

after I set the config to use websocket only, I can keep around 9000 connections.

Set this config at client side:

const socket = require('socket.io-client')
const conn = socket(host, { upgrade: false, transports: ['websocket'] })