How to attach to multiple instances? #5476

Yes for http servers
One Server can be attached many times

const io = new Server();
io.attach(httpServer1);
io.attach(httpServer2);

Handlers stay shared so io.on("connection") fires for both

Across separate node processes it is different
Each process needs its own Server plus an adapter so rooms and broadcasts cross processes

import { createAdapter } from "@socket.io/redis-adapter";
io.adapter(createAdapter(pubClient, subClient));

And sticky sessions on the load balancer if polling is enabled