Difference between pingInterval: 1000 and setInterval(1000) for recovery

[minchanhan]

https://socket.io/docs/v4/connection-state-recovery#usage,
https://codesandbox.io/p/sandbox/github/socketio/socket.io/tree/main/examples/connection-state-recovery-example/esm?file=index.js

I'm following this doc and example to test out the connection state recovery feature in socket.io but I'm confused because I thought socket.io handled ping pong emits automatically, so I figured setting pingInterval: 1000 would replace the line used in the example:

setInterval(() => {
  io.emit("ping", new Date().toISOString());
}, 1000);

But the recovery isn't working when using:

const io = new Server(server, { // for work with socket.io
  cors: corsOptions,
  pingInterval: 1000,
  connectionStateRecovery: {
    maxDisconnectionDuration: 2 * 60 * 1000, // 2 mins backup
    skipMiddlewares: true,
  },
});

without the setInterval...

With some digging I found that socket.on("ping") isn't triggered from pingInterval like I expected. So do I still need this setInterval if I want to handle temporary reconnections rather than relying on pingInterval?:

const socket = io(
  process.env.REACT_APP_SERVER || "http://localhost:3001",
  {
    reconnectionDelay: 5000, // defaults to 1000
    reconnectionDelayMax: 5000, // defaults to 5000
  }
);
socket.on("connect", () => {
  console.log(socket.recovered); // true only for setInterval method
});
socket.io.on("ping", () => {
  console.log("ping from pingInterval only");
});
socket.on("ping", () => {
  console.log("ping from setInterval only");
});

[darrachequesne]

Hi!

So do I still need this setInterval if I want to handle temporary reconnections rather than relying on pingInterval?:

The io.emit() is done in order to init the offset on the client side, so that the connection state can be successfully recovered afterwards.

io.emit("ping") is indeed not related to the heartbeat mechanism, we could have used any other event name like io.emit("hello") instead.

[minchanhan]

Hello, in that case, wouldn't it be sufficient to just do io.emit("offset_ping") on connection? I removed the setInterval and added this and the recovery still seems to be working.