Fallback to polling does not work

[RonnieRocket147]

We have a couple of customers who are unable to connect with the server due to the websocket being blocked by their network. Our client transports are configured as:

["websocket", "polling"]

I would suspect the client to fallback to polling however this does not work in their situation. When the following transports order is used the polling connection IS working for them:

["polling", "websocket"]

See the image of what the customer is seeing in the browser console when the websocket is specified as first option.

Why is the fallback not working in this case?

[darrachequesne]

Hi! I think the word "fallback" may be a bit misleading here, as the order of the transports in the transports option matters:

• ["websocket", "polling"] means: establish a WebSocket connection first (or use HTTP long-polling if WebSocket is not available, for example in older browsers)
• ["polling", "websocket"] means: establish a HTTP long-polling connection first, and then upgrade to WebSocket

Related: https://github.com/socketio/engine.io-client/issues/575

[RonnieRocket147]

Hmm interesting!

["websocket", "polling"]: so in a modern browser polling will never be used even if the Websocket connection cannot be established (like in my example above)?

Is there a scenario possible like try Websocket first, if not possible then use polling? I think that would be better than poll-and-upgrade-if-possible nowadays.

Thanks again!

[darrachequesne]

["websocket", "polling"]: so in a modern browser polling will never be used even if the Websocket connection cannot be established (like in my example above)?

Yes, that's right, unfortunately.

Is there a scenario possible like try Websocket first, if not possible then use polling? I think that would be better than poll-and-upgrade-if-possible nowadays.

I think the easiest way to do this is to catch the connect_error event:

const socket = io({
  transports: ["websocket", "polling"]
});

socket.on("connect_error", () => {
  // revert to classic upgrade
  socket.io.opts.transports = ["polling", "websocket"];
});

Reference: https://socket.io/docs/v4/client-socket-instance/#connect-error

[RonnieRocket147]

Thanks Damien,

That looks like it could work, I'll give that a try.

Many thanks for taking the time to respond.

[darrachequesne]

You're welcome! I updated the documentation here: https://socket.io/docs/v4/client-initialization/#transports