How to determine that an SSE connection was closed?

Question

We have an SSE (Server-Sent Events) connection open in JavaScript which can time to time get closed, either because of server restarts or other causes. In that case it would be good to reestablish the connection. How to do it? Is there a way to find out on the client side that the connection was closed?

Here: https://developer.mozilla.org/en-US/docs/Web/API/EventSource I found only a way to close the connection, but no callback or a test method for determining whether the connection is still alive.

Thank you for your help.

Answer 1

If the connection is closed (in such a way that the browser can realize it), it will auto-connect. And it tends to do this quickly (default is 3 seconds in Chrome, 5 seconds in Firefox). readyState will be CONNECTING (0) while it is doing this. It is only ever CLOSED (2) if there was some problem connecting in the first place (e.g. due to a CORS issue). Once CLOSED, it does not retry.

I prefer to add a keep-alive mechanism on top, as the browser cannot always detect dead sockets (not to mention a remote server process that is locked up, etc.). See ch.5 of Data Push Apps with HTML5 SSE for detailed code, but basically it involves having the server send a message every 15 seconds, then a JavaScript timer that runs for 20 seconds, but is reset each time a message is received. If the timer ever does expire, we close the connection and reconnect.