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Comment by upghost | Hacker News Reader

Does anyone have a good trick for figuring out when the client side connection is closed? I just kill the connection on the server every N minutes and force the client to reconnect, but it's not exactly graceful.

Secondly, on iOS mobile, I've noticed that the EventSource seems to fall asleep at some point and not wake up when you switch back to the PWA. Does anyone know what's up with that?

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The socket closes. Most languages bubble this back up to you with a connection closed exception. In python async world, it would be a cancelled error.

I haven't seen a library that does that yet, including Python. Usually you just throw messages into the void. Do you know of a specific library that does that?

I use starlette + SSE

Thanks, I will check that out. If they properly handle client disconnects I'll pull this in.

Send a dummy event and see if you get an ACK in response. Depends on the library you're using.

There's no ack on a raw SSE stream, unfortunately -- unless you mean send an event and expect the client to issue an HTTP request to the server like a keepalive?

There should be an ACK on the tcp packet (IIRC it’s not a lateral ACK but something like it) and the server should handle a timeout on that as the connection being “closed” which can be returned to the connection opener.

You might want to look into timeouts or error callbacks on your connection library/framework.

Interesting, hadn't checked at the TCP level. Will need to look into that.

I remembered wrong. In most circumstances a tcp connection will be gracefully terminated by sending a FIN message. The timeout I talked about is on an ACK for a keepalive message. So after x time of not receiving a keepalive message the connection is closed. This handles cases where a connection is ungracefully dropped.

All this is done at the kernel level, so at the application level you should be able to just verify if the connection is open by trying a read from the socket.

Thanks for clarifying, that would've sent me on a long wild goose chase. Most libraries only provide some sort of channel to send messages to. They generally do not indicate any FIN or ACK received at the TCP level.

If anyone knows any library or framework in any language that solves this problem, I'd love to hear about it.

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