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If a client connects to a server over a normal tcp connection, and then later on the client's connection cuts out, the server will get (assuming active mode) {tcp_closed,Socket}. But there are cases where the server won't know that the client has disconnected, such as power failure or crashing and such (I believe, I could be wrong). In these cases, the client is gone but the server still believes it's connected. If the server attempts to send the client a message in these cases, will it assume that the client gets the message or will the tcp stack sort that out on the low level and the server gets back some kind of error?

I know this is a simplistic question, but I've been having trouble testing it myself, as I can't get a client to catastrophically fail like I need it to (even kill -9 isn't doing it). Does anyone have any experience with this?

Mediocre Gopher
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    TCP will know that the message wasn't delivered, but whether Erlang will pick up on it or not is a different issue. Did you try unplugging the client's ethernet cable? That's pretty low-impact and easy to test. – nmichaels Apr 07 '11 at 17:50
  • I'm using only my own machine to test using localhost, I don't have access to another machine at the moment. – Mediocre Gopher Apr 07 '11 at 17:51
  • Ah, no fun...I guess you could use a VM, but that's way more work than asking on SO. – nmichaels Apr 07 '11 at 17:53

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The answer depends. When you try to send out data, the kernels TCP window will slowly fill until it can't take any more data. Then your send will block because the internal kernel buffer is full. TCP has some timers which will trigger after some time. When that happens, the kernel will error the send request, Erlangs VM runtime will transform it into {error, Reason}, where Reason is the posix() error message from the underlying system.

If you want to be sure the data got through, you have to acknowledge it on the stream the other way. Or you can make the data idempotent so you can resend it without trouble. It is especially important if the other endpoint, the client, is a device like a mobile phone where disconnects will happen all the time.

To test it, you can block the communication with a firewall rule on lo.

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