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we're on Apache/2.4.7 on Ubuntu 14.04.

We're trying to solve a mystery of http requests getting lost somewhere.

Our payments provider sends us notifications/request about payments made by our customers. Time to time it happens that they get no confirmation/response from our side.

This behavior is reproducible by sending a test/check notification from payments providers side. You can send e.g. 14 test notifications that all get confirmation in form of response from us with code 200. Then the 15-th notification would be shown as timed-out and there is no response from our side.

After verifying number of requests sent from our payments provider (they see 15) we could only see 14 in our access log. The number doesn't have to be 15 though, it happens arbitrarily.

Can you think about any circumstances, when a request wouldn't make it to access log (overloaded server, networking issues, crash in processing)? Can a http request simply get lost somewhere on it's way? Can you thing of any reason why something like this would be happening? Is there a way to debug this?

Thank you very much for your help!

Tomas

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    I think you've pretty much answered your own question: overloaded server, network issues, crash (although a crash would be recorded in the apache error log and / or kernel log). Access log entries are written when the processing of the request is complete. For debugging you could use tcpdump / wireshark or use iptables to log incoming SYN packets from the payments provider. – wurtel Jan 24 '18 at 10:03
  • consider logs get into access.log once they have been replied successfully, so for example, if requests comes but return to client is not possible it won't be logged in access.log. Check error log for clues too. – ezra-s Jan 24 '18 at 11:51
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Thank you for your answers! I just got an email from our payments provider saying that after investigation they've just fixed an issue with one of the hops of one of the ISP's they are using. Problems on our side got fixed by this.. so the requests really were getting lost it seems.

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