DB hangs with postgres and broken network link

[Curtis Dutton]

I'm in the process of writing a simple daemon that periodically checks in with a postgres database. I'm trying to make it resilient with network outages. 

If you connect to the database.

Unplug the network cable.

Run a query.

The query will hang for what appears to be forever without throwing an exception. 

Is this expected behavior?

Thanks,

    Curtis

[Curtis Dutton]

Ok well on additional investigation this appears only to happen while using virtual connections. If a raw postgresql-connect is used it does raise a tcp timeout error.

[Ryan Culpepper]

Hi Curtis, thanks for the report. No, that isn't supposed to happen. I haven't managed to reproduce a silent hang, and looking over the virtual-connection code it looks like any errors (eg, TCP timeout) should be propagated to the caller. On the other hand, I don't have a way of simulating a network outage for testing, so maybe in that case the connection never realizes it is no longer connected, unlike the things I tried.

Could you run your program with db logging at the debug level (eg, set the environment variable PLTSTDERR to "debug@db error") and also add `#:debug? #t` to your call to `postgresql-connect` and see what gets logged/printed before the hang occurs? (You may want to redact the output.)

Ryan

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[James Platt]

I wonder if this is a more general TCP/IP issue rather than anything Postgres specific. I'm troubleshooting an issue where a TCP connection to a server is unexpectedly going down, once in a while, with no obvious indication. The solution, for now, is to have the client send a sort of ping over the TCP connection to the server every so often and then establish a new connection if it doesn't get a response. In other words, when the connection goes down, trying to send something over it from the client does not generate an error. I haven't found an indication on either the client or server. So, to detect the connection failure, you have to look for a lack of response. This is an intermittent failure and, as such, very annoying to troubleshoot.