Re: [racket-users] Re: DB hangs with postgres and broken network link

[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.

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Re: [racket-users] Re: DB hangs with postgres and broken network link

[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

On Fri, Jul 3, 2020 at 5:07 PM Curtis Dutton  wrote:

> 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.
>
> On Fri, Jul 3, 2020 at 10:51 AM Curtis Dutton  wrote:
>
>> 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
>>
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> .
>

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[racket-users] New package: place-controllers

[Sage Gerard]

Here's a way to encapsulate places in a generator-like procedure: 
[https://docs.racket-lang.org/place-controller/index.html](https://docs.racket-lang.org/place-controller/index.html?q=place%20con)

I ended up writing this to support a parallel build model, such that a single 
(apply sync (p1) (p2) ...) returns a value or status from a place, along with 
helpful context.

Source code is available at https://github.com/zyrolasting/place-controller

~slg

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[racket-users] Re: DB hangs with postgres and broken network link

[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.

On Fri, Jul 3, 2020 at 10:51 AM Curtis Dutton  wrote:

> 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
>

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[racket-users] 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

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