VSL Timeout and Varnishkafka

[Luca Toscano]

2016-06-21 20:27 GMT+02:00 Geoff Simmons <geoff at uplex.de>:

First of all thanks a lot for the answer!

On 06/21/2016 07:27 PM, Luca Toscano wrote:
> >
> > -   VSL            timeout -   End            synth
>
> The timeout (-T arg of the VSL tools) starts when the Begin tag for a
> transaction is read from shared memory, and ticks until the End tag is
> read for that transaction. If End is not encountered before the
> timeout expires, you get the synthesized VSL record.
>
> So in your example, VSL isn't seeing End in the log before the timeout
> elapses, probably 120 seconds if you haven't changed the default. This
> could happen, for example, if you have a response that takes that long
> to complete.
>

My understanding is that a Varnish worker will flush all the tags related
to a specific request only when it is done with it (this is what I got from
https://info.varnish-software.com/blog/understanding-timeouts-varnish-cache).
As written in my past email I tried to set up a basic backend timeout
scenario but I wasn't able to reproduce the VSL error. More details:

- Varnish -> Apache -> PHP-FPM <-> php script that sleeps for 200 seconds
before emitting HTML
- Varnish is configured with default parameters plus "-p
first_byte_timeout=150" (note: bigger than the 120 seconds set by default
for VSL timeout, the default one is 60)
- a simple curl request for the URL that runs the PHP script correctly
triggers a backend fetch timeout, but Begin|End SLT Timestamps are there.
- I also tried with varnishlog -g request -T 5 to force a VSL Timeout, but
didn't succeed.

Is it possible that the VSL timeout is something related only to the data
flow between SHM and varnishkafka (so not related to the time taken by a
Varnish worker to serve a request) ?

If this is completely false and incorrect, would you mind to give me a more
specific example about how to trigger a VSL timeout?


>
> To set the timeout in a VSL client, you can use:
>
> VSL_Arg(vsl, 'T', timeout)
>
> ... where timeout is a string containing decimal digits.
>
> varnishevent is very similar to varnishkafka and supports -T, you can
> see some working code here:
>
>
> https://code.uplex.de/uplex-varnish/varnishevent/blob/master/src/varnishevent.c#L641


Thanks for the link, I implemented the -T exactly in the same way yesterday
for Varnishkafka, really good to have a working example to confirm my code.
Also, Varnishevent looks really awesome, nice work! Looking forward to test
it :)

Regards,

Luca
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