Varnish error for failed backend

[Danila Vershinin]

Hi Geoff,

Thank you. I understand about vcl_synth being used when getting 500 & 503 better :)
However, I’m also interested how can we distinguish backend errors while using grace mode and failed probe.

Suppose that you have configured grace mode and a health check against homepage.  And homepage fails with 500.
The backend is marked as sick, then synthetic response (guru meditation) is delivered from vcl_backend_error with resp.status pre-filled to 503.
So there seems to be no way to do the same conditional error display in that case?

Best Regards,
Danila

> On 31 Mar 2018, at 14:25, Geoff Simmons <geoff at uplex.de> wrote:
> 
> On 03/31/2018 10:21 AM, Danila Vershinin wrote:
>> 
>> If a constant PHP error occurs in the backend and there is no cache -
>> we see Backend fetch failed.
> [...]
> 
>> ... it always takes time to explain to the clients that the Varnish
>> is not to blame ...
> Welcome to Varnish administration! Such is the life.
> 
> When the backend fetch fails, always look to the FetchError entry in the
> backend log, which diagnoses the problem. It helps with those conversations.
> 
>> ... and ideally there would be a way to deliver different Varnish
>> error page for these 3 cases:>
>> * 500 error-ed backend
>> * 404-ed backend
>> * actual problem talking to the backend (HTTP etc.)
> 
> Varnish can generate the response itself with vcl_synth -- when you see
> the Guru Meditation and "Backend fetch failed", you're seeing the
> buitlin.vcl version of vcl_synth.
> 
> vcl_synth is called automatically for response codes 500 & 503, for 404
> or anything else, your VCL has to direct to there:
> 
> sub vcl_deliver {
> 	if (resp.status == 404) {
> 		return (synth(404));
> 	}
> }
> 
> So you could have something like this:
> 
> sub vcl_synth {
> 	if (resp.status == 500) {
> 		synthetic("500 error-ed backend");
> 	}
> 	elsif (resp.status == 404) {
> 		synthetic("404-ed backend");
> 	}
> 	elsif (resp.reason == "Backend fetch failed") {
> 		synthetic("actual problem talking to the backend");
> 	}
> }
> 
> Since I believe about Varnish 5.0 or so you can use also this syntax to
> generate the synthetic response (synthetic() works as well):
> 
> 	set resp.body = "mumble";
> 
> Of course you'll probably want to have HTML markup in the generated
> response -- look to the Guru Meditation in builtin.vcl's vcl_synth for
> an example.
> 
> 
> HTH,
> Geoff
> --
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