Varnish and time out on backend (first_byte_timeout).

[Mickaël GERVAIS]

Thanks a lot!! Apparently it works... (I've taken my hearplugs...)
Here is my config is somebody needs it:

backend fake {
     .host = "xxxxxxxxx";
     .port = "80";
     .probe = {
        .url = "/fake.html";
        .interval = 60s;
        .timeout = 0.1s;
        .window = 1;
        .threshold = 1;
        .initial = 1;
     }
}

sub vcl_recv {
[...]
   if ( req.http.magicmarker && req.http.magicmarker == "fake" ) {
       unset req.http.magicmarker;
       set req.backend = fake;
   } else {
       set req.backend = yyyy;
   }
[...]
}

sub vcl_error {
     log "[Error ]  ( ) " req.url "(Status: " obj.status ", Restarts: "
req.restarts ")";
     if (obj.status == 503 && req.restarts < 5) {
        log "--- Restart url: " req.url "(Status: " obj.status ", Restarts:
" req.restarts ")";
        set obj.http.X-Restarts = req.restarts;
        if ( req.restarts == 0 ){
                log "--- First restart add fake.";
                set req.http.magicmarker = "fake";
        }
        restart;
    }
}

On Wed, 12 Jan 2011 14:31:58 +0100, Kristian Lyngstol
<kristian at varnish-software.com> wrote:
> On Wed, Jan 12, 2011 at 11:34:37AM +0100, Mickaël GERVAIS wrote:
>> If a timeout occurs (first_byte_timeout reached) the function vcl_error
>> is
>> called, I'd like to use the saint mode to retreive the response from the
>> cache, but saint mode is only avaliable on beresp. 
>> 
>> Is there a way to tell varnish use a dirty object from the cache? Maybe
>> is
>> not the correct way to handle this kind of error. 
> 
> You are correct - that is a weakness. I have a nasty hack, though.
> 
> 1. Declare a second, bogus backend which will always be sick.
> 2. In vcl_error if restarts is 0, set a magic marker and restart.
> 3. Look for the magic marker in vcl_recv - if it's present, tell Varnish
to
>    use the bogus backend. Grace will then kick in because that backend is
>    marked as sick.
> 4. If the object exists in cache (graced) - it will be used. Otherwise,
you
>    will hit vcl_error again. (Thus the check of req.restarts in step 2).
> 
> It's a nasty, yet brilliant hack, if I might say so myself ;)
> 
> It adds latency and doesn't utilize saintmode, but it gets the job done
in
> a way that will also make little children cry.
> 
> - Kristian

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MICKAL GERVAIS

Agaetis
10 allée Evariste Galois
63 000 Clermont-Ferrand

Courriel : mgervais at agaetis.fr
Téléphone : 04 73 44 56 51
Portable : 06 82 35 52 82
Site : http://www.agaetis.fr
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