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Trying multiple backends for a resource

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oddur at ccpgames

Aug 1, 2012, 12:04 PM

Post #1 of 5 (506 views)
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Trying multiple backends for a resource

I“ve got multiple backends which each might have the file I“m requesting.

Would it be possible to configure varnish in such a way that is tries multiple backends and uses the first one it finds with a 200 response ?

Best regards,

Oddur Snęr Magnśsson
Senior Programmer | Reykjavķk | Iceland

[Description: Description: Description: Description: cid:DB5C0F43-387B-4CF8-BC6F-B17512BB8251 [at] ccp]

:wq

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hugo.cisneiros at gmail

Aug 1, 2012, 1:04 PM

Post #2 of 5 (490 views)
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Re: Trying multiple backends for a resource [In reply to]

On Wed, Aug 1, 2012 at 4:04 PM, Oddur Snęr Magnśsson <oddur [at] ccpgames> wrote:
> I“ve got multiple backends which each might have the file I“m requesting.
>
> Would it be possible to configure varnish in such a way that is tries multiple backends and uses the first one it finds with a 200 response ?

Maybe using restarts...

sub vcl_recv {
if (req.http.fallbackmarker) {
if (req.restarts == 1) {
set req.backend = backend_one;
} elsif (req.restarts == 2) {
set req.backend = backend_two;
} elsif (req.restarts == 3) {
set req.backend = backend_two;
}
[...]
}
}

sub vcl_fetch {
if (req.url == "/file.ext" && beresp.status != 200) {
set req.http.fallbackmarker = 1;
return (restart);
}
}

And setting option max_restarts to the number of backends you may use.

--
[]'s
Hugo
www.devin.com.br

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oddur at ccpgames

Aug 2, 2012, 4:55 AM

Post #3 of 5 (490 views)
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RE: Trying multiple backends for a resource [In reply to]

Seems like the fallback director is what I want, if I can get it to try to continue trying on 404.
I could possibly map 404 to 503(temp unavailable)

https://www.varnish-cache.org/docs/trunk/reference/vcl.html#the-fallback-director

From: rzuidhof [at] gmail [mailto:rzuidhof [at] gmail] On Behalf Of Richard Zuidhof
Sent: Wednesday, August 01, 2012 7:26 PM
To: Oddur Snęr Magnśsson
Subject: Re: Trying multiple backends for a resource

No problem at all.
Op 1 aug. 2012 21:06 schreef "Oddur Snęr Magnśsson" <oddur [at] ccpgames<mailto:oddur [at] ccpgames>> het volgende:
I“ve got multiple backends which each might have the file I“m requesting.

Would it be possible to configure varnish in such a way that is tries multiple backends and uses the first one it finds with a 200 response ?

Best regards,

Oddur Snęr Magnśsson
Senior Programmer | Reykjavķk | Iceland

[Description: Description: Description: Description: cid:DB5C0F43-387B-4CF8-BC6F-B17512BB8251 [at] ccp]

:wq

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enno at tty

Aug 2, 2012, 6:37 AM

Post #4 of 5 (485 views)
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Re: Trying multiple backends for a resource [In reply to]

On 08/02/2012 01:55 PM, Oddur Snęr Magnśsson wrote:
>
> Seems like the fallback director is what I want, if I can get it to
> try to continue trying on 404.
>
> I could possibly map 404 to 503(temp unavailable)
>
> https://www.varnish-cache.org/docs/trunk/reference/vcl.html#the-fallback-director
>
>
the fallback director means that your secondairy backends don't get any
requests, unless the first backend is unhealthy. I'm not sure if that's
what you want.

I'm using the following setup to allow retries on different backends:

director cluster1 random {
{ .backend = backend1; .weight = 10; }
{ .backend = backend2; .weight = 10; }
...
}

sub vcl_recv {
...
set req.backend = cluster1;
...
}

sub vcl_fetch {
if (! beresp.cacheable) {
if (beresp.status == 404 || beresp.status == 500 ||
beresp.status == 503) {
if (req.restarts < 3) {
return (restart);
}
}
return (pass);
}

return (deliver);
}

Enno

apj at mutt

Aug 2, 2012, 6:45 AM

Post #5 of 5 (486 views)
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Re: Trying multiple backends for a resource [In reply to]

On Wed, Aug 01, 2012 at 07:04:50PM +0000, Oddur Snęr Magnśsson wrote:
>
> Would it be possible to configure varnish in such a way that is tries
> multiple backends and uses the first one it finds with a 200 response ?

Use saint mode and restarts.
Remember to set saintmode_threshold to something appropriate though.

--
Andreas

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