Varnish and max-age=0

[Guillaume Quintard]

beresp.uncacheable == false is the default, ie. "cache the object and serve
it next time someone ask for it"

-- 
Guillaume Quintard

On Fri, Jul 21, 2017 at 2:17 PM, Girouard, Yanick <Yanick.Girouard at stm.info>
wrote:

> So in which case would you want to force it to false? I read about it and
> it's mainly used to force a hit for pass, but I haven't read about a
> scenario where the opposite would be useful.
>
>
>
> *De :* Guillaume Quintard [mailto:guillaume at varnish-software.com]
> *Envoyé :* vendredi 21 juillet 2017 03:49
> *À :* Girouard, Yanick <Yanick.Girouard at stm.info>
> *Cc :* Andrei <lagged at gmail.com>; varnish-misc at varnish-cache.org
>
> *Objet :* Re: Varnish and max-age=0
>
>
>
> Common mistake, beresp.uncacheable isn't the opposite of beresp.ttl>0.
> "uncacheable" tells Varnish that if it gets a HIT for that object, it
> should convert it to a PASS/MISS (depending on the versions) and avoir
> request coalescing. In that scenario too, the ttl is the time the object
> will live in cache. ie. how long do you retain the memory that it's not
> cacheable.
>
>
> --
>
> Guillaume Quintard
>
>
>
> On Fri, Jul 21, 2017 at 12:09 AM, Girouard, Yanick <
> Yanick.Girouard at stm.info> wrote:
>
> That's a good thought, but what would really be the impact of this setting
> if I've already set the ttl to a positive value after stripping all headers
> that would make Varnish consider the object as being uncacheable to begin
> with? Is there a case where it would be required?
>
>
> ________________________________
> De : Andrei <lagged at gmail.com>
> Envoyé : 20 juillet 2017 15:22
> À : Girouard, Yanick
> Cc : Reza Naghibi; varnish-misc at varnish-cache.org
> Objet : Re: Varnish and max-age=0
>
> Just a thought, if you're going to force an otherwise uncacheable request
> to be cached, you should probably: set beresp.uncacheable = false;
>
>
>
> On Thu, Jul 20, 2017 at 9:03 PM, Girouard, Yanick <
> Yanick.Girouard at stm.info<mailto:Yanick.Girouard at stm.info>> wrote:
> Hi Reza,
>
> Yes we are. Here's the default we apply. Those two subs are called in
> order in vcl_backend_response:
>
> /* REMOVE CACHE-CONTROL AND SURROGATE-CONTROL FROM BACKEND *
> ***********************************************************/
> sub stm_backend_resp_unset_cache_control_headers {
>                 unset beresp.http.Surrogate-Control;
>                 unset beresp.http.Cache-Control;
>                 unset beresp.http.Expires;
> }
>
> /* DEFAULT ALL TO: TTL 30MIN + GRACE 15MIN *
> *******************************************/
> sub stm_backend_resp_expiration_default {
>                 set beresp.ttl = 30m;
>                 set beresp.grace = 15m;
> }
>
> That doesn't seem to have any impact when the backend responds with a
> Cache-Control: max-age=0 header.
>
> Any idea?
>
>
> De : Reza Naghibi [mailto:reza at varnish-software.com<mailto:reza at varnish-
> software.com>]
> Envoyé : jeudi 20 juillet 2017 13:58
> À : Girouard, Yanick <Yanick.Girouard at stm.info<mailto:Yanick.Girouard at stm.
> info>>
> Cc : varnish-misc at varnish-cache.org<mailto:varnish-misc at varnish-cache.org>
> Objet : Re: Varnish and max-age=0
>
> The TTL is calculated before entering vcl_backend_response. So eventhough
> you unset the Cache-Control header, the value of TTL will be calculated
> based on it. Are you setting a new value for beresp.ttl? You need to do
> that:
>
> sub vcl_backend_response
> {
>   unset beresp.http.Cache-Control;
>   set beresp.ttl = 120s;
> }
>
> --
> Reza Naghibi
> Varnish Software
>
> On Thu, Jul 20, 2017 at 1:44 PM, Girouard, Yanick <
> Yanick.Girouard at stm.info<mailto:Yanick.Girouard at stm.info>> wrote:
> Hi,
>
> We use Varnish to cache for multiple backends and need Varnish to always
> control what is cached despite what backends could respond. In other words,
> even if a backend sets Cache-Control headers to never cache its pages, we
> still want Varnish to cache them based on defined rules (i.e. certain URL
> patterns or hosts have different TTLs).
>
> We have recently realized that one of our backend always set the following
> header: Cache-Control: max-age=0, private, must-revalidate
>
> Our VCL unsets the Cache-Control header in vcl_backend_response and sets
> its own before delivering. By unsetting the Cache-Control header in
> vcl_backend_response I would expect Varnish to ignore the max-age=0 value
> and still cache the page as per our other rules, but it seems that the
> second it sees max-age=0 in the response header, that it makrs the object
> as not cacheable.
>
> Other than by changing the backend's response to never set max-age=0, is
> there a way to force Varnish to cach pages even if it returned max-age=0?
>
> Is this even by design or is it a bug?
>
> Thanks,
> Yanick Girouard
>
>
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