Varnish and max-age=0

[Girouard, Yanick]

Thanks but that doesn't really answer my question. Being the default, you'd only want to set it to false explicitly if it was set to true. My question was when would you ever want or need to do this? I can see cases where you'd want to force it to true, but not the opposite.


De : Guillaume Quintard [mailto:guillaume at varnish-software.com]
Envoyé : vendredi 21 juillet 2017 08:38
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

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<mailto: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<mailto:guillaume at varnish-software.com>]
Envoyé : vendredi 21 juillet 2017 03:49
À : Girouard, Yanick <Yanick.Girouard at stm.info<mailto:Yanick.Girouard at stm.info>>
Cc : Andrei <lagged at gmail.com<mailto:lagged at gmail.com>>; varnish-misc at varnish-cache.org<mailto: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<mailto: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<mailto:lagged at gmail.com>>
Envoyé : 20 juillet 2017 15:22
À : Girouard, Yanick
Cc : Reza Naghibi; varnish-misc at varnish-cache.org<mailto: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><mailto: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><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><mailto: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><mailto: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><mailto: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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