PATCH: stale-while-revalidate support

[Per Buer]

On Mon, Jun 23, 2014 at 11:44 AM, Federico Schwindt <fgsch at lodoss.net>
wrote:

> Hi,
>
> After Per's recent post (1) and discussing it with him here's a patch that
> uses this to initialize beresp.grace.
> I've reused cache_rfc2616.c but perhaps this should be moved into its own
> file, i.e. cache_rfc5861.c.
> Comments?
>
>
RFC5861 ads two subheaders to Cache-control, stale-while-revalidate and
stale-while-error. I think it makes sense to parse these in Varnish because
it is much more convenient to have the backend issue the policy on each
response, instead of having lots and lots of VCL.

The semantics would be to parse both stale-while-revalidate and
stale-while-error, set beresp.grace to the biggest.

The problematic part is delivery, where one would have to have a bit more
logic in vcl_hit. So vcl_hit would maybe look something like this:

if (std.healthy(req.backend_hint)) {
 # backend healthy.
if (obj.ttl + obj.cachecontrol.stale-while-revalidate > 0s) {
 return(deliver);
} else {
 return(fetch);
}
} else {
# backend is not healthy, do stale-while-error
 if (obj.ttl + obj.cachecontrol.stale-while-error > 0s) {
 return(deliver);
} else {
 return(fetch);
}
}

One of the problems here is that the logic relies on a probe being active
on the backend. In my experience the added complexity is worth it, a lot of
the "low end" users out there have mental barriers writing VCL but have no
problems adding a header or two to their PHP application. The
stale-while-revalidate functionality has been in Squid for quite some time
and I believe there is support for ATS as well. Nginx or Apache mod_proxy
does not seem to have any support for this, but those aren't very ambitious
proxies, imho.




-- 
 <http://www.varnish-software.com/> *Per Buer*
CTO | Varnish Software
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