[PATCH] stale-while-revalidate updated

[Geoff Simmons]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

On 8/27/14 7:11 PM, Federico Schwindt wrote:
> 
> Attached is an updated patch for s-w-r support that I originally
> sent in June 23.
> 
> Before this turns into another bike-shedding, a few things worth
> clarifying:
[...]

This might be a bit of bike-shedding, and I know that there's been
some discussion going that I haven't kept up with. So apologies in
advance.

I think I'm unsure about what we're striving for in Varnish 4 --
wasn't the goal to move as much caching policy as possible out to VCL,
with good defaults in builtin.vcl? The patch implements a solution for
s-w-r in the binary, by magic as it were, but it seems to me that it
could be done in VCL.

My stab at a VCL solution does turn out to be cumbersome (just typing
the idea here, haven't tested it).

if (beresp.http.Cache-Control ~ "stale-while-revalidate\s*=\s*\d+") {
    set beresp.grace
        = std.duration(regsub(beresp.http.Cache-Control,
                             "^.*stale-while-revalidate\s*=\s*(\d+).*$",
                             "\1s"), 120s);
}

That is admittedly pretty ugly. (We really need to get that VMOD for
easier backref capture upgraded for Varnish 4, then it would be much
simpler.)

Is the verbosity of a VCL solution the reason for wanting varnishd to
take care of s-w-r?

I'm hesitating here because s-w-r would give application developers a
way to set grace. It can be hard enough to get them to understand how
to set TTL (and do it well), and grace needs even more explanation.
But grace can be critical for getting Varnish to run smoothly and
perform well. App developers might get the idea of setting grace to 0,
but as an admin in charge of Varnish and the owner of VCL, I think I'd
prevent them from doing so. That thought tends to persuade me of the
wisdom of implementing as much caching policy in VCL as we can,
especially where grace is concerned.


Best,
Geoff
- -- 
UPLEX Systemoptimierung
Scheffelstraße 32
22301 Hamburg
http://uplex.de/
Mob: +49-176-63690917
-----BEGIN PGP SIGNATURE-----
Version: GnuPG/MacGPG2 v2.0.14 (Darwin)
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=i60z
-----END PGP SIGNATURE-----