Refreshing modified content

[Jonathan Leibiusky]

I think that conditional GETs could be really useful here. I just don't know
how varnish works in this scenario. Maybe someone from the varnish
development team can shed some light here :)

On Fri, Dec 17, 2010 at 12:42 PM, James A. Robinson <
jim.robinson at stanford.edu> wrote:

> On Thu, Dec 16, 2010 at 12:31, Paulo Paracatu <paulo at aliancaproject.com>
> wrote:
> > If I understood it, the purging method isn't automatic, right? I'd need
> to
> > purge the content everytime it is modified.
> > This is kinda stupid... I host more than 10k sites, modifying files
> > everytime. If I set a high TTL, the backend will be happy and the
> webmaster
> > will be angry. If I set a low TTL, the webmaster will be happy, but the
> > backend will die. Plus, there is no point using a cache if the TTL is
> low.
>
> In a later post you ask about whether or not varnish could be
> configured to send a conditional GET on every request
>
> Basically varnish would be looking up an item in its own cache, seeing
> if it had a Last-Modified or ETag header from the backend, and sending
> a conditional GET -- if it got an entity back it'd store that entity
> as the new version, otherwise serve the old.  I'd be curious if
> anyone's put together VCL logic that is capable of that.  It'd be good
> to know how to do it.
>
> If it's possible to due, this technique might work well when you are
> fronting a backend that is very fast at computing conditional GETs,
> e.g., static files that can be examined to see if its inode, size, or
> last modified time has changed.  I imagine most people using varnish
> have slower backend servers, ones that build dynamic content and
> aren't able to respond to conditional GETs any more efficiently than
> they could respond to an unqualified GET.
>
> One of the places we use varnish at my dept is fronting a large (half
> terabyte of about six million files) static file server.  Instead of
> using some form of conditional GET, what we use are cache channels.
>
>  http://tools.ietf.org/id/draft-nottingham-http-cache-channels-01.txt
>
> When various programs are updating the backend server static files
> they POST the filepath to our cache channel server.  We have another
> program running that monitors the cache channel once a minute for
> updates, and when it sees a new entry show up it turns around and
> sends a PURGE request to varnish.
>
> Jim
>
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