Sharing a cache between multiple Varnish servers

[Jonathan Matthews]

On 21 April 2012 09:25, Neil Saunders <n.j.saunders at gmail.com> wrote:
> Hi all -
>
> A two part question on cache sharing:
>
> a) I've got 3 web servers each with a 3.5Gb memory cache. I'd like them to
> share a cache but don't want to use the experimental persistant storage
> backend - Are there any other options?

As Per's said, this isn't possible.

> b) We run a cache warming script to ensure a certain set of URL's are always
> cached, but at the moment the script requests to all 3 web heads to ensure
> cache consistency - I see that Varnish supports PUT operations - Would it be
> feasible for the cache warmer to request content from webhead 1 and make a
> "PUT request to servers 2 & 3? I've searched high and low for documentation
> on this but can't find anything.

If Per's suggestion of hashing content across the caches doesn't fit
in with what you're trying to do, how about this.

Put some logic in your VCL that looks out for a "X-Cache-Warming:
True" (or whatever) request header, which your warming script will
explicitly set.

If this header is present, then use VCL to get Varnish to switch over
to another cache as its backend such that, instead of just going
cache1->origin, the request instead goes
cache1->cache2->cache3->origin. You'll need this logic on N-1 of your
caches (the last one doesn't need to know it's part of this scheme) ,
and it should enable you to make 1 request to warm N caches.

You might even be able abstract this so it works for cache flushes,
too, and not just warming operations.

HTH,
Jonathan
-- 
Jonathan Matthews
Oxford, London, UK
http://www.jpluscplusm.com/contact.html