Multi varnish setup and purging

[Andy Fase]

Hi there
We're in the process of taking our 3.0.X single varnish instance into a HA 
multi
varnish setup.
My plan for caching was to initially use a hash director to pass requests 
from
whatever the incoming varnish instance is to a specific one (based on the 
URL)
so that the cache for a given URL will only be kept in one place.
Hence the flow would be
Varnish -> varnish -> backend
However our overall application makes heavy use of wildcard purging via the 
ban
command. As each URL in a wildcard ban could be on different varnish 
instances I
don't see how this could actually work?
Does anyone have any experience in supporting purging multiple varnish 
servers
while supporting wildcard purges without having to get the application to 
send
separate purge requests to all varnish instances?
I was thinking of maybe a round robin director with all varnish instances
defined as backends and some vcl to actually do the ban but return a error 
(to
the calling varnish instance) so that the ban gets sent to all varnish 
instances
(because each of them return an error after the ban)
Any thoughts appreciated
Cheers Andy
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