Wido den Hollander wrote:
Now, when running just one Varnish instance which does loadbalancing
over multiple RGW instances is not a real problem. When it sees a PUT
operation it can "purge" (called banning in Varnish) the object from
it's cache.
When looking at the scenario where you have multiple caches you run into
the cache-consistency problem. If an object is modified the caches are
not notified and will continue to serve an outdated object.
Looking at the Last-Modified header is not an option since the cache
will not contact RGW when serving out of it's cache.
To handle this there has to be some kind of "hook" inside RGW that can
notify Varnish (or some other cache) when an object changes.
For nginx, it appears there is a well-tested production module that does
this: http://labs.frickle.com/nginx_ngx_cache_purge/ (see the examples
at the end of the README)
--Jeff
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