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 If an object is modified the caches are not notified and will continue to serve an outdated object.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)
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