Xkey, Varnish's open source, tag-based invalidation 0 feature has been deprecated in favor of the proprietary YKey.. Which requires a $10k+/year subscription to "Varnish Cache Plus" to use.
Is there another HTTP cache that supports similar tag-based cache invalidation?
Basically, an upstream server will add an HTTP response header with a number of tags. Those tags can be used to evict items from the cache with an HTTP PURGE request. It's useful for caching lists of items and invalidating the specific pages an item appears in when it's modified.