The idea behind the HttpCache namespace was that the Varnish (and Nginx, etc.) classes are clients to those HTTP caches that now do invalidation only, but can do more in the future (such as getting configuration from  Varnish). If we can make sure the Varnish etc. classes will only do invalidation tasks, we can put them in the Invalidation namespace. And if we do that, should we have the interfaces in the same directory, which may be a bit messy, or move them to a Strategy or Capability or ... directory?