RFC 2616 with Apache 2.4

Solution 1:

You give a couple of examples like this:

Cache-Control: public, s-maxage=0
Expires: ... (+1 day)
X-Group: A
Vary: X-Group
ETag: W/"foo1"

I notice in particular the Vary: X-Group header. Is X-Group a request header or a response header?

As per RFC 2616 section 14.44, Vary should list any request headers that will result in a different content representation from an origin server. From your example, I suspect X-Group might be a response header, in which case mod_cache won't be storing multiple versions of the given resource.

One thing you could try would be to set Vary: If-None-Match, which would let mod_cache store a version of the resource for each ETag. Admittedly, I haven't tried this. Assuming it works, one downside is that the first request from any client would be a cache miss.