This class produces secondary keys for a response from an origin server that contains a Vary header.

The cache must store the response with the requested header from the original request, so that subsequent requests match. This is a little confusing, so an example follows:

A request comes in with:

GET /some/path HTTP/1.1
Host: example.com
Accept-Encoding: gzip

The response comes back with a Vary header on the Accept-Encoding:

HTTP/1.1 200 OK
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Type: application/json
Content-Length: 230
Cache-Control: max-age=10000000

The cache has to look at the Vary header, see that "Accept-Encoding" is a header on the request, and then store "Accept-Encoding: gzip" as a secondary key on the cache entry.

The implication is that the primary key entry of "GET http://example.com/some/path" is insufficient, and in order to get that specific response from cache, any subsequent requests should have a "Accept-Encoding: gzip" header in the same way.