I've just created a configuration composed of two servers.
Both just forwards traffic from different ports (and protocols) to different servers, and keep responses in cache. It works nice.

For both servers "GET /" request is sent, in the response there is proper resource. Thus (to distinguish responses in the cache) I added for both cache_key:

proxy_cache_key "$upstream_addr*$request";

But it doesn't work. I found that upstream states are not available and thus "proxied" address is not known when cache key is computed. And "proxied" address is computed only when additional stream must be used..

Is there any way to get proper key? Or any other way to keep all copies in the cache?

I plan to add "load balancing" module, which would be able to forward request on (multiple) source addresses to multiple proxied addresses, with caching.. Will it work properly?