Fleet of Apache 2.4 reverse proxies on an AWS EC2 instance, directed to cache to an ElastiCache memcache cluster. Pretty typical stuff.
The CacheDetailHeader directive is on, and regularly sends the value "cache miss: cache unwilling to store response" from hostname.my.domain which is unusually resistant to Google searches.
I've verified connectivity between the instances and the cache cluster, and I can watch periodic "stats" interactions between an instance and the cluster, which seem successful. I can interact with the server over telnet, so that part's solid.
Thing is, the Apache server rarely even tries to "set" a value, which makes me think Apache's "willingness" to store a response is the problem.
A typical subset of response header from the upstream is like this for and HTTPS request of random-image.png
Cache-Control: public Cache-Control: max-age=31535000 ETag: "*(a value)*" Expires: Tue, 27 Nov 2018 18:38:04 GMT *(24 hours from now)*
No Set-Cookie, no request headers that might prevent a cache.
Ultimately the question is this: what else could persuade Apache to be "unwilling" to cache?