Re replacement policy - that's why I mentioned that the file can be changed. You'll need an mtime/inode/time check on each request / periodically.
Cache can miss? I don't think that's required. It can miss in a general sense, as in it needs to be lazy loaded. But I'd still call it caching if you're getting a single value. For example, you can still cache whole front page with server-side push into the cache. You also can't miss in that case.
But yeah, that's just details. Cache/memory structure is a rather vague separation.
