image/svg+xml some text some text has if-unmodified-since? has if-none-match? some text ... generate_etag has if-modified-since? has if-match? generate_etag last_modified true match* true not modified* true no match* false false, orinvalid modified* false 412 precondition failed middlewares no match* date is in the future? last_modified 304 not modified ... false, orinvalid match* method is GET/HEAD? true false true false true modified* not modified* generate_etag expires