A cached source and its variants were served until eviction, so a
changed image at the same URL was never picked up.
After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews the source with no download
or conversion. New bytes replace the source, and older variants are
converted again. A failed check serves the cached image and puts the
URL in the negative cache. This includes the full fetch made for a
variant whose source was evicted. Images not checked for 30 days are
evicted.
A variant file's modification time is its source's check time, so a
hit still makes no database call.
Model: opus-5-5
