ETag.
also: ETags · conditional request · conditional requests · If-None-Match · 304 Not ModifiedA version tag on a response; the client sends it back in If-None-Match and gets a body-less 304 Not Modified when nothing changed.
the two-minute refresher
- First response: 200 with ETag "abc123" and a max-age.
- After expiry: the request carries If-None-Match: "abc123".
- Unchanged: 304 with no body, so the client reuses its copy. Changed: 200 with a new ETag.
- Saves bandwidth; it still costs a round trip.