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ETag.

also: ETags · conditional request · conditional requests · If-None-Match · 304 Not Modified

A 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.

Where the course teaches it

W04 · Day 5Multi-Tier Cachinglearn it →

Related concepts

Cache-Control
Versioned keys