ETag

A fingerprint of the response body, so the next request can ask “has it changed?” and get a tiny 304 instead of the whole thing.

response header

Example

ETag: "5f2b-1a2c3d4e"

What it does: A fingerprint of the response body, so the next request can ask “has it changed?” and get a tiny 304 instead of the whole thing.
In practice: The client sends the fingerprint back in If-None-Match; if it still matches, the server answers 304 Not Modified with no body. Two gotchas: a load-balanced fleet must compute identical ETags for identical content or caching silently dies (some servers derive them from inode numbers, which differ per machine); and a weak ETag (prefixed W/) promises only semantic equality, which byte-range requests cannot use.

Works with