ETag: version identifiers for conditional requests

The value is opaque: clients never parse it, they only compare it. A server can build it from a content hash, a database row version, or a modification timestamp plus a size, and the choice is invisible to the client. A hash is the most reliable because it cannot say unchanged when the bytes have in fact changed.

A weak tag, written with the W/ prefix, says two representations are equivalent for caching even if the bytes differ. That is what you want when a page embeds a timestamp or a rotating advertisement that nobody needs to refetch. Strong tags promise byte identity, which is what range requests need before they can stitch a partial download onto a stored copy.

The tag drives two very different flows. Sent back in If-None-Match on a GET, it produces 304 Not Modified and saves the body. Sent back in If-Match on a PUT or PATCH, it produces 412 Precondition Failed when somebody else has already written, which is the standard way to stop two editors silently overwriting each other.

A representation includes its encoding, so a gzipped and an uncompressed copy are different representations and should not share a strong tag. Servers that compress at the edge often weaken the tag automatically for exactly this reason, which is why an ETag can gain a W/ prefix in production that it never had locally.