Backend & systems·Caching and invalidation
the client sends back the version tag it already has and you answer 304 with no body instead of resending the whole thing.
ETag
Also calledentity tag, conditional request, If-None-Match
An opaque version identifier for a response, which clients echo back so the server can answer "unchanged" rather than resend. It saves bandwidth without giving up freshness, because the client still asks every time. The same tag doubles as an optimistic concurrency token on writes via `If-Match`, which is the cheapest way to reject a lost update over HTTP.