HTTP ETags: revalidation and optimistic concurrency

Revalidate a stored representation

An ETag identifies a selected representation according to the server's rules. A client can send If-None-Match to check whether its stored response is still usable.

On an unchanged GET, a 304 response carries no new representation body; the client uses its stored content.

GET /document
If-None-Match: "v3"
Unchanged response: 304 Not Modified

Protect an update with If-Match

For optimistic concurrency, send the validator of the version you edited. If it no longer matches, the server can reject the update with 412.

This prevents overwriting a concurrent change only when the server actually enforces the precondition.

PUT /document
If-Match: "v3"
Server now has "v4" → 412 Precondition Failed

Understand weak validators

An ETag beginning W/ is weak. It can support semantic revalidation, but If-Match requires strong comparison.

Do not remove W/ yourself to turn a weak validator into a strong one. Preserve the exact server-provided quoted value.

Inspect from an authorized client

Use the HTTP client only when explicitly running a request to a destination you control or may test. Cross-origin scripts may need ETag exposed through CORS.

A browser hiding the header is different from a server omitting it. Compare the network response and the readable client headers.