ETag and Conditional Requests

A validator that lets clients revalidate or protect updates to a representation.

Use ETag with If-None-Match for efficient reads and If-Match to prevent lost updates during writes.

httpcacheconcurrency

Reference (http)

ETag: "user-42-v7"
If-Match: "user-42-v7"

Use ETag with If-None-Match for efficient reads and If-Match to prevent lost updates during writes.

Common mistakes

Generating an unstable ETag or ignoring If-Match on a write endpoint that needs optimistic concurrency.

IANA registry: http-fields/field-names

Sources