How the If-Match HTTP Header Prevents Concurrent Write Conflicts

The If-Match HTTP header helps prevent lost updates when multiple clients edit the same resource simultaneously, a common problem in web applications. A server assigns an ETag version token to a resource on every GET request, and clients must include that token in subsequent PUT requests via the If-Match header. If the server's current ETag no longer matches the one sent by the client, it returns a 412 Precondition Failed response and rejects the write. This mechanism ensures that a client editing stale data cannot silently overwrite changes made by another client in the meantime. The comparison is strict and byte-for-byte, meaning weak ETags prefixed with W/ will never satisfy an If-Match condition.