HTTP conditional request

An HTTP conditional request carries one or more preconditions to be evaluated before applying its HTTP request method. The condition can ask whether the selected HTTP representation still has a previously observed entity tag. This makes the request depend on the state the client knows, rather than applying an operation unconditionally.^[ref]

For HTTP cache validation, suppose an earlier retrieval of /color stored the text blue with entity tag "v7". A later GET with If-None-Match: "v7" asks for the representation only if its tag no longer matches. If the selected representation still matches, the condition is false and the server returns status 304 instead of transferring the representation data again. If it has changed, an otherwise successful retrieval can return the new data.^[ref]

A second use prevents overwriting another client's work. Suppose two clients read version "v7". The first makes an update and the server assigns version "v8". The second submits a different PUT with If-Match: "v7". The stale tag does not match, so the server must not apply that update; in this example it returns 412. Checking and applying the condition as part of the operation prevents the second client from silently replacing the first client's changes.^[ref]

The two field names deliberately use different comparisons: If-Match requires strong entity tag comparison, while If-None-Match uses weak comparison and can match a weak entity tag. Failed If-Match need not always yield 412: the specification permits a success response when the requested change appears already applied. Authentication, redirects and other normal request checks also precede precondition evaluation. An HTTP conditional request therefore combines method semantics, a particular condition and the specified evaluation order.