Request HeaderIf-None-Match
Beispiel:
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
Original Beschreibung (engl.):
The If-None-Match request-header field is used with a method to make
it conditional. A client that has one or more entities previously
obtained from the resource can verify that none of those entities is
current by including a list of their associated entity tags in the
If-None-Match header field. The purpose of this feature is to allow
efficient updates of cached information with a minimum amount of
transaction overhead. It is also used to prevent a method (e.g. PUT)
from inadvertently modifying an existing resource when the client
believes that the resource does not exist.
As a special case, the value "*" matches any current entity of the
resource.
If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
If any of the entity tags match the entity tag of the entity that
would have been returned in the response to a similar GET request
(without the If-None-Match header) on that resource, or if "*" is
given and any current entity exists for that resource, then the
server MUST NOT perform the requested method, unless required to do
so because the resource's modification date fails to match that
supplied in an If-Modified-Since header field in the request.
Instead, if the request method was GET or HEAD, the server SHOULD
respond with a 304 (Not Modified) response, including the cache-
related header fields (particularly ETag) of one of the entities that
matched. For all other request methods, the server MUST respond with
a status of 412 (Precondition Failed).
See section 13.3.3 for rules on how to determine if two entities tags
match. The weak comparison function can only be used with GET or HEAD
requests.
If none of the entity tags match, then the server MAY perform the
requested method as if the If-None-Match header field did not exist,
but MUST also ignore any If-Modified-Since header field(s) in the
request. That is, if no entity tags match, then the server MUST NOT
return a 304 (Not Modified) response.
If the request would, without the If-None-Match header field, result
in anything other than a 2xx or 304 status, then the If-None-Match
header MUST be ignored. (See section 13.3.4 for a discussion of
server behavior when both If-Modified-Since and If-None-Match appear
in the same request.)
The meaning of "If-None-Match: *" is that the method MUST NOT be
performed if the representation selected by the origin server (or by
a cache, possibly using the Vary mechanism, see section 14.44)
exists, and SHOULD be performed if the representation does not exist.
This feature is intended to be useful in preventing races between PUT
operations.
Examples:
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
The result of a request having both an If-None-Match header field and
either an If-Match or an If-Unmodified-Since header fields is
undefined by this specification.
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