If-None-Match HTTP request header makes the request conditional. For
HEAD methods, the server will send back the requested resource, with a
200 status, only if it doesn't have an
ETag matching the given ones. For other methods, the request will be processed only if the eventually existing resource's
ETag doesn't match any of the values listed.
When the condition fails for
HEAD methods, then the server must return HTTP status code 304 (Not Modified). For methods that apply server-side changes, the status code 412 (Precondition Failed) is used. Note that the server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary.
The comparison with the stored
ETag uses the weak comparison algorithm, meaning two files are considered identical if the content is equivalent — they don't have to be identical byte for byte. For example, two pages that differ by the date of generation in the footer would still be considered as identical.
When used in combination with
If-None-Match has precedence (if the server supports it).
There are two common use cases:
HEADmethods, to update a cached entity that has an associated
If-None-Matchused with the
*value can be used to save a file not known to exist, guaranteeing that another upload didn't happen before, losing the data of the previous put; this problem is a variation of the lost update problem.
If-None-Match: "<etag_value>" If-None-Match: "<etag_value>", "<etag_value>", … If-None-Match: *
"675af34563dc-tr34") and may be prefixed by
W/to indicate that the weak comparison algorithm should be used (This is useless with
If-None-Matchas it only uses that algorithm).
PUT, to check if another resource with the identity has already been uploaded before.
If-None-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d" If-None-Match: W/"67ab43", "54ed21", "7892dd" If-None-Match: *
|RFC 7232, section 3.2: If-None-Match||Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests|
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