If-Match HTTP request header makes the request
HEAD methods, the server
will send back the requested resource only if it matches one of the listed
PUT and other non-safe methods, it will only
upload the resource in this case.
The comparison with the stored
ETag uses the strong comparison
algorithm, meaning two files are considered identical byte to byte only. If a
ETag has the
W/ prefix indicating a weak entity tag, it
will never match under this comparison algorithm.
There are two common use cases:
HEADmethods, used in combination with a
Rangeheader, it can guarantee that the new ranges requested comes from the same resource than the previous one. If it doesn't match, then a
416(Range Not Satisfiable) response is returned.
For other methods, and in particular for
If-Matchcan be used to prevent the lost update problem. It can check if the modification of a resource that the user wants to upload will not override another change that has been done since the original resource was fetched. If the request cannot be fulfilled, the
412(Precondition Failed) response is returned.
|Header type||Request header|
|Forbidden header name||no|
If-Match: <etag_value> If-Match: <etag_value>, <etag_value>, …
Entity tags uniquely representing the requested resources. They are a string of ASCII characters placed between double quotes (like
"675af34563dc-tr34"). They may be prefixed by
W/to indicate that they are "weak", i.e. that they represent the resource semantically, but not byte-for-byte. However, in an
If-Matchheader, weak entity tags will never match.
The asterisk is a special value representing any resource.
If-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d" If-Match: "67ab43", "54ed21", "7892dd" If-Match: *
|Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests (HTTP/1.1)|
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