If-Match
The If-Match
HTTP request header makes the request
conditional. For GET
and HEAD
methods, the server
will return the requested resource only if it matches one of the listed
ETags
. For 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 by byte only. If a
listed ETag
has the W/
prefix indicating a weak entity tag, this comparison
algorithm will never match it.
There are two common use cases:
-
For
GET
andHEAD
methods, used in combination with aRange
header, it can guarantee that the new ranges requested come from the same resource as the previous one. If it doesn't match, then a416
(Range Not Satisfiable) response is returned. -
For other methods, and in particular for
PUT
,If-Match
can 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, the412
(Precondition Failed) response is returned.
Header type | Request header |
---|---|
Forbidden header name | no |
Syntax
If-Match: <etag_value> If-Match: <etag_value>, <etag_value>, …
Directives
- <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 byW/
to indicate that they are "weak", i.e. that they represent the resource semantically but not byte-by-byte. However, in anIf-Match
header, weak entity tags will never match. *
-
The asterisk is a special value representing any resource.
Examples
If-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d" If-Match: "67ab43", "54ed21", "7892dd" If-Match: *
Specifications
Specification |
---|
Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests # header.if-match |
Browser compatibility
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