If-Match
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The HTTP If-Match
request header makes a request conditional.
A server will return resources for GET
and HEAD
methods, or upload resource for PUT
and other non-safe methods, only if the resource matches one of the ETag
values in the If-Match
request header.
If the conditional does not match, the 412 Precondition Failed
response is returned instead.
The comparison with the stored ETag
uses the strong comparison algorithm, meaning two files are considered identical byte-by-byte.
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. - 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.
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. Note that this must match as
false
if the origin server does not have a current representation for the target resource.
Examples
If-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d"
If-Match: "67ab43", "54ed21", "7892dd"
If-Match: *
Specifications
Specification |
---|
HTTP Semantics # field.if-match |
Browser compatibility
BCD tables only load in the browser
See also
ETag
If-None-Match
,If-Modified-Since
,If-Unmodified-Since
conditional request headers412 Precondition Failed