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:
GET and HEAD methods, used in combination with a Range header, it can guarantee that the new ranges requested come from the same resource as the previous one.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 request header | No |
If-Match: <etag_value>
If-Match: <etag_value>, <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-by-byte. However, in an If-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.
If-Match: "bfc13a64729c4290ef5b2c2730249c88ca92d82d"
If-Match: "67ab43", "54ed21", "7892dd"
If-Match: *ETagIf-None-Match, If-Modified-Since, If-Unmodified-Since conditional request headers412 Precondition Failed