If-Match

The If-Match HTTP request header makes the request conditional. For GET and HEAD methods, the server will send back 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 to byte only. If a listed ETag has the W/ prefix indicating a weak entity tag, it will never match under this comparison algorithm.

There are two common use cases:

For GET and HEAD methods, used in combination with a Range header, 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 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, the 412 (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 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-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	Title
RFC 7232, section 3.1: If-Match	Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

Browser compatibility

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