The If-Match HTTP request header makes a request conditional.

A server will only return requested resources for GET and HEAD methods, or upload resource for PUT and other non-safe methods, if the resource matches one of the listed ETag values. If the conditional does not match then the 412 (Precondition Failed) response is returned.

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 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.
• 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.