412 Precondition Failed
The HyperText Transfer Protocol (HTTP)
412 Precondition Failed client error response code
indicates that access to the target resource has been denied. This happens with
conditional requests on methods other than GET or
HEAD when the condition defined by the
If-Unmodified-Since or If-None-Match headers is not
fulfilled. In that case, the request, usually an upload or a modification of a resource,
cannot be made and this error response is sent back.
Status
412 Precondition Failed
Examples
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
ETag: W/"0815"
Avoiding mid-air collisions
With the help of the ETag and the If-Match headers, you
can detect mid-air edit collisions.
For example, when editing MDN, the current wiki content is hashed and put into an
Etag in the response:
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
When saving changes to a wiki page (posting data), the POST request
will contain the If-Match header containing the ETag
values to check freshness against.
If-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"
If the hashes don't match, it means that the document has been edited in-between and a
412 Precondition Failed error is thrown.
Specifications
| Specification |
|---|
| HTTP Semantics # status.412 |