412 Precondition Failed
The HTTP 412 Precondition Failed
client error response status code indicates that access to the target resource was denied.
This happens with conditional requests on methods other than GET
or HEAD
when the condition defined by the If-Unmodified-Since
or If-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
Precondition failed
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
ETag: W/"0815"
Avoiding mid-air collisions
With the help of the ETag
and the If-Match
headers, you can prevent conflicts or mid-air collisions.
For example, when editing some wiki pages, content is hashed and put into an ETag
in successful responses:
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
When saving changes to a wiki page (posting data), the POST
request will contain the If-Match
header containing the ETag
values that the client stored from the last edit to check freshness of the resource on the server:
If-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"
If the hashes don't match, the document has been edited in-between and a 412 Precondition Failed
error is thrown.
Specifications
Specification |
---|
HTTP Semantics # status.412 |