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
http
412 Precondition Failed
Examples
http
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:
http
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.
http
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 |
Browser compatibility
BCD tables only load in the browser
The information below has been pulled from MDN's GitHub (https://github.com/mdn/browser-compat-data).