Takeaways
- One or more conditions given in the request header fields evaluated to false when tested on the server.
- This response code allows the client to place preconditions on the current resource state (its current representations and metadata).
- Thus, this prevents the request method from being applied if the target resource is in an unexpected state.
Information
The 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.
Example
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 a wiki, 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.