412 Precondition Failed
When the server responds with the HTTP status code 412 Precondition Failed, it means that the client-specified conditions in the request were not met, leading to the failure of the request.
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Introduction
In Action
Sample
Conclusion
Introduction
When the server responds with the HTTP status code 412 Precondition Failed, it means that the client-specified conditions in the request were not met, leading to the failure of the request.
This error informs the client that the server cannot process the request until the specified conditions are satisfied.
In Action
When the client receives the HTTP status code 412 Precondition Failed, it means that the conditions specified in the request were not met, and therefore, the resource was not in the expected state for further processing.
This error typically occurs for HTTP requests other than HEAD or GET methods. It is similar to the 304 Not Modified response, which occurs when the client already has the most recent version of a resource, and a new transmission would be unnecessary.
Sample
In the example, the client wants to post a reply to a blog post, but it needs to ensure that the post hasn’t been changed since the client last viewed it. The server checks this condition using the If-Unmodified-Since header, and if the post has been modified, it responds with the 412 Precondition Failed status to signal that the request cannot be completed.
By using this protocol, the problem of “lost updates” is avoided. Lost updates occur when multiple people are editing the same resource, and someone is working with an outdated version.
Ensuring that the resource hasn’t been modified guarantees that no one overwrites or modifies data that has changed since they last accessed it. This ensures that updates are properly managed, preventing conflicts and preserving the integrity of the content.
Request
POST /blog/update?postid=111&task=reply HTTP/1.1
Host: www.example.com
If-Unmodified-Since: Fri, 1 Jan 2021 00:00:00 GMT
Content-Type: text/plain
Content-Length: 45
< Message body contains reply-text from the client>
Response
HTTP/1.1 412 Precondition Failed
Content-Type: text/html
Content-Length: 182
< html>
< head>
< title>Blog Update Error< /title>
< /head>
< body>
< p>The post has changed since you last viewed it, so your reply will not be recorded.< /p>
< /body>
< /html>
Conclusion
The 412 Precondition Failed status code indicates that the client’s request cannot be completed because one or more conditions specified in the request have not been met. It is useful for preventing conflicts and “lost updates” when multiple users are accessing and modifying the same resource simultaneously.
By using this status code, the server ensures that clients have access to the most recent version of the resource before making changes, thus maintaining data integrity and avoiding potential data overwrite issues.
