The 412 status code, also known as “412 Precondition Failed”, is a client error response indicating that access to a requested resource has been denied. When a client makes a request with specific conditions and these conditions are not met, the server returns HTTP status 412 Precondition Failed.
This commonly occurs during resource modifications or uploads, where the server needs to ensure data consistency and prevent conflicts between multiple users trying to modify the same resource simultaneously.
What Causes the 412 Status Code?
The 412 response code typically occurs in several scenarios:
- Resource Version Mismatch
When the ETag (entity tag) sent in the request does not match the current version on the server, indicating the resource has been modified since the client last accessed it.
- Conditional Headers
When requests containing If-Match or If-Unmodified-Since headers fail their conditions. This is particularly important in preventing “mid-air collisions”, where many users attempt to modify the same resource simultaneously.
- Resource State Changes
When the requested resource’s state does not match the client’s expectations, it often occurs during content updates or file uploads.
Example
Here’s a practical example of a request and response resulting in a 412 Precondition Failed:
Request:
POST /document/edit/123 HTTP/1.1
Host: example.com
If-Match: "abc123def456"
Content-Type: application/json
Content-Length: 50
{
"title": "Updated Document Title",
"content": "New content"
}
Response:
HTTP/1.1 412 Precondition Failed
Content-Type: application/json
Content-Length: 89
{
"error": "Document has been modified since the last retrieval"
"current_etag": "xyz789uvw321"
}
In this example, the client attempts to update the document but includes an outdated e-tag. The server recognises the document has been modified and responds with 412 status, preventing potential loss or conflicts.
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