HTTP 412 Precondition Failed vs 501 Not Implemented
HTTP 412 (Precondition Failed) is a 4xx Client Error response, while 501 (Not Implemented) is a 5xx Server Error response. 412 indicates that one or more conditions in the request headers (If-Match, If-Unmodified-Since) evaluated to false. In contrast, 501 means that the server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it.
Description
One or more conditions in the request headers (If-Match, If-Unmodified-Since) evaluated to false.
When You See It
When using conditional requests with ETags and the resource has changed.
How to Fix
Refresh the ETag/Last-Modified value and retry with updated preconditions.
Description
The server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it.
When You See It
When using an HTTP method (like PATCH) that the server doesn't support.
How to Fix
Use a different HTTP method that the server supports. Check server documentation.
Key Differences
412 is a 4xx Client Error response, while 501 is a 5xx Server Error response.
HTTP 412: One or more conditions in the request headers (If-Match, If-Unmodified-Since) evaluated to false.
HTTP 501: The server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it.
You encounter 412 when when using conditional requests with ETags and the resource has changed.
You encounter 501 when when using an HTTP method (like PATCH) that the server doesn't support.
When to Use Which
For 412 (Precondition Failed): Refresh the ETag/Last-Modified value and retry with updated preconditions. For 501 (Not Implemented): Use a different HTTP method that the server supports. Check server documentation.