If a requester accesses data in your bucket but the data does not exist in your bucket, 404 Not Found is returned. If you configure back-to-origin rules that contain a valid origin for the bucket, the requester can obtain the data based on the rules. You can configure mirroring-based or redirection-based back-to-origin rules for hot migration and specific request redirection.
Principles and scenarios of the mirroring-based and redirection-based back-to-origin rules:
Mirroring-based back-to-origin: After you configure mirroring-based back-to-origin rules for a bucket, if the object that a requester accesses does not exist in your bucket, OSS obtains the object from the origin specified by the back-to-origin rules. OSS returns the object obtained from the origin to the requester and stores the object in the bucket. Mirroring-based back-to-origin is used to seamlessly migrate data to OSS. For more information, see Configuration examples of mirroring-based back-to-origin.
Redirection: After you configure redirection-based back-to-origin rules for a bucket, if an error occurs when a requester accesses the bucket, OSS redirects the request to the specified URL or path. You can use this feature to redirect requests for objects and develop various services based on redirection.
For more information, see FAQ about back-to-origin.