Callback URL can be used during file import and will be fetched after successful or failed file import. It can be specified while importig files via API

http://wiki.ahcdn.com/doku.php?id=en:api_file_add

For example, callback URL like

http://www.example.com/cdn_callback.php?file=/1/2.mp4

If file has been fetched successfully, callback URL is appended by parameters

status=success&size=$file_size&sha-256=$chksum

http://www.example.com/cdn_callback.php?file=/1/2.mp4&status=success&size=1234567&sha-256=<chksum>

In case of failed import callback URL is appended by

status=fail&message=$error_text

http://www.example.com/cdn_callback.php?file=/1/2.mp4&status=fail&message=404%20error

'message' param will depend on error type.