According to the document(Microsoft SQL Server Virtual Backup Device Specification), a VDI program have both client and server side.

But the sample is only the client side programing
(including simple,osimple,mthread,mprocess,snapshot etc.).

so my question is :
Where is the server side? Does the SQL Server Agent do everything?

If so,what's the use of those server side functions??? Is it useless??

Where could we use them??

Thanks for reply,^v^.