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A high availability SQL Server BDC uses the Virtual Device Interface (VDI) to seed databases to secondary replicas. (https://docs.microsoft.com/en-us/sql/big-data-cluster/
VDI was introduced to SQL Server allowing custom providers to backup/restore data to/from SQL Server like a file interface would. The VDI interface is a client/server event driven model used for commands such as Read, Write, Flush, … When the SQL Server backup would have called the Win32 WriteFile API backup instead calls the server instance, WriteCmd and the client instance accepts and processes the write event.