The "extern" medium name serves to perform database backups under the administration of a backup tool. xbackup writes the data to a named pipe. xrestore reads the data from a named pipe. An external archiver tool must call xbackup or xrestore as a synchronous function to have the data transferred.
Some archiver tools, e.g. NetVault, do not provide for calling synchronous functions, they only support a pre-function to initiate the data transfer and a post-function to terminate it. To connect xbackup or xrestore to such a tool, there must be an interface that is started as a pre-function of the tool and then calls xbackup or xrestore without waiting for termination. Then CONTROL and the archiver run in parallel, and when transfer is finished, the archiver calls a post-function.
The ADABAS version for Windows NT contains a NetVault.txt file in the %DBROOT%\misc directory. This file is a template for pre- and post-functions to interface between NetVault (or another similar archiver tool) and xbackup or xrestore. It contains a description of the steps the user must perform to transfer this template into the actual .bat files. For more information on this subject, see the "netvault reference information", Chapter "using named pipes".