During normal operation, the zenon Service Engine continuously accesses different runtime files for reading and writing. A backup software that attempts to create a backup of a file that is currently accessed by the Service Engine may receive an error message when that file is currently in use by the zenon Service Engine. The backup of the file will fail.
A backup software may have a retry counter for files that are in use, which may allow successful backup of such files. It may also use a feature of the Windows Operating System called "Volume Shadow Copy Service" or VSS, which provides a Windows mechanism for creating backups of files that are in use. The use of VSS requires NTFS volumes, local drives and administrator or backup operator privileges.
A backup software that uses VSS is a VSS requestor. The Windows VSS provider handles the request from e.g. the backup software. Application software that supports VSS and that has constant open files, like database applications, can register itself as VSS writer with the VSS provider.
The Windows VSS provider will contact on behalf of the requestor (backup software) the VSS writer (e.g. database application) to request to flush its data buffer to disk and halt IO write operations until the shadow copy is created. The VSS requestor (backup software) can then access the shadow copy to create a consistent backup.
zenon software platform products are not "VSS aware" (they are not VSS writers). This means that if VSS creates a volume shadow copy, zenon does not write unsaved data to the disk prior to the creation of the volume shadow copy. For a fully consistent backup, where also unsaved data has been written to the disk, the Service Engine must therefore be closed.
VSS however can still be used to create a backup of Service Engine data including files that are currently in use. This can be done using backup software that utilizes VSS, manually by using Microsoft tools like "dosdev.exe", "robocopy.exe", "vshadow.exe" and "mountvol.exe" or by using open source tools like ShadowSpan, that integrate the functions of these tools.