Volume Shadow Copy Service (VSS) is a Windows service for capturing and creating snapshots called shadow copies. VSS, which operates at the block level of the file system, provides a backup infrastructure for Microsoft operating systems.
Windows VSS has three major components in addition to the service -- writer, requester and provider. The service sits logically in the center of the other components and handles communication between them.
VSS writer - Each VSS-aware application installs its own VSS writer to a computer during the initial installation.
VSS requestor - Any application that needs to quiesce data for capture can play the role of VSS requestor.
VSS provider - The provider creates and manage the shadow copies of data on the system.
Here's how VSS works: The VSS requestor announces that it needs to create a server snapshot. Prior to creating that snapshot, it queries the server to determine which VSS writers have been installed. (It needs this list so it can later instruct each writer to quiesce its associated application). Then, the VSS requestor instructs each VSS writer to accomplish whichever task is needed for data quiescence. After each VSS writer reports that it has completed pre-backup tasks, the VSS requestor instructs the VSS provider to create a snapshot. The provider tells the requestor where to go to locate the data it needs and the backup begins. When the VM backup is complete, the VSS requestor announces that it has completed its activities. This announcement instructs each VSS writer to perform any post-backup tasks necessary so the computer and its applications can return to regular operation.