Use of Components by the Requester
In addition to performing a backup or restore, and supervising shadow copies, a requester must manage information about the components of the writers it interacts with. Component selectability and logical path are used to include or exclude data from a backup, and to decide what component information is included in the Backup Components Document.
During backup operations, a requester imports writer metadata component data using the IVssBackupComponents::GatherWriterMetadata and IVssBackupComponents::GetWriterMetadata methods (see Overview of Backup Initialization for more information).
After examining writer information with the IVssExamineWriterMetadata interface, a requester decides which writers it will back up and, to a limited extent, which of a given writer's components it will back up.
When backing up a writer, a requester:
- Must explicitly include all of a writer's nonselectable for backup components without selectable for backup ancestors using IVssBackupComponents::AddComponent to add the component to the Backup Components Document
- May explicitly include any of a writer's selectable for backup components using IVssBackupComponents::AddComponent to add the component to the Backup Components Document
- If a selectable for backup component defines a component set, its explicit inclusion implicitly includes all the component set's members—whether selectable for backup or not. These components are not added to the Backup Components Document.
In adding a selectable for backup component or a nonselectable for backup components without selectable for backup ancestors to its Backup Components Document, a requester specifies the following:
- The instance of the writer managing the component
- The writer's class identifier
- The logical path of the component (which may be NULL)
- The component's name
If a component does not match the specification, an error will be returned.
If such a component exists, VSS creates an IVssComponent interface for the component in the Backup Components Document. This information will be accessible and modifiable by the writer and requester. For a selectable component that defines a component set, it describes not only properties of the component but also all the subcomponents it contains.
Information about implicitly added components is not available in the Backup Components Document. In addition, no file information is available in the Backup Components Document. To obtain that information, the requester will have to examine the Writer Metadata Documents (which will have already been read) in the context of the selected stored components in the Backup Components Document.
During restore operations, a requester should not import component information from the writers currently active on the system via IVssBackupComponents::GatherWriterMetadata, because the state of currently executing processes will not necessarily reflect the state of processes when a backup was made.
It should still generate an Identify event via IVssBackupComponents::GatherWriterMetadata, both to create an Identify event and to determine which writers are currently on the system and their status.
The requester retrieves the stored Backup Components Document during its initialization as well as stored Writer Metadata Documents (see Overview of Restore Initialization for more information) .
There are, however, some differences:
- If a component has already been explicitly included to the Backup Components Document during backup, if it is included for restore (either explicitly or implicitly), IVssBackupComponents::SetSelectedForRestore is used to explicitly add it to the Backup Components Document for restore.
- If a component was implicitly included to the backup, and is nonselectable for restore with no selectable for restore ancestors—which in the backup case would imply the need for explicit inclusion—the component is not explicitly included (that is, it is not added to the Backup Components Document using IVssBackupComponents::SetSelectedForRestore). Such a component should be considered implicitly selected for restore.
- Of those components implicitly selected for backup (whether that component was selectable for backup or not), only those that are selectable for restore can be added to the Backup Components Document using IVssBackupComponents::AddRestoreSubcomponent.
- Selectable for restore components may define a component set for restore—just as selectable for backup components do. This selectable for restore component then defines this component set for the restore operation.
A writer with no components explicitly selected for restore prior to the generation of a PreRestore event will not receive any VSS events.
Requesters and writers can access stored component information using the IVssComponent interface. Through the IVssComponent interface, writers can modify some of the settings of those of its components explicitly included in the Backup Components Document to support a restore (such as the restore target). If it defines a component set, writer modifications of an explicitly included component will propagate to its subcomponents. In addition, the interface provides a mechanism for passing information about restore success and failure between writer and requester.
As during backup, there is insufficient information in the Backup Components Document itself to implement the restore. Again, the Writer Metadata Documents will be required to supply information about the actual paths of files to be restored and to discover what nonselectable components are part of the selectable components component set and therefore need to be restored.
See Working with Selectability and Logical Paths for information about the types of selectability and their usage.
Information about each stored selectable component's parent writer can be found using IVssWriterComponentsExt::GetWriterInfo.
With this information, the Writer Metadata Documents can be queried and the matching document identified. Then, by using the logical path, the requester can identify the dependent nonselectable components for each selectable component—that is, identify all members of the selectable component's component set.
Using the IVssExamineWriterMetadata interface, the requester now has full component information—including path specification (from the IVssWMComponent interface)—for both selectable and nonselectable components it needs to back up or restore.
This is one reason why it is vital for a requester to save both the state of its own Backup Components Document and the Writer Metadata Documents of the writers it is backing up.
See Working with Selectability and Logical Paths for more detailed information.