Overview of Restore Initialization
In initializing a VSS restore operation, a requester needs to retrieve the Backup Component Document and each relevant Writer Metadata Document created and saved during the backup operation. The writer will have its current state queried in handling the Identify event that the requester generates. For more information, see Overview of Processing a Restore Under VSS.
The following table shows the sequence of actions and events that are required to initialize a restore operation.
| Requester action | Event | Writer action |
|---|---|---|
| Create an IVssBackupComponents interface, initialize it to manage a restore, and load stored requester metadata (see CreateVssBackupComponents, IVssBackupComponents::InitializeForRestore). | None | None |
| Call CreateVssExamineWriterMetadata to create IVssExamineWriterMetadata interfaces and load them with stored writer metadata. | None | None |
| Initiate asynchronous contact with writers (see IVssBackupComponents::GatherWriterMetadata.) | Identify | The writer begins event handling in support of the restore. Creates the Writer Metadata Document (see Working with the Writer Metadata Document, CVssWriter::OnIdentify, IVssCreateWriterMetadata). |
| The requester waits for writers to initialize by calling IVssAsync. | None | None |
Requester Actions during Restore Initialization
During the initialization phase of a restore, the requester needs to have access to the stored Backup Components Document and all the Writer Metadata Documents.
Depending on the implementation, this will mean either that the requester will require that backup media be mounted and readable, or that some other mechanism for accessing the stored metadata be available.
The requester uses the stored XML document containing the Backup Components Document of the requester that performed the backup to initialize its Backup Components Document using IVssBackupComponents::InitializeForRestore can access the information.
As was the case during the backup, the Backup Components Document has insufficient information to support a restore; therefore, the requester needs access to those Writer Metadata Documents stored during backup (see Use of Components by the Requester).
The requester retrieves the stored writer metadata by calling CreateVssExamineWriterMetadata for each writer whose data was backed up and now is to be restored. This function creates an IVssExamineWriterMetadata object for each writer and loads the writer's Writer Metadata Document into the object.
As was the case during the backup, to initiate cooperation between itself and the system's writers, a requester must generate an Identify event by calling IVssBackupComponents::GatherWriterMetadata. It is not necessary to call IVssBackupComponents::GatherWriterStatus following the completion of GatherWriterMetadata. Writers that fail to process the Identify event will not be included in the list of writers providing the metadata to be returned by IVssBackupComponents::GetWriterMetadataCount and IVssBackupComponents::GetWriterMetadata (see Determining Writer Status).
As with the backup operation, a requester will need to query and parse the information in the Backup Components Document and compare it to data in the Writer Metadata Documents to determine which components were backed up and to choose those to be restored (see Overview of Preparing for Restore). In addition, the requester will need to generate a detailed list containing information about the files on the backup media selected for restore, as well as how and where they are to be restored. (See Generating a Restore Set.)
Therefore, some backup applications may find it useful to have stored on the backup media their own list (in their own optimized format) of the files and their associated writer, component, restore procedure, and location information. This list can be used to minimize the amount of parsing and comparing of Writer Metadata Documents and the Backup Component Documents required to support a restore.
Writer Actions during Restore Initialization
Just as is done during a restore operation, in response to the Identify event, VSS calls each writer's virtual handler method CVssWriter::OnIdentify.
Note that applications other than the current requester (for instance, system applications) can generate Identify events, which must be handled by the writer. In addition, there is no way for a writer to determine from within CVssWriter::OnIdentify which application has generated the Identify event.
Given that a writer may receive several Identify events while processing a restore operation, writers should never set state information in the CVssWriter::OnIdentify handler. Instead, they must use the same algorithm for creating their Writer Metadata Document as was done during backup operations (see Writer Actions during Backup Initialization for more information).