Working with Alternate Locations during Restore
There are many reasons why a requester either should not, or may not, be able to restore files from backup media to their original location. For example, a restore method or target may require such a restoration, or the current restoration location may be occupied and unwritable.
To handle such cases, a writer may have defined an alternate location mapping, a nonstandard restore destination to be used for special circumstances.
The term alternate location mapping, as used with VSS, should not be confused with the term alternate path. Alternate location mappings are used only during restore operations, and refer to an alternate destination for restore operations. Alternate paths are used only during backup operations, and refer to an alternate source from which to back up.
To use alternate location mappings during restore, a requester would do the following (typically following the generation of a PreRestore event):
Using an instance of the IVssExamineWriterMetadata interface obtained by retrieving a stored writer, a requester uses the IVssExamineWriterMetadata::GetAlternateLocationMapping method to obtain a writer's alternate location mappings as instances of the IVssWMFiledesc interface.Note The requester uses IVssExamineWriterMetadata::GetAlternateLocationMapping, not IVssComponent::GetAlternateLocationMapping. The former returns those alternate location mappings available for use by a requester. The latter is used to indicate those alternate location mappings actually used by a requester.
The call to the IVssExamineWriterMetadata::GetAlternateLocationMapping returns an instance of the IVssWMFiledesc interface. This instance contains file set information—a path specified by IVssWMFiledesc::GetPath, a file specification returned through IVssWMFiledesc::GetFilespec, and a recursion flag obtained from IVssWMFiledesc::GetRecursive—matching one of the file sets added (using IVssCreateWriterMetadata::AddDatabaseFiles, IVssCreateWriterMetadata::AddDatabaseLogFiles, or IVssCreateWriterMetadata::AddFilesToFileGroup) to one of the components managed by the writer.
The value returned by IVssWMFiledesc::GetAlternateLocation is the alternate location mapping for this file set.
Alternate location mappings do not contain component information, so it will be necessary to compare the file set information (path, file specification, and recursion flag) obtained by calling IVssExamineWriterMetadata::GetAlternateLocationMapping to that contained by the writer's components.
This information can be found by iterating over the writer's components and calling IVssExamineWriterMetadata::GetComponent to get an instance of the IVssWMComponent interface and use IVssWMComponent::GetFile to obtain a IVssWMFiledesc instance containing the component file set information.
When the file set information returned by the instance of IVssWMFiledesc obtained from IVssExamineWriterMetadata::GetComponent and IVssWMComponent::GetFile matches that obtained from the IVssWMFiledesc instance derived from IVssWMFiledesc::GetAlternateLocation, the component managing the files with the specific alternate location mapping has been found.
Having located the component, the requester can determine the conditions under which an alternate location mapping should be used by doing the following:
- Examining the component's restore method, which is obtained by calling IVssExamineWriterMetadata::GetRestoreMethod.
- Checking to see if a restore target overrides the restore method, by calling
If the component found in the Writer Metadata Document had been explicitly included in the backup, the instance of the IVssComponent interface will correspond to that component. If the component had been implicitly included in the backup, then the instance of IVssComponent will correspond to the component defining the component set of which the component in the Writer Metadata Document is a subcomponent.
With this information, the requester can determine on a component-by-component basis if it needs to restore a given file set of a given component to a destination defined by the alternate location mapping.
When using an alternate location mapping, the requester respects the file set's file descriptor and recursive flag and uses the path provided by the alternate location mapping.
The requester indicates that it has used an alternate location mapping during a restore operation by calling the IVssBackupComponents::AddAlternativeLocationMapping with the file set's default location information, the alternate restore destination used, and a component name.
If the file set was managed by a component that was explicitly included in the backup, that component name will be used. If the file set was managed by a component that was implicitly included in the backup, then the name used will be that of the component defining the component set of which the component managing the file set is a subcomponent.
Writers verify whether file sets from one of their components were restored to an alternate location mapping by calling IVssComponent::GetAlternateLocationMapping.