A restore is a multiphase process. The possible phases of a restore include the data copy, redo (roll forward), and undo (roll back) phases:

• The data copy phase involves copying all the data, log, and index pages from the backup media of a database to the database files.

• The redo phase applies the logged transactions to the data copied from the backup to roll forward that data to the recovery point. At this point, a database typically has uncommitted transactions and is in an unusable state. In that case, an undo phase is required as part of recovering the database.

• The undo phase, which is the first part of recovery, rolls back any uncommitted transactions and makes the database available to users. After the roll back phase, subsequent backups cannot be restored.