 Each standby database applies (replays) the arrived redo, resulting in synchronization with the primary database.If a database crashes, the recovery process has to apply all transactions, both uncommitted as well as committed, to the data-files on disk, using the information in the redo log files.Oracle must re-do all redo-log transactions that have both a codice_8 and a codice_2 entry (roll forward), and it must undo all transactions that have a codice_8 entry but no codice_2 entry (roll back).