A shadow database is a copy of an original database to which all updates are applied after a time delay. If the original database is destroyed, a shadow database can be used to quickly resume full database operation after it has been updated using the most recent after images (see section “Shadow database”).
During live database operation, you can perform so-called online backups using BS2000 operating system functions e. g. with the BS2000 utility routines ARCHIVE or HSMS . This makes it possible to create backups without impairing live operation.
Online backups created in this way are generally inconsistent unless administrative measures are undertaken to ensure that the database is consistent at the start of the backup operation and that no changes are made to it during the entire period of the backup.
When a backup has been made using BS2000, it can be updated by means of the corresponding log files and thereby be transformed into a shadow database. UDS/SQL knows internally which log files are required for this update (see section “Saving a database online”).