Media recovery includes all the measures which serve to safeguard the data for the entire life of the database. These measures include a preventive backup of the database, the maintenance of ALOG files, and database reconstruction.
A preventive backup involves saving all the data of a database at regular intervals. If, for example, a database disk fails, you can then return to this older backup status without losing all your data.
To avoid losing any updates made to the data between each backup, ALOG (archive log) files are maintained during operation. These files contain the data after it has been updated (after-images). If updates are made to the database but no ALOG file is maintained, a "logging gap" exists for this period of time.
ALOG files allow you to reconstruct database consistency, e.g. for the period between two backups for the entire database. This is done by using the UDS/SQL utility routine BMEND (UPDATE-DATABASE statement) to apply updates in the ALOG file(s) to a backup status of the database until the desired consistent status is achieved.