Database recovery

The primary methods for recovering a database after a failure are forward recovery and backward recovery or backout. Two methods of database recovery are possible for each type of recovery.

Forward recovery involves reconstructing the database from a copy of the database made prior to the database failure. Forward recovery uses the information you have been keeping, such as image copies, logs, and so forth, and reapplies it to the database copy. It is based on the notion that if you knew what the data was like at one time and you know what you have done to it since then, you can process the data to return the database to the state it was in just before it was lost.

IMS supplies two methods for the forward recovery of databases:

• The Database Recovery utility (DFSURDB0).
• If you have the Database Recovery Facility or Online Recovery Service products installed, you can use the /RECOVER command.

Backout allows you to remove incorrect or unwanted changes from existing information or work without rebuilding from a prior copy of the database.

There are two types of database backout:

• Dynamic backout, in which IMS automatically backs out changes to a database, usually after an application program error.
• Database batch backout, in which the Batch Backout utility (DFSBBO00) removes database changes made by IMS batch jobs and online programs.