IMS catalog support for database versioning

The IMS catalog is required for database versioning.

Except for the version of the DBD that is designated in the IMS catalog as the active DBD, all prior versions of the DBD of a database exist only in the IMS catalog in the DBD record of the database.

When an application program makes a database call that requires a prior version of the DBD, IMS transforms the data that is retrieved from the database by using both the prior version of the DBD and the active DBD. When an application program makes a database call that does not require a prior version of the DBD, IMS uses only the active DBD to return the data.

Attention: If a prior version of a DBD is deleted from the catalog, that definition of the database is lost and application programs that use that version of the DBD can no longer access the database. Moreover, if the entire IMS catalog becomes unavailable, IMS can no longer provide access to any prior version of the database; only the application programs that use the version of the DBD that is active in the IMS catalog can access the database.

Back up and recover the IMS catalog by using the same backup and recovery procedures that are used for other databases. However, when database versioning is used, recovering the IMS catalog is more complex and can take longer because of the multiple DBD versions, PSBs, and ACB members that are involved.