Database versioning migration considerations

Database versioning introduces some specific considerations for migration.

If you are migrating an IMS system that uses database versioning to IMS 15.6, you must preserve in the catalog the DBD segment instances that define the prior version of any DBD version in use.

CAUTION:

If the segment instances for the prior version of a DBD are not preserved, any application programs that use that prior version of the DBD can no longer access the database.

For example, if your IMS system uses version 0 and version 1 of DBD DBD1A in IMS 14, both versions of the DBD1A DBD are stored as separate segment instances in the same DBD1A record in the IMS catalog. Version 1 of DBD1A reflects the actual physical structure of the database. Version 0 of DBD1A is a "virtual" structure that exists only in the IMS catalog.

In this case, when you migrate to IMS 15.6:

You regenerate version 1 of DBD1A by using the IMS 15.6 generation utilities. When you populate the IMS catalog, version 1 of DBD1A gets inserted as a new set of segments in the same record as all of the segment instances of DBD1A from IMS 14. The IMS 15.6 instance of version 1 now becomes the active version of the DBD and the IMS 14 instance of version 1 is no longer used.
You do not regenerate version 0 of DBD1A. Instead, you preserve the IMS 14 instance of the DBD1A version 0 in the IMS catalog. Application programs that use version 0 in IMS 15.6 continue to use the segment instances that were generated in IMS 14.