IMS catalog fallback

An IMS system in which the IMS catalog is enabled can fall back to an earlier version of IMS.

If you are falling back to a prior version of IMS in which the IMS catalog was enabled, unless the DBD and PSB instances from the prior version were purged from the IMS catalog, you do not need to change the IMS catalog.

If the DBD and PSB instances from the prior version were not purged, fallback to the IMS catalog of the prior version is achieved by making the ACB library (ACBLIB) of the prior version the active ACBLIB again. The active ACBLIB determines which DBD and PSB instances are active in the IMS catalog.

If the DBD and PSB instances from the prior version were purged from the IMS catalog, you must repopulate the IMS catalog with the DBD and PSB instances from the prior version after you make the ACBLIB from the prior version active.

After fallback is complete, you can leave the IMS 15.4 DBD and PSB instances in the IMS catalog or you can purge them to save storage.

The following figure shows the IMS catalog before fallback. The ACBLIB for the new version is active. The new DBDs are used in the DMB pool of the new version of IMS and the new DBD instances in the DBD record in the IMS catalog are used.

Figure 1. DBDs that IMS uses before fallback
The figure shows two ACBLIBs: one for the new version of IMS and one for the prior version of IMS. The new version of the ACBLIB is active. An arrow points from the new version of the ACBLIB to the new version of IMS. The new version of IMS contains new DBDs in the DMB pool, which point to corresponding new DBD records in the IMS catalog.

The following figure shows the IMS catalog after fallback. The ACBLIB for the prior version is made active again. The prior version of IMS loads the prior DBDs into the DMB pool and uses the prior DBD instances in the DBD records in the IMS catalog.

Figure 2. DBDs that IMS uses after fallback
The figure shows two ACBLIBs: one for the new version of IMS and one for the prior version of IMS. The prior version of the ACBLIB is active. An arrow points from the prior version of the ACBLIB to the prior version of IMS. The prior version of IMS contains the prior DBDs in the DMB pool, which point to corresponding prior instances of the DBDs in the DBD record in the IMS catalog.