VSAM assumes alternate indexes are always synchronized with the base cluster and does not check synchronization during open processing. Therefore, all structural changes made to a base cluster must be reflected in its alternate index or indexes. This is called index upgrade.

You can maintain your own alternate indexes or have VSAM maintain them. When the alternate index is defined with the UPGRADE attribute of the DEFINE command, VSAM updates the alternate index whenever there is a change to the associated base cluster. VSAM opens all upgrade alternate indexes for a base cluster whenever the base cluster is opened for output. If you are using control interval processing, you cannot use UPGRADE. See Processing Control Intervals.

You can define a maximum of 255 alternate indexes in a base cluster with the UPGRADE attribute.

All the alternate indexes of a given base cluster that have the UPGRADE attribute belong to the upgrade set. The upgrade set is updated whenever a base data record is inserted, erased, or updated. The upgrading is part of a request and VSAM completes it before returning control to your program. If upgrade processing is interrupted because of a machine or program error so that a record is missing from the base cluster but its pointer still exists in the alternate index, record management will synchronize the alternate index with the base cluster by letting you reinsert the missing base record. However, if the pointer is missing from the alternate index, that is, the alternate index does not reflect all the base cluster data records, you must rebuild your alternate index to resolve this discrepancy.

Note that when you use SHAREOPTIONS 2, 3, and 4, you must continue to ensure read/write integrity when issuing concurrent requests (such as GETs and PUTs) on the base cluster and its associated alternate indexes. Failure to ensure read/write integrity might temporarily cause "No Record Found" or "No Associated Base Record" errors for a GET request. You can bypass such errors by reissuing the GET request, but it is best to prevent the errors by ensuring read/write integrity.

If you specify NOUPGRADE in the DEFINE command when the alternate index is defined, insertions, deletions, and changes made to the base cluster will not be reflected in the associated alternate index.

When a path is opened for update, the base cluster and all the alternate indexes in the upgrade set are allocated. If updating the alternate indexes is unnecessary, you can specify NOUPDATE in the DEFINE PATH command and only the base cluster is allocated. In that case, VSAM does not automatically upgrade the alternate index. If two paths are opened with MACRF=DSN specified in the ACB macro, the NOUPDATE specification of one can be nullified if the other path is opened with UPDATE specified.