Call sequence results

The same call sequence against MSDBs and other IMS databases might bring different results.

For parallel access to MSDB data, updates to MSDB records take place during sync point processing. Changes are not reflected in those records until the sync point is completed. For example, the sequence of calls GHU (Get-Hold-Unique), REPL (Replace), and GU (Get-Unique) for the same database record results in the same information in the I/O area for the GU call as that returned for the GHU.

The postponement of an updated database record to the point of commitment is also true of FLD/CHANGE calls, and affects FLD/VERIFY calls. You should watch for multiple FLD/VERIFY and FLD/CHANGE calls on the same field of the same segment. Such sequences can decrease performance because reprocessing results.

For terminal-related dynamic MSDBs, the following examples of call sequences do not have the same results as with other IMS databases or DEDBs:

• A GHU following an ISRT receives a 'segment not found' status code.
• An ISRT after a DLET receives a 'segment already exists' status code.
• No more than one ISRT or DLET is allowed for each MSDB in processing a transaction.

The preceding differences become more critical when transactions update or refer to both full function DL/I and MSDB data. Updates to full function DL/I databases and DEDBs are immediately available while MSDB changes are not. For example, if you issue a GHU and a REPL for a segment in an MSDB, then you issue another get call for the same segment in the same commit interval, the segment IMS returns to you is the “old” value, not the updated one.

If processing is not single mode, this difference can increase. In the case of multiple mode processing, the sync point processing is not invoked for every transaction. Your solution might be to ask for single mode processing when MSDB data is to be updated.

Another consideration for MSDB processing is that terminal-related MSDB segments can be updated only by transactions originating from the owners of the segment, the LTERMs. Programs that are non-transaction-driven BMPs can only update MSDBs that are declared as non-terminal-related.