DELTA IMS and IMS Online Change facility

Although DELTA IMS can coexist with the standard IMS Online Change facility, most sites rely solely on DELTA IMS for changes to databases, applications, transactions, and route codes. To ensure that DELTA IMS updates are not regressed by an online change, it is important to understand the relationship of these two facilities.

The standard IMS Online Change facility is invoked by the /MODIFY command. /MODIFY PREPARE compares the current system (in main storage) to the new system (in the new MODBLKS data set). When the comparison is made and the changing components have been quiesced, the /MODIFY COMMIT command is used to make the appropriate updates to the current IMS system. The result is a new current system.

DELTA IMS uses a much simpler technique. As changes are received, service routines in the IMS control region update the current system to the desired result on an item-by-item basis. The result is a new current system after each update.

IMS Online Change does not support changing VTAM terminals, LTERMs, and Fast Path databases. The status of DELTA IMS updates to these elements is not affected by an IMS Online Change. However, IMS Online Changes can affect the status of DELTA IMS changes to databases (including full function databases), applications, transactions, and route codes.

If you use DELTA IMS to change the IMS configuration and then use the IMS Online Change facility to implement other changes to the IMS configuration, all DELTA IMS changes must be temporarily regressed and reapplied for IMS Online Change to successfully complete. IMS automatically regresses and reapplies its changes except when messages are queued to elements added through DELTA IMS. In this case, the Online Change facility does not complete and IMS issues the message DFS3452, 'WORK IN PROGRESS.'

Warning

Mixing DELTA IMS with IMS Online Change requires careful consideration. Although system integrity is always preserved, element attribute settings implemented by DELTA IMS can be wiped out by a MODBLKS change. You can avoid these problems by incorporating DELTA IMS changes into the new MODBLKS IMSGEN.

When performing a MODBLKS IMSGEN, keep in mind that information in the MODBLKS data set that DELTA IMS uses to ensure system integrity contains a date stamp with no time stamp. Therefore, when you execute a DELTA list, you can perform only one MODBLKS IMSGEN or CTLBLKS IMSGEN on that day. Performing more than one MODBLKS IMSGEN or CTLBLKS IMSGEN in one day with active entries on the DELTA Log will produce unpredictable results. Switching the active MODBLKS data set several times with active entries on the DELTA Log will also produce unpredictable results.

Warning

DELTA IMS is designed to prevent automatic simple checkpoints during a /MODIFY by making a temporary override of 2,147,483,647 to the CPLOG parameter. However, an operator initiated checkpoint, (for example, /CHE, /DBR) taken during online change processing will result in an invalid checkpoint that will abend on the next IMS restart.

If an operator initiated checkpoint does occur during a /MODIFY, do not restart your system using that checkpoint. Restart IMS using a prior, valid checkpoint.

Related topic

Introduction-to-DELTA-Lists

 

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