When is change accumulation required?

Running the Database Change Accumulation utility is not required if, during the time period in question, only one system updated the database; using it periodically speeds any database recovery that becomes necessary. Alternately, you can run the Database Change Accumulation utility only when the need for recovery arises (just before running DB Recovery utility).

Change accumulation would not be required when non-concurrent data set update information exists in various logs. The database changes are received in the correct order if the logs are input serially to DB Recovery utility.

Change accumulation can be required when concurrent data set update information exists in various logs. The logs cannot be input to DB Recovery utility in a way that the change records are seen in the correct order.

Related reading: See IMS Version 15.5 Database Utilities for detailed instructions on running the Database Change Accumulation utility.

Input to the database change accumulation utility

In addition to stored SLDSs and RLDSs, you can use a previous change accumulation data set and other IMS log volumes as input. The utility writes the accumulated changes in a new change accumulation data set.

You can specify all log volumes or a subset of log volumes as input to the Database Change Accumulation utility. When you specify a subset of log volumes, DBRC determines whether the subset is complete for each DBDS or area. A subset of log volumes is complete for a DBDS or area when all of the following conditions are true:

The first volume in the subset is the earliest volume, that could possibly have changes to the DBDS, that were not included in the last change accumulation or in the last image copy.
The remaining volumes are in sequence.
In a data sharing environment, logs from all updating subsystems containing changes and any open data streams for a DBDS are included.

The DBRC LIST.CAGRP command indicates whether the log subset for each DBDS of the change accumulation group is complete.

You can use the change accumulation data set as input to a later run of the Database Change Accumulation utility whether your subset of log volumes is complete or incomplete; however, you can use a change accumulation data set as input to DB Recovery utility only if it represents a complete log subset.

Recommendation: If DBRC is being used, perform the change accumulation process using the GENJCL.CA command. This command creates the correct JCL and includes all unprocessed log data sets. If you use your own JCL instead, verify the specifications for change accumulation before execution.

An image copy of the specified database data set is needed and must be identified to the RECON data set in order to create a valid starting point for change accumulation records.

All changes since the last valid image copy are collected by the utility. If a time-stamp recovery has occurred since the last image copy, the change accumulation that is created is invalid for use in future recoveries. No error messages are generated by GENJCL.CA or by the execution of the utility.

You can run the Change Accumulation utility with a valid log subset at any time to reduce data to a minimum.

When the most recent image copy is used as input to the Change Accumulation utility and that image copy is a concurrent image copy, changes already made to the database by active applications might be missing from the copy because the changes might not have been physically written to the data set. These changes, however, have been written to the log. In this case, it is necessary to go back to some earlier point in the logs to ensure that all changes are applied. How far to go back depends on the type of database and which image copy utility was used.

The point-in-time selected to start the Change Accumulation utility is called the purge time.

Related reading: See IMS Version 15.5 Database Utilities for more information on the Database Change Accumulation utility.

Change accumulation groups

You can use the Database Change Accumulation utility to process some or all of your recoverable, registered databases as change accumulation (CA) groups. You can, for example, organize the DBDSs into the following groups:

Application-associated databases
Physical database clusters
Logical database clusters
Volatile (critical data) databases

You can add or delete members of a CA group after you have created it. A database can be a member in only one CA group. To move a member from one CA group to another CA group, you must first delete it from its current CA group and then add it to the new CA group.

Recommendation: Create an image copy of a CA-group member before you move that member to a new CA group. The change accumulation utility processes all logs for the member that are needed for recovery, based on the last good image copy.

A Change Accumulation group can contain from one to 32767 members, which must be registered DBDSs or areas.

Use the INIT.CAGRP command to create your Change Accumulation group. To modify your Change Accumulation group, use the CHANGE.CAGRP. A DBDS or area can belong to only one Change Accumulation group.

DBRC supports the Change Accumulation utility only by Change Accumulation group. Likewise, GENJCL.CA generates JCL only for a Change Accumulation group. The DBRC verification exit routine verifies for a whole Change Accumulation group.

You can run the Change Accumulation utility for DBDSs that do not belong to a Change Accumulation group even with DBRC=YES in effect. However, DBRC does not verify the input to the utility nor record its output.

Defining change accumulation data sets for future use

You can use the REUSE keyword on the INIT.CAGRP command to inform DBRC that you want to define a pool of data sets to receive the output from the Change Accumulation utility. Define the data sets using the INIT.CA command. The number of change accumulation data sets that you can define, can equal up to the value of the INIT.CAGRP command GRPMAX parameter that defined the group.

If you define a Change Accumulation group with the REUSE parameter and also use a GENJCL.CA command to generate the job for the Database Change Accumulation utility, data set reuse can occur. When all available change accumulation data sets for this Change Accumulation group have been used and the maximum number of change accumulation data sets has been reached, the next run of the Database Change Accumulation utility for this group reuses the change accumulation data set containing the oldest change records. To reuse a change accumulation data set means that its data set name, volumes, and physical space are used as if they were for an empty change accumulation data set.

If you define the Change Accumulation group with NOREUSE:

You must specify the output data set name for the utility either in the skeletal JCL member used for the GENJCL.CA command or in the JCL that you produce.
When you run the Change Accumulation utility and the number of data sets specified by GRPMAX has been reached, DBRC deletes the record of the change accumulation data set (from the RECON data set) that contains the oldest change records. The data set itself is not scratched. The data set must be manually scratched or monitored if you want to keep it, because DBRC no longer recognizes its existence.

Related reading: See IMS Version 15.5 Database Utilities and IMS Version 15.5 Commands, Volume 1: IMS Commands A-M for details on the Change Accumulation JCL specifications.