CAP - Change Accumulation Program

Enhanced Definition

A Change Accumulation Program (CAP) is an IBM IMS utility that processes IMS log records (OLDS/SLDS) to consolidate database changes that have occurred since a prior image copy or change accumulation run. Its primary purpose is to create a Change Accumulation Data Set (CADS), which significantly reduces the number of log records that need to be processed during a forward recovery of an IMS database.

Key Characteristics

- Input: CAP takes one or more IMS log data sets (Online Log Data Sets - OLDS, or System Log Data Sets - SLDS) as input, which contain all database updates, additions, and deletions.
- Output: The primary output is a Change Accumulation Data Set (CADS), which is a condensed record of all database changes for a specified period, effectively an "accumulated log."
- Efficiency for Recovery: By consolidating multiple log records for the same database block or segment into a single entry, CAP drastically reduces the I/O and CPU overhead required during a subsequent forward recovery operation.
- Database Scope: It operates on a per-database or per-database-area basis within the IMS environment, allowing granular control over which parts of the database are processed.
- DBRC Integration: CAP relies heavily on the Database Recovery Control (DBRC) component of IMS to manage log records, image copies, and CADS, ensuring data integrity and proper recovery chain management.
- Processing Logic: It applies changes in chronological order to a conceptual "base" (often an image copy point), effectively creating a cumulative update file that can be applied to an older image copy.

Use Cases

- Expedited Forward Recovery: The most critical use case is to accelerate the forward recovery of an IMS database. Instead of applying potentially millions of individual log records, recovery utilities can apply a single CADS and then only subsequent log records not included in the CADS.
- Database Refresh/Replication: CAP can be used to efficiently bring a backup copy of a database up to a more current state, or to refresh a test environment with recent production data without processing the entire log history.
- Disaster Recovery Planning: Regular execution of CAPs and shipment of CADS to a disaster recovery site ensures that databases can be recovered quickly and efficiently in the event of a catastrophic failure.
- Reducing Log Retention Requirements: While not its primary purpose, by consolidating changes, it can indirectly help manage the volume of log data needed for long-term retention for recovery purposes, as CADS can replace a large range of log records.

Related Concepts

CAP is intrinsically linked to IMS (Information Management System), specifically its database recovery mechanisms. It works in conjunction with IMS Log Data Sets (OLDS/SLDS), which are the source of all database changes. The output, a Change Accumulation Data Set (CADS), is then used by IMS Database Recovery Utilities for forward recovery. DBRC (Database Recovery Control) is central to CAP's operation, as it tracks all log data sets, image copies, and CADS, providing the necessary metadata for recovery and ensuring that the correct inputs are used. CAP effectively bridges the gap between raw log data and efficient database recovery, often starting from an Image Copy (IC).

Best Practices:

Regular Execution: Run CAP frequently enough to keep the volume of log records processed by each run manageable and to ensure CADS are current for recovery needs. The frequency depends on transaction volume and recovery point objectives (RPO).
DBRC Registration: Always ensure that CAP output (CADS)