Introduction to DELTA Logs
A key component of DELTA IMS is the DELTA Log. The DELTA Log is a pair of formatted BSAM data sets used to record all DELTA IMS updates to an IMS control region. When an existing IMS element is revised, the log record contains both the before and the after update attributes of the element. The DELTA Log can be used to record IMS operator commands and responses issued from DELTA IMS. Log records are used during IMS initialization to reapply all active DELTA IMS updates to IMS and are also used in a variety of reports that provide a comprehensive audit trail of DELTA IMS activity.
There is a pair of DELTA Logs for each IMSID defined to DELTA IMS, each serving as a backup for the other. Both data sets must be available and be without errors to perform DELTA IMS updates to the IMS control region. Only one log data set need be available for IMS restart, log reporting, and log utility functions.
You supply the data set names of the log data sets on the IMSID Options panel. You can allocate and format the DELTA Logs as a sub-function of this panel or by using the Log utility. When DELTA IMS has been activated on an IMS system, the DELTA Log data sets must be available before an IMS restart. Refer to the installation guide for information on naming the DELTA Logs.
You can allocate the log data sets for DELTA IMS use in two ways. The first way is to let DELTA IMS determine which log data sets are required for the IMSID in question by obtaining log data set names from the IMSID basic options module for the respective IMSID. This way, you need not be concerned about data set allocation. DELTA IMS dynamically allocates the required data sets and frees them when no longer needed. The second way is for you to specifically allocate log data sets (for example, with DD cards or the TSO ALLOCATE command). The ddnames used for this allocation are BMCDLAL1 and BMCDLAL2. If these ddnames are specified, DELTA IMS uses these data sets instead of using dynamic allocation.
DELTA IMS updates to IMS are written to the log data sets by BMCLINK’s BMP task (either VTAM or non-VTAM). To ensure integrity, a temporary log record is written before the IMS update. After the update, the temporary record is replaced with the before and after log records that reflect the IMS element’s before update and after update attributes. If the update was unsuccessful, the temporary record is erased.
If the DELTA IMS log data sets are to be accessed from multiple CPUs in a shared-DASD environment, they must be protected from simultaneous modifications. A systems-level ENQUEUE using the 8-byte qname of DELTA IMS and a 44-byte rname containing the log dsname is used for brief periods to synchronize individual I/O operations within each system. This same ENQUEUE must be propagated to all systems in a shared-DASD environment using a product such as IBM’s GRS package or its equivalent.
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