IMS DB Application control blocks (ACBs)
An ACB is the run-time control block which is a consolidation of PSBs and DBDs.
In another words, Application control blocks (ACBs) are created by merging information from PSBs and DBDs.
For online program ACB must be build before using ACB maintenance utility.
ACBGEN ran for online programs, which merges the DBD and PSB and creates the ACB.
For Batch programs, ACBs can be built dynamically.
IMS dynamically creates the ACB from PSBLIB and DBDLIB as input (PARM=DL/I) for batch programs.
IMS can also use prebuilt ACB from ACBLIB for batch programs (PARM=DBB).
While IMS executing an IMS program, it uses ACB.
Using prebuilt ACBs require less time to schedule an application program and use less storage.
The ACB maintenance utility also provides some error-checking capability.
ACBs can be built for all PSBs reference a particular DBD or ACBs can be built for a specific PSB reference t particular DBD.
Prebuilt ACBs are stored in the IMS ACBLIB library.
ACBGEN is known as Access Control Blocks Generator.
It is used to generate ACBs.
During ACB generation, the ACB maintenance utility should have exclusive control of the IMS ACBLIB.
The utility must be executed using an IMS ACBLIB that is not currently allocated to an active IMS system.
Otherwise, the process of ACB generation will fail.
So to correct this, execute the ACB maintenance utility against an inactive copy of ACBLIB, then use the IMS Online Change function to make the new members available to an active IMS online system.
Below diagram explains about the process in detail.
IMSDB ACBGEN Utility
IMSDB Program Specification Block IMSDB Program Coding