When IBM made IMS Managed ACBs mandatory with the release of IMS 15.6 in June 2025, it marked the end of a long transition period and the beginning of a new operational reality for IMS customers.
At a recent Planet Mainframe Virtual IMS User Group, Anshul Agrawal, Senior Manager of Product Development at BMC Software, stepped back to examine where the IMS community stands today—what the mandate has changed in practice, where teams are still adjusting, and what comes next as IMS continues its modernization journey.
Six months in (or close enough), one thing is clear: IMS Managed ACBs are no longer a future requirement. They are now the foundation for how IMS environments are managed.
Why IMS Managed ACBs Exist
IMS Catalog was introduced with IMS Version 12 in 2011 to address a long-standing challenge: IMS metadata lived in too many places. Database and program definitions were spread across DBD and PSB source, COBOL copybooks, PL/I includes, assembler structures, and ACB libraries. Over time, this fragmented model became increasingly difficult to manage.
As organizations adopted Java access, modern tooling, and mixed-language development, the need for a centralized, authoritative source of metadata became unavoidable. IMS Catalog addressed that need by consolidating metadata into a single system of record, implemented as a HALDB to support long-term growth and scale.
IMS Managed ACBs extend this model by moving control block management out of static libraries and into catalog-driven directory datasets. This is not a cosmetic change—it is a structural shift that enables IMS to evolve.
The Real Driver: DDL
While centralized metadata and Java access were early motivations, the long-term driver behind IMS Managed ACBs is support for DDL.
IBM’s direction is to allow IMS databases and program views to be managed using CREATE and ALTER statements, much like Db2 schemas today. Traditional workflows—DBDGEN, PSBGEN, ACBGEN, and library promotion—are transitional. The intended end state is a DDL-driven IMS, where schema changes are defined declaratively, staged through the catalog, tested safely, and activated in a controlled manner.
IMS 15.6 does not require customers to use DDL yet, but it does require the infrastructure that makes DDL possible.
What Changed with IMS 15.6
With IMS 15.6, IMS Managed ACBs became mandatory. If an IMS control region is not configured with a valid catalog and directory setup, it will not initialize.
To help manage the transition, IBM introduced marker PTFs that control when IMS 15.6 behavior is activated. This allowed customers to remain on IMS 15.5 behavior while continuing to receive maintenance, provided they had not applied the marker PTFs. Once applied, however, configuration validation becomes significantly stricter. Issues that might have gone unnoticed in earlier releases can now result in startup failures.
The takeaway for customers is straightforward: catalog discipline is no longer optional.
Life Between Classic IMS and Full DDL
Most IMS organizations today are operating in a hybrid state. They have migrated to IMS Catalog and enabled Managed ACBs, but they are not yet using DDL for everyday schema changes.
In this transitional phase, teams often continue to update traditional source definitions, generate control blocks, populate the catalog, stage changes in the directory, test those changes using unload/reload or vendor tools, and then activate them using IMPORT commands or change management utilities.
This approach is more complex than the historical library-based model, but it represents an intentional bridge between classic IMS operations and a fully DDL-driven future.
Shared and Non-Shared Catalog Considerations
IMSplex environments introduce additional decisions around whether catalogs and directories should be shared across IMS systems or maintained separately.
Shared catalogs simplify administration and consistency, while non-shared catalogs can reduce perceived single points of failure at the cost of greater operational complexity. Neither approach is universally correct. What matters is discipline—ensuring that catalogs and directories remain synchronized, recoverable, and well-governed in multi-IMS environments.
Treat the Catalog as Mission-Critical
One of the strongest themes from the session was the need to treat IMS Catalog like any other mission-critical IMS database.
That means planning for regular backups, reorganization to manage fragmentation, maintenance for structural updates, recovery procedures, and the cleanup of obsolete metadata. Many organizations discovered during migration that decades of unused DBDs and PSBs were carried forward into the catalog, creating unnecessary clutter and risk.
The move to Managed ACBs provides an opportunity to clean house before DDL becomes the only supported change mechanism.
The Bottom Line
IMS Managed ACBs are not simply a technical requirement tied to a specific release level. They represent a fundamental shift in how IMS systems are defined, changed, and governed.
Six months into the mandate, most organizations are still adapting. But the direction is clear: catalog-first IMS is here, DDL is the future, and operational rigor around metadata matters more than ever.
The good news is that IMS retains its core strengths—performance, resilience, and scalability—while gaining a modernization path that aligns it more closely with contemporary database practices.
This article is based on “IMS Managed ACB Mandate After Six Months: Adapting to the New Normal” presented by Anshul Agrawal at the Planet Mainframe Virtual IMS User Group.
Watch the session and download a copy of the presentation here!
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