Chapter 26: Key Takeaways - IMS Database and Transaction Management

Chapter Summary

IMS (Information Management System) is one of the oldest and most enduring database and transaction management systems in the mainframe world, first deployed by IBM in the late 1960s to support the Apollo space program. This chapter explored how COBOL programs interact with IMS through DL/I (Data Language/I) calls to navigate, retrieve, and update data stored in hierarchical databases. Unlike the relational model used by DB2, IMS organizes data as a tree of segments, where each segment type has a parent-child relationship that mirrors real-world hierarchies such as customers with orders, or departments with employees. Understanding IMS is essential for any COBOL developer working on legacy mainframe systems, as billions of transactions per day still flow through IMS databases worldwide.

The chapter covered the foundational building blocks of IMS programming: the Database Descriptor (DBD), which defines the physical structure of the database; the Program Specification Block (PSB), which defines the logical view a particular program has of one or more databases; and the Program Communication Block (PCB), which provides the status feedback mechanism after each DL/I call. We examined how COBOL programs issue DL/I calls such as GU (Get Unique), GN (Get Next), GNP (Get Next within Parent), REPL (Replace), DLET (Delete), and ISRT (Insert) to perform all database operations. Segment Search Arguments (SSAs) were covered in depth, showing how unqualified and qualified SSAs guide IMS to the correct segment occurrence within the hierarchy.

We also examined IMS DC (Data Communications), which provides transaction management for online IMS applications through Message Processing Programs (MPPs). The chapter discussed how MPPs receive messages from terminals, process them against IMS databases, and return responses -- all within the IMS transaction manager's control. Concepts such as commit points, rollback, and the role of the IMS control region in managing resources were addressed, providing a complete picture of how IMS functions as both a database manager and a transaction processing system.

Key Concepts

  • Hierarchical Data Model: IMS organizes data as an inverted tree of segment types, where each database has a single root segment and zero or more dependent (child) segment types beneath it, supporting up to 15 levels of hierarchy.
  • Database Descriptor (DBD): The DBD defines the physical layout of an IMS database, including segment types, field definitions, parent-child relationships, and the access method (HISAM, HIDAM, HDAM, HSAM) used for storage and retrieval.
  • Program Specification Block (PSB): The PSB defines the logical view of one or more databases that a particular application program is authorized to access, acting as a security and scope boundary.
  • Program Communication Block (PCB): Each PCB within a PSB represents a single database view and contains the status code field that the program must check after every DL/I call to determine success or failure.
  • DL/I Call Interface: COBOL programs interact with IMS through CALL 'CBLTDLI' statements, passing a function code, the PCB address, an I/O area, and optional Segment Search Arguments.
  • Segment Search Arguments (SSAs): SSAs direct IMS to the desired segment occurrence -- unqualified SSAs specify only the segment name, while qualified SSAs add field-level criteria using relational operators.
  • Get Calls (GU, GN, GNP): GU retrieves a specific segment by fully qualifying the path; GN retrieves the next segment in hierarchical sequence; GNP retrieves the next segment within the established parent.
  • Update Calls (REPL, DLET, ISRT): REPL replaces a segment's data after a successful Get-Hold call; DLET deletes a segment and all its dependents; ISRT adds a new segment occurrence to the database.
  • Status Codes: IMS returns a two-character status code in the PCB after each call -- spaces indicate success, 'GE' means segment not found, 'GB' means end of database, and other codes signal specific error conditions.
  • IMS DC and MPPs: Message Processing Programs run under IMS DC to handle online transactions, receiving input messages from the I/O PCB and sending responses back through ISRT calls to the I/O PCB.
  • Commit and Sync Points: IMS uses checkpoint and sync point mechanisms to ensure data integrity, committing all database changes as a unit of work or rolling them back if a failure occurs.
  • Secondary Indexing: Secondary indexes allow access to segments through fields other than the natural hierarchy path, improving retrieval flexibility without restructuring the database.
  • Logical Relationships: IMS supports logical relationships between segments in different physical databases, enabling data sharing and reducing redundancy across hierarchical structures.

Common Pitfalls

  1. Forgetting to check the PCB status code after every DL/I call. Unlike DB2's SQLCODE, IMS status codes are only two characters and are easily overlooked, but ignoring them can lead to processing invalid data or missing error conditions entirely.
  2. Using REPL or DLET without a preceding Get-Hold call. IMS requires that you issue GHU, GHN, or GHNP (the Hold variants) before any Replace or Delete operation. Attempting to update after a regular Get call results in an 'DJ' status code.
  3. Misunderstanding the hierarchical sequence. GN calls traverse the entire database in top-down, left-to-right order. Developers accustomed to relational databases often expect set-based retrieval and are surprised when GN walks through all segment types, not just the one they want.
  4. Building incorrect SSA qualification strings. SSAs have a rigid format with specific column positions for segment names (bytes 1-8), the command code area, and the qualification statement. Misalignment by even one byte produces unpredictable results or status code 'AJ'.
  5. Not understanding the difference between GNP and GN. GNP restricts retrieval to children of the current parent, while GN continues through the entire database. Using GN when GNP is intended causes the program to process segments belonging to unrelated parent occurrences.
  6. Failing to handle the 'GE' status code in loops. When iterating through segments, a 'GE' status code indicates no more occurrences were found. Not testing for this condition leads to infinite loops or attempts to process empty I/O areas.
  7. Neglecting to define the I/O area large enough for the largest segment. If the I/O area in WORKING-STORAGE is smaller than the segment being retrieved, IMS will overwrite adjacent memory, causing data corruption that may not manifest until much later in processing.
  8. Issuing ISRT calls with segments in the wrong hierarchical order. When inserting a path of segments, you must insert from root to leaf. Attempting to insert a child segment before its parent exists results in a 'GE' status code on the insert.

Quick Reference

      * DL/I CALL SYNTAX
       CALL 'CBLTDLI' USING DLI-FUNCTION
                             DB-PCB-MASK
                             IO-AREA
                             SSA1
                             SSA2.

      * FUNCTION CODES
       01  DLI-GU       PIC X(4) VALUE 'GU  '.
       01  DLI-GN       PIC X(4) VALUE 'GN  '.
       01  DLI-GNP      PIC X(4) VALUE 'GNP '.
       01  DLI-GHU      PIC X(4) VALUE 'GHU '.
       01  DLI-GHN      PIC X(4) VALUE 'GHN '.
       01  DLI-GHNP     PIC X(4) VALUE 'GHNP'.
       01  DLI-ISRT     PIC X(4) VALUE 'ISRT'.
       01  DLI-REPL     PIC X(4) VALUE 'REPL'.
       01  DLI-DLET     PIC X(4) VALUE 'DLET'.

      * PCB MASK
       01  DB-PCB-MASK.
           05  DBD-NAME          PIC X(8).
           05  SEG-LEVEL         PIC XX.
           05  STATUS-CODE       PIC XX.
           05  PROC-OPTIONS      PIC X(4).
           05  FILLER            PIC S9(5) COMP.
           05  SEG-NAME-FB       PIC X(8).
           05  LENGTH-FB-KEY     PIC S9(5) COMP.
           05  NUM-SENS-SEGS     PIC S9(5) COMP.
           05  KEY-FB-AREA       PIC X(50).

      * UNQUALIFIED SSA
       01  UNQUAL-SSA.
           05  SEG-NAME      PIC X(8)  VALUE 'CUSTOMER'.
           05  FILLER        PIC X     VALUE SPACE.

      * QUALIFIED SSA
       01  QUAL-SSA.
           05  SEG-NAME      PIC X(8)  VALUE 'CUSTOMER'.
           05  FILLER        PIC X     VALUE '('.
           05  FLD-NAME      PIC X(8)  VALUE 'CUSTID  '.
           05  REL-OPER      PIC XX    VALUE ' ='.
           05  FLD-VALUE     PIC X(10) VALUE '0001234567'.
           05  FILLER        PIC X     VALUE ')'.

      * STATUS CODE CHECKING
           IF STATUS-CODE = SPACES
               PERFORM PROCESS-SEGMENT
           ELSE IF STATUS-CODE = 'GE'
               SET END-OF-SEGMENTS TO TRUE
           ELSE
               PERFORM DLI-ERROR-HANDLER
           END-IF.

What's Next

Chapter 27 moves from database access to the operational environment that surrounds every COBOL program on the mainframe: Job Control Language (JCL). You will learn how JCL defines, schedules, and controls the execution of COBOL batch programs, including how to specify datasets, manage job steps, pass parameters, and handle conditional execution. Mastering JCL is essential because even a perfectly written COBOL program cannot run without a well-constructed JCL job stream to allocate resources and direct the operating system.