DBD - Database Description
A DBD (Database Description) is an IMS (Information Management System) control block that defines the physical and, optionally, logical structure of an IMS database. It specifies the database's organization, segments, fields, access methods, and relationships between segments, serving as the blueprint for how IMS stores and retrieves data. A Database Description (DBD) is an IMS control block that defines the physical and logical structure of an IMS database. It specifies the database organization, access method, segment types, field definitions, and hierarchical relationships between segments, enabling IMS to manage and access the data.
Key Characteristics
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- IMS Specific: Exclusively used by IBM's IMS database management system to manage its hierarchical databases.
- Physical Structure Definition: Details the physical layout, including segment names, field names, data types, lengths, segment hierarchy, and the access method (e.g., HDAM, HIDAM, HISAM, HSAM).
- Logical Structure Definition: Can define logical relationships between segments, potentially across different physical databases, enabling complex data models and reducing data redundancy.
- Source Code Assembly: Created from source statements written in IMS control language (e.g.,
DBD,SEGM,FIELD,LCHILD), which are then assembled and link-edited into an executable load module. - Stored in ACBLIB/DBDLIB: The resulting load module is stored in an IMS
ACBLIB(Application Control Block Library) orDBDLIB(Database Description Library), which IMS uses at runtime. - Offline Utility Creation: DBDs are generated and maintained using the IMS
DBDGENutility, an offline process, rather than being defined dynamically.
Use Cases
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- New Database Creation: Defining the initial structure for any new IMS database, specifying its segments, fields, and how data will be physically stored and accessed.
- Database Structure Modification: Implementing changes to an existing IMS database's structure, such as adding new segments or fields, altering field lengths, or modifying logical relationships.
- Application Program Interface: Providing IMS with the necessary metadata for application programs (e.g., COBOL, PL/I) to interact with the database via DL/I calls, ensuring data integrity and correct access paths.
- Database Utilities: Used by IMS utilities (e.g., database recovery, reorganization, unload/reload) to understand the database's physical and logical layout for their operations.
Related Concepts
The DBD is fundamental to IMS and works closely with other components. A PSB (Program Specification Block) defines an application program's specific view of one or more IMS databases, referencing the underlying DBDs. Both DBDs and PSBs are assembled into load modules and stored in the ACBLIB (Application Control Block Library), which IMS loads into memory at runtime. DL/I (Data Language/I) calls from application programs are interpreted by IMS, which then uses the information from the DBDs (via the PSB) to navigate and manipulate the database's physical data.
- Version Control: Maintain DBD source code in a robust version control system (e.g., PDS, Endevor, CA-Librarian) to track changes, facilitate auditing, and enable quick rollback if issues arise.
- Thorough Documentation: Document the purpose of each segment, field, and logical relationship within the DBD source and external documentation to aid understanding and maintenance.
- Impact Analysis: Before making any changes to a DBD, perform a comprehensive impact analysis to identify all affected application programs, PSBs, and other related DBDs, especially for logical relationships.
- Rigorous Testing: Always test DBD changes extensively in a non-production environment, including running affected applications and relevant IMS utilities (e.g., unload/reload) to ensure data integrity and application compatibility.
- Minimize Changes: Design IMS databases carefully to reduce the need for frequent structural changes, as DBD modifications can be complex, time-consuming, and often require database reorganization and application retesting.