Database Descriptor
A Database Descriptor (DBD) is a control block in IBM's Information Management System (IMS) that defines the physical structure of an IMS database. It specifies the database organization, access method, segments, fields, and their hierarchical relationships, enabling IMS to manage and access the data. A Database Descriptor (DBD) is an IMS control block that defines the physical structure of an IMS database. It specifies the database's organization, access method, segments, fields, and the hierarchical relationships between them. The DBD is generated from a source definition using the `DBDGEN` utility and is essential for IMS to manage and access the database.
Key Characteristics
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- Physical Database Definition: Describes the physical layout of an IMS database, including its segments, fields, and the pointers linking them.
- Hierarchical Structure: Reflects the inherent hierarchical organization of IMS databases, defining parent-child relationships between segments.
- Source Statements: Created from a set of IMS control language statements (DBD statements) that are assembled and link-edited into an executable load module.
- Access Method Specification: Defines the IMS access method to be used for the database, such as HDAM (Hierarchical Direct Access Method), HIDAM (Hierarchical Indexed Direct Access Method), HISAM (Hierarchical Indexed Sequential Access Method), or HSAM (Hierarchical Sequential Access Method).
- DBDLIB Storage: The compiled DBD load module resides in the IMS Database Definition Library (
DBDLIB), which is accessed by the IMS control region. - Mandatory for IMS: An IMS database cannot be accessed or managed without its corresponding DBD.
Use Cases
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- Database Creation: Defining the initial structure for a new IMS database before it can be loaded with data.
- Database Modification: Implementing changes to an existing database's structure, such as adding new segments, fields, or modifying pointer types.
- Application Development: Providing the necessary structural information to application programmers for developing programs that interact with the IMS database using DL/I calls.
- Database Utilities: Used by IMS utilities (e.g., database reorganization, recovery, or load utilities) to understand and process the database's physical layout.
Related Concepts
The DBD is fundamental to IMS, working in conjunction with other key components. It defines the physical database structure, while the Program Specification Block (PSB) defines the logical view (subset) of the database that an application program can access. Both DBDs and PSBs are combined during ACBGEN (Application Control Block Generation) to create ACBs (Application Control Blocks), which are optimized for performance and stored in the ACBLIB. DL/I (Data Language/I), the API for IMS, relies on the DBD to interpret and process database requests from application programs.
- Version Control: Maintain DBD source code under a robust version control system to track changes and facilitate rollback if necessary.
- Impact Analysis: Thoroughly analyze the potential impact of any DBD changes on existing application programs, other databases, and IMS utilities before implementation.
- Rigorous Testing: Test all DBD changes extensively in development and test environments to ensure data integrity and application compatibility before deploying to production.
- Documentation: Keep comprehensive documentation for each DBD, detailing its purpose, segment/field definitions, and any specific considerations or dependencies.
- ACB Generation (ACBGEN): Always perform
ACBGENafter any DBD or PSB changes to ensure that theACBLIBis up-to-date and consistent, preventing runtime errors.