IMS DB DBD- Mainframe

Learn how to create DBDs (Database Descriptions) in IMS (Information Management System) with this detailed explanation. This video, sourced from an IMS DB live training session, walks you through the process of defining databases for application programs.

Understanding DBDs & DBDGEN

A Database Description (DBD) is a crucial DL/I control block containing all the information an application needs to access data within an IMS database. The DBDGEN utility is the key tool for generating these descriptions. This utility is essential for IMS systems utilizing DBD libraries.

When is DBDGEN Used?

DBDGEN is primarily used in IMS systems that manage runtime application control blocks via a catalog. It may not be necessary in systems managing control blocks through a different method.

Creating a DBD: The Process

You create a DBD by coding specific macro instructions. These macros serve as input for the DBDGEN utility. The process involves defining:

• Database Information: The core details needed for application access.
• Physical and Logical Relationships: How different segment types relate to each other.
• Database Organization & Access Method: Specifying how data is structured and accessed.
• Physical Characteristics: Defining the database's physical attributes.
• Exit Routines: Defining the name and data options of selected exit routines.
• Metadata: Descriptive information about the database and its data.

Important Considerations:

• Each physical database can only have one physical DBD. Attempting to define multiple DBDs for the same physical database will likely result in an IMS abend (e.g., 0850, 0852, or 0853).
• At runtime, DL/I uses the DBD to construct internal control blocks, enabling data access.

Example DBD Command:

DBD IMS Database Name ROOTFIELD NAMEACCESS=HISAM BYTESSTARTTYPE

This example shows a basic DBD command structure. Understanding this structure is vital for effective DBD creation.

This video provides a practical understanding of DBD creation, a fundamental skill for working with IMS databases. Mastering DBDGEN is essential for developers working with IMS applications.