IMS DB Interview Questions (11 - 20)

11. What are Twin Segments?

Twin segments are multiple segments of the same type under the same parent. For example - If a SCHOOL has many STUDENT segments, each STUDENT is a twin of the others that are in the same class.

12. What are logical twins?

Logical twins are segments that appear to be twins in a logical relationship (used when data is connected across different databases). For example - If a SCHOOL has many STUDENT segments, each STUDENT is a twin of the other student who is in different CLASS.

13. Define Segment Occurrence.

A segment occurrence is a specific instance of a segment type within the database. While a segment type defines the structure and attributes of the data, a segment occurrence contains the actual data values. For example - If STUDENT segments contain data for "John" and "Alice," each is a segment occurrence.

14. What is a PSB?

Program Specification Block (PSB): Defines which parts of the database a program can access. It contains PCBs (Program Communication Blocks) that specify permissions (read, insert, delete, etc.).

15. What is a PCB?

Program Communication Block (PCB): Part of the PSB that tells IMS:

  • Which database the program can use.
  • What operations it can perform (using PROCOPT).

16. What is PROCOPT? What is its use?

PROCOPT stands for Processing Option. It is a parameter used in PCBs and SENSEG statements within a PSB to specify the types of operations an application program is allowed to perform on a segment. Common PROCOPT values include:

  • G (Get) = Read data.
  • I (Insert) = Add new data.
  • D (Delete) = Remove data.
  • R (Replace) = Update data.

By setting appropriate PROCOPT values, you control the program's interaction with the database segments.

17. What is a DBD?

A Database Descriptor (DBD) defines the physical structure ("blueprint") of an IMS database. It describes the organization of data, including segment types, their hierarchical relationships, and storage details. The DBD is created using a process called DBDGEN (Database Generation). ​

18. What information does a DBD contain?

A DBD contains detailed information about the database structure, such as:

  • Segment definitions: Names and attributes of each segment type.
  • Field definitions: Details about the fields within each segment.
  • Hierarchical relationships: Parent-child relationships between segments.
  • Access methods: How data is stored and accessed (e.g., HDAM, HIDAM).

This information guides IMS in managing and accessing the database efficiently. ​

19. What is DBDGEN?

DBDGEN stands for Database Descriptor Generation. It's a process in IMS (Information Management System) used to define the physical structure of a database. During DBDGEN, you create a Database Descriptor (DBD), which specifies details like segment types, their relationships, and storage information.

20. Explain the DBDGEN process.

DBDGEN is the process of creating a Database Descriptor (DBD) using a series of macro statements that define the database's structure. The steps involved include:

  • Coding DBDGEN statements: Write macro statements specifying segment types, fields, relationships, and storage details.
  • Assembling the DBD: Use an assembler to process the macro statements and create the DBD module.
  • Storing the DBD: Place the assembled DBD into the IMS DBD library for use by IMS during database operations.