IMS DB Realtime (Scenario based) Interview Questions (1 - 10)

1. What is the difference between a hierarchical database and a relational database?

Hierarchical (IMS DB)	Relational (DB2)
Data is stored in a tree-like structure (parent-child).	Data is stored in tables (rows & columns).
Faster for fixed, predictable queries (e.g., bill generation).	Better for flexible, complex queries (e.g., reports).
Uses pointers to link segments.	Uses keys (primary/foreign) to link tables.
Example: IMS DB.	Example: DB2, Oracle, MySQL.

2. What is the difference between a clustered index and a non-clustered index?

Clustered Index	Non-Clustered Index
Reorders the actual data on disk (like a phone book).	Creates a separate lookup structure (like an index at the back of a book).
Only one per table (since data is stored in order).	Multiple allowed per table.
Faster for range queries (e.g., "Get employees A-Z").	Faster for specific lookups (e.g., "Find employee ID 123").

3. What is the use of Sparse Sequencing?

Sparse Sequencing is a technique in IMS DB used to create a secondary index that includes only selected occurrences of a segment, rather than indexing every occurrence. This approach helps in:

• Reducing Index Size.​​
• Improving Performance.

For example - if you have a database of customers and only want to index those with active accounts, sparse sequencing allows you to exclude inactive accounts from the index. ​

4. What is the function of the IMS DB Control Region?

The IMS DB Control Region serves as the central component responsible for managing and coordinating database operations. Its functions include:​

• Running programs.
• Managing data buffers.
• Handling input/output (I/O).
• Controlling access to the database.

5. What is a Database Log?

A log is a record of all changes made to the database (like a diary of updates). Types of logs are:

• Update logs (records inserts, deletes, updates).
• Checkpoint logs (marks recovery points).

Used for:

• Recovery (undo/redo changes after a crash).
• Auditing (track who changed what).

6. What is database recovery?

Database recovery refers to the processes and procedures employed to restore an IMS DB database to a correct and consistent state following a system crash, hardware failure, or other disruptions. Recovery mechanisms utilize database logs and checkpoints to:​

• Rollback Unfinished Transactions: Undo partial changes from incomplete transactions.​
• Redo Completed Transactions: Reapply changes from completed transactions that have not yet been written to the main database.

7. What is the Role of the IMS DB Online Region?

The online region handles real-time transactions (like user requests from ATMs or websites). ​

• Processes MPPs (Message Processing Programs).
• Manages message queues (holds requests before processing).​
• Ensures fast responses for users.​

8. What is the syntax of an ISRT call in IMS DB?

ISRT = Insert (adds a new segment to the database). For example -

CALL 'CBLTDLI' USING ISRT, 
    PCB-MASK, 
	SEGMENT-AREA, 
	[ssa-list] 

• 'CBLTDLI' → The IMS DB interface module..
• ISRT → The function code indicating an insert operation.
• PCB-MASK → Specifies the database.
• SEGMENT-AREA → Contains the new segment data.
• SSA (Optional) → Segment Search Arguments specifying where the new segment should be placed within the database hierarchy.

9. What is a program-controlled transaction?

A transaction where the program decides when to commit (save) changes.

Difference from auto-commit:

• Auto-commit → IMS saves changes immediately.
• Program-controlled → The program can roll back changes if something goes wrong.

10. What is the Function of the PSBGEN Utility in IMS DB?

PSBGEN is a utility that generates a PSB (Program Specification Block). The PSB defines which parts of the database a program can access. PSBGEN compiles the PSB source code into a format IMS can use.

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