IMS DB interview questions - Basics of DLI

Q1 : What do you mean by DBD and PSB?
A1. DBD stands for database descriptor. It defines the structure of database. Each database has one and only one DBD. PSB stands for Program specification block. Each program that accesses the database has its own PSB. DBD and PSB are created by the IMS DBA.

Q2 : How is the batch DL/I program executed?
A2 : In the JCL one needs to run the batch initialization module DFSRRC00 and then pass the PSB as the parameter. This way the batch module is run.

Q3 : How does COBOL module communicate with the DL/I database?
A3 : It communicates by calling the subroutine CBLTDLI. Following is a sample format.

      CALL 'CBLTDLI' USING               DLI-FUNCTION
                                                       PCB-MASK
                                                       SEGMENT-IO-AREA
                                                       SSA


Q4: What are the different DL/I functions? Explain them.
A4 : GU , GHU , GN , GHN , GNP , GHNP , ISRT , DLET , REPL , CHKP , XRST , PCB

GU : Get unique - It lets DL/I to retrieve a particular segment. Like the COBOL read.
GN : Get next - It lets DL/I to retrieve the next segment. Like COBOL read next.
GNP : Get next within parent. It lets retrieve segment occurences in sequence , but only subordinate to an established parent segment. This does not have a COBOL corresponding statement.

GHU , GHN , GHNP : These are corresponding commands that are used with the intent of updating the segments.

ISRT : Insert a segment.
DLET : Deelete a particular segment
REPL : Replace a partcular segment - Like COBOL Rewrite.

CHKP , XRST : IMS recovery functions
PCB : Used in CICS programs.

Q5 : Describe a PCB mask.
A5 : Following is a sample PCB mask.

      01  SAMPLE-PCB-MASK.
                  05     IPCB-DBD-NAME                     PIC X(8).
                  05     IPCB-SEGMENT LEVEL           PIC X(2).
                  05     IPCB-STATUS-CODE               PIC X(2).
                  05     IPCB-PROC-OPTIONS             PIC X(4).
                  05     FILLER                                       PIC S9(5)        COMP.
                  05     IPCB-SEGMENT-NAME          PIC X(8).
                  05     IPCB-KEY-LENGTH                PIC S9(5)         COMP.  
                  05     IPCB-NUMB-SENS-SEGS       PIC S9(5)         COMP.
                  05     IPCB-KEY                                 PIC X(11).

This is defined in the linkage section of the program.

(i) IPCB-DBD-NAME : Its the name of the database the program is going after.

(ii) IPCB-SEGMENT-LEVEL : Its the level of the segment that the system is processing at any given time.

(iii) IPCB-STATUS-CODE : Its the status code that's returned after the database processing is done.

(iv) IPCB-PROC-OPTIONS : Its the process options that indicates what processing the program is authorized to do on the database.

(v) FILLER : Its reserved for DL/I's use.

(vi) IPCB-SEGMENT-NAME : After each call , DL/I stores the eight character name of the segment just processed in this field.

(vii) IPCB-KEY-LENGTH :  DL/I stores in this field the length of the concatenated key of the lowest level segment processed during the previous call.

(viii) IPCB-NUMB-SENS-SEGS : Its value is determined by PSBGEN. DL/I uses this field to report the number of SENSEG macros subordinate to the PCB macro for this data base.

(ix) IPCB-KEY : Unlike all other fields, this one varies in length from one PCB to another.Its as long as the longest possible concatenated key that can be used with the program's view of the database.

Q6 : What do the status code GB and GE mean?
A6 : GB means that the end of database has been reached. GE means that the segment is not found.

Q7. What is the difference between un-qualified and qualified SSA?
A7: Unqualified SSA is 9 bytes long. First 8 bytes has the segment name and nineth byte is blank. Qualified SSA consists of the (i) Segment name , (ii) Field name (iii) arithmetic operator (iv) Variable that will have the value which would be compared to that of field name. Qualified SSA is used for getting a particular segment occurence.