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Q: When should I use COMMIT in DB2?
Answer:
COMMIT saves all changes since the last COMMIT and releases locks.
When to COMMIT:
- After processing a logical unit of work
- Periodically in long-running batch (every N records)
- Before ending the program successfully
Frequency Guidelines:
- Too frequent: Performance overhead
- Too rare: Lock contention, large log
- Typical: Every 100-1000 records in batch
Best Practice:
MOVE 0 TO WS-COMMIT-COUNT.
PERFORM PROCESS-RECORD.
ADD 1 TO WS-COMMIT-COUNT.
IF WS-COMMIT-COUNT >= 500
EXEC SQL COMMIT END-EXEC
MOVE 0 TO WS-COMMIT-COUNT
END-IF.
Note: In CICS, syncpoint (COMMIT) happens automatically at task end. Explicit SYNCPOINT is rarely needed.
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Q: What is the difference between SECTION and PARAGRAPH in COBOL?
Answer:
SECTION:
- Contains one or more paragraphs
- Ends with next SECTION or end of program
- Used for logical grouping
- Can be performed as a unit
PARAGRAPH:
- Basic unit of code
- Named block of statements
- Ends at next paragraph name or SECTION
PERFORM SECTION-NAME executes all paragraphs in the section.
PERFORM PARA-NAME executes only that paragraph.
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Q: What is SQLCODE and what are common values?
Answer:
SQLCODE is a return code in SQLCA indicating the result of SQL execution.
Common SQLCODE values:
| 0 | Successful execution |
| 100 | No data found / End of cursor |
| -803 | Duplicate key on insert |
| -811 | Multiple rows returned for singleton SELECT |
| -904 | Unavailable resource |
| -911 | Deadlock/timeout |
| -922 | Authorization failure |
| -927 | DB2 not available |
Negative = Error, 0 = Success, 100 = No data
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Q: Explain SECTION vs PARAGRAPH
Answer:
SECTION groups related paragraphs, terminated by next SECTION or end of program. PARAGRAPH is a named block of code, terminated by next paragraph/section. PERFORM SECTION executes all paragraphs within. SECTION needed for segmentation, PARAGRAPH for simple procedures.
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Q: What is RECORDING MODE for files?
Answer:
RECORDING MODE specifies record format: F=fixed, V=variable, U=undefined, S=spanned. RECORDING MODE IS V for variable records. Affects how records are blocked and how length is tracked. Must match JCL DCB specifications.
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Q: What is CI and CA in VSAM?
Answer:
CI (Control Interval) is I/O unit, like block. Contains records, free space, control info. CA (Control Area) is group of CIs. CISIZE affects performance. CI split when full. CA split more expensive.
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Q: What causes file status 97?
Answer:
Status 97 indicates OPEN problem, often password or integrity. May be: dataset locked, open elsewhere without sharing, damaged dataset. Check SHAREOPTIONS, verify cluster status, recovery may be needed.
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Q: Explain SPACE parameter options
Answer:
SPACE=(unit,(primary,secondary,directory)). Unit: TRK/CYL/block-size. Primary allocated first, secondary in 15 increments. Directory only for PDS. RLSE releases unused. CONTIG requires contiguous. Example: SPACE=(CYL,(10,5,20),RLSE)
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Q: Explain DCB parameter
Answer:
DCB defines Data Control Block: RECFM (F/FB/V/VB/U), LRECL (record length), BLKSIZE (block size). DCB=(RECFM=FB,LRECL=80,BLKSIZE=27920). Can inherit from existing dataset. LRECL required for new datasets. BLKSIZE=0 lets system optimize.
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Q: What is EIB?
Answer:
EIB (Execute Interface Block) contains execution info. EIBCALEN=COMMAREA length. EIBTRNID=transaction. EIBAID=attention key. EIBDATE/EIBTIME=date/time. EIBRESP/EIBRESP2=response codes. Automatically available in COBOL.
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Q: Explain RECFM options
Answer:
RECFM defines record format. F=fixed, V=variable, U=undefined. B=blocked, A=ASA control, M=machine control, S=spanned. Combinations: FB=fixed blocked, VBS=variable blocked spanned. Match program expectations.
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Q: How to use ENQ/DEQ?
Answer:
ENQ serializes resources. EXEC CICS ENQ RESOURCE(name) LENGTH(len). DEQ releases. Prevents concurrent access. NOSUSPEND returns immediately if busy. Use for data integrity.
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Q: Explain EROPT parameter?
Answer:
EROPT specifies error handling. DCB=(EROPT=ABE/ACC/SKP). ABE=abend on error, ACC=accept and continue, SKP=skip block. For tape I/O errors. Program may handle differently. Default is ABE.
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Q: Explain DB2 locking
Answer:
DB2 locks at row, page, table, tablespace level. X (Exclusive) for writes, S (Share) for reads. IX/IS for intent. Lock escalation moves to higher level. LOCK TABLE statement forces mode. Timeout if wait too long.
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Q: Explain COMMIT and ROLLBACK
Answer:
COMMIT makes changes permanent, releases locks. ROLLBACK undoes changes since last COMMIT. Implicit COMMIT at program end (normal). Implicit ROLLBACK on abend. Frequent COMMIT reduces lock duration and log usage.
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Q: How to handle RLS (Record Level Sharing)?
Answer:
RLS allows VSAM access without exclusive control. Define with SHAREOPTIONS and LOG. Enable RLS at VSAM level. CF lock structure coordinates. Better than old batch/CICS conflicts.
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Q: What is EXEC CICS SEND CONTROL?
Answer:
SEND CONTROL sends control functions. EXEC CICS SEND CONTROL ERASE FREEKB. Clears screen, unlocks keyboard. No map or data. Prepare terminal for next operation.
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Q: How to handle CICS timeout?
Answer:
Set RTIMOUT on transaction. EXEC CICS HANDLE CONDITION ERROR. DTIMOUT for deadlock. Program design for cleanup. TIMEOUT condition raised.
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Q: How to monitor DB2 performance?
Answer:
Use OMEGAMON, DB2PM, or built-in monitor. Check: buffer pool hit ratios, lock waits, CPU time, elapsed time. DISPLAY commands show real-time. Statistics trace for analysis. EXPLAIN for query level.
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Q: What is BLOCK CONTAINS clause?
Answer:
BLOCK CONTAINS defines blocking factor. BLOCK CONTAINS 10 RECORDS or BLOCK CONTAINS 800 CHARACTERS. 0 RECORDS means system-determined. Affects I/O performance-larger blocks fewer I/Os. Must match JCL/VSAM definition.
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Q: How to define PDS?
Answer:
//PDSOUT DD DSN=MY.PDS,DISP=(NEW,CATLG),SPACE=(CYL,(5,1,10)),DCB=(RECFM=FB,LRECL=80). Directory blocks (10) required for PDS. Or DSNTYPE=LIBRARY for PDSE. PDSE allows dynamic directory expansion, member-level sharing.
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Q: How to SEND MAP to terminal?
Answer:
EXEC CICS SEND MAP(mapname) MAPSET(mapsetname) FROM(symbolic-map) ERASE. CURSOR option positions cursor. FREEKB unlocks keyboard. ALARM sounds alarm. MAPONLY sends without data. DATAONLY sends only modified fields.
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Q: What causes -911 SQLCODE?
Answer:
-911 is deadlock or timeout. Two processes waiting for each other's resources. Solutions: consistent lock order, shorter transactions, COMMIT frequently, appropriate isolation level. -913 is similar (deadlock victim). Retry transaction.
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Q: Explain DB2 isolation levels
Answer:
UR (Uncommitted Read) reads dirty data. CS (Cursor Stability) locks current row. RS (Read Stability) locks all accessed rows. RR (Repeatable Read) locks range, prevents phantom. Higher isolation = more consistency but more locking. CS most common.
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Q: What is BDAM conversion to VSAM?
Answer:
BDAM is block-oriented, VSAM is record-oriented. Convert: analyze BDAM access, choose KSDS/RRDS/ESDS. REPRO or IDCAMS copy. Program changes for VSAM access. Test thoroughly.
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Q: Explain NOFOR option in SELECT?
Answer:
NOFOR prevents FOR UPDATE locking. SELECT ... NOFOR. Read-only access without lock overhead. For display-only queries. Cannot UPDATE WHERE CURRENT OF cursor with NOFOR.