COBOL Tutorial

RETRY - Implementing Retry Logic

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Overview

Retry is a pattern you implement to handle transient failures safely. Keep attempts bounded and add backoff.

🔁 Analogy

Like calling a busy office: you try again a few times, waiting longer between calls, and eventually leave a message if no answer.

Simple Retry Loop

Example (File Open)

cobol

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WORKING-STORAGE SECTION.
01  ATTEMPTS        PIC 9 VALUE 0.
01  MAX-ATTEMPTS    PIC 9 VALUE 3.
01  OPEN-STATUS     PIC XX.
...
PROCEDURE DIVISION.
OPEN-TRY.
    ADD 1 TO ATTEMPTS
    OPEN INPUT MYFILE
    MOVE FILE-STATUS OF MYFILE TO OPEN-STATUS
    IF OPEN-STATUS NOT = '00'
        IF ATTEMPTS < MAX-ATTEMPTS
            PERFORM BACKOFF
            GO TO OPEN-TRY
        ELSE
            DISPLAY 'Open failed after retries, RC=8'
            STOP RUN
        END-IF
    END-IF.
...
BACKOFF.
    *> Implement wait (site-specific)
    DISPLAY 'Waiting before retry # ' ATTEMPTS.

Adjust for your compiler; use proper sleep APIs where available.

Exponential Backoff (Concept)

Attempt 1: wait 1s
Attempt 2: wait 2s
Attempt 3: wait 4s

Best Practices

Bound attempts and use backoff
Log each attempt and reason
Rollback/close resources between attempts
Make thresholds configurable
Coordinate with JCL restart/automation

Common Mistakes

Mistake	Problem	Fix
Infinite loops	Hangs and resource waste	Add max attempts
No delay	Hammering a busy resource	Use backoff
Retrying non-transient errors	Wastes time	Check error codes; fail fast

Quick Reference

Concept	Tip	Example
Max attempts	Keep small (3-5)	MAX-ATTEMPTS = 3
Backoff	Wait longer each time	1s, 2s, 4s
Failover	Return code, message	RC=8

Test Your Knowledge

1. Is RETRY a standard COBOL verb?

2. What is a good strategy for transient errors?

Retry immediately forever
Exponential backoff with max attempts
Never retry
Random jumps

3. Where do you store attempt counters?

WORKING-STORAGE
FILE SECTION
REPORT SECTION
LINKAGE only

4. What should happen after max attempts?

Loop forever
Graceful fail or escalate with return code
Restart IPL
Ignore error

5. Which JCL concept relates to post-failure recovery?

EXEC PGM
STEP RESTART/COND
DD DUMMY
IEBGENER

Frequently Asked Questions

Related Concepts

File Status Codes

Interpreting file errors for retry decisions.

JCL Restart

Operational restart complements in-program retries.

Error Handling

Patterns for handling exceptions and errors.

REPORT-WRITER SCREEN SECTION