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COBOL Exception Handling Concepts

Exception handling in COBOL involves detecting and managing exceptional conditions that occur during program execution. Unlike regular error handling, exception handling deals with unexpected system conditions, data anomalies, and exceptional circumstances that require special processing. Understanding exception handling concepts is essential for building robust, fault-tolerant COBOL applications.

Understanding Exception Handling

Exception handling in COBOL encompasses all methods of detecting, managing, and recovering from exceptional conditions including system exceptions, data exceptions, and operational exceptions. Proper exception handling prevents program termination, ensures data integrity, and provides graceful recovery mechanisms. Different exception handling techniques are appropriate for different types of exceptional conditions.

ON EXCEPTION Clauses

1. Basic Exception Handling

ON EXCEPTION clauses provide exception handling for specific operations, allowing programs to detect and respond to exceptional conditions during execution. These clauses are commonly used with file operations and system calls.

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       PROCEDURE DIVISION.
       FILE-OPERATIONS.
           DISPLAY 'Performing file operations with exception handling'
           
           OPEN INPUT INPUT-FILE
           ON EXCEPTION
               DISPLAY 'EXCEPTION: Cannot open input file'
               PERFORM FILE-EXCEPTION-HANDLING
               STOP RUN
           END-OPEN
           
           READ INPUT-FILE
           ON EXCEPTION
               DISPLAY 'EXCEPTION: File read error'
               PERFORM READ-EXCEPTION-HANDLING
           END-READ
           
           CLOSE INPUT-FILE
           ON EXCEPTION
               DISPLAY 'EXCEPTION: File close error'
               PERFORM CLOSE-EXCEPTION-HANDLING
           END-CLOSE.
       
       FILE-EXCEPTION-HANDLING.
           DISPLAY 'Handling file exception'
           MOVE 'FILE-ERROR' TO EXCEPTION-TYPE
           PERFORM LOG-EXCEPTION

ON EXCEPTION clauses provide immediate exception handling for specific operations. They catch exceptions as they occur and allow programs to respond appropriately without terminating.

2. Advanced Exception Handling

Advanced exception handling involves multiple exception conditions, nested exception handling, and complex recovery procedures for sophisticated error management scenarios.

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       PROCEDURE DIVISION.
       COMPLEX-OPERATIONS.
           DISPLAY 'Performing complex operations with exception handling'
           
           PERFORM PROCESS-DATA
           ON EXCEPTION
               DISPLAY 'EXCEPTION: Data processing error'
               PERFORM DATA-EXCEPTION-HANDLING
               PERFORM RECOVERY-PROCEDURES
           END-PERFORM
           
           PERFORM CALCULATE-RESULTS
           ON EXCEPTION
               DISPLAY 'EXCEPTION: Calculation error'
               PERFORM CALCULATION-EXCEPTION-HANDLING
               PERFORM USE-DEFAULT-VALUES
           END-PERFORM.
       
       DATA-EXCEPTION-HANDLING.
           DISPLAY 'Handling data processing exception'
           MOVE 'DATA-ERROR' TO EXCEPTION-TYPE
           ADD 1 TO DATA-ERROR-COUNT
           PERFORM LOG-EXCEPTION.
       
       CALCULATION-EXCEPTION-HANDLING.
           DISPLAY 'Handling calculation exception'
           MOVE 'CALC-ERROR' TO EXCEPTION-TYPE
           ADD 1 TO CALC-ERROR-COUNT
           PERFORM LOG-EXCEPTION

Advanced exception handling provides comprehensive error management for complex operations. Multiple exception handlers can be nested and coordinated to provide robust error recovery.

USE Statements

1. Declarative Exception Handling

USE statements provide declarative exception handling for file operations, allowing programs to handle file-related exceptions automatically without explicit ON EXCEPTION clauses.

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       DATA DIVISION.
       FILE SECTION.
       FD INPUT-FILE.
       01 INPUT-RECORD        PIC X(100).
       
       PROCEDURE DIVISION.
       DECLARATIVES.
       FILE-ERROR-SECTION.
           USE AFTER EXCEPTION PROCEDURE ON INPUT-FILE.
       FILE-ERROR-HANDLER.
           DISPLAY 'File exception occurred'
           DISPLAY 'File status: ' FILE-STATUS
           
           EVALUATE FILE-STATUS
               WHEN '23'
                   DISPLAY 'File not found exception'
                   PERFORM FILE-NOT-FOUND-HANDLING
               WHEN '24'
                   DISPLAY 'File locked exception'
                   PERFORM FILE-LOCKED-HANDLING
               WHEN '30'
                   DISPLAY 'Permission denied exception'
                   PERFORM PERMISSION-DENIED-HANDLING
               WHEN OTHER
                   DISPLAY 'Unknown file exception'
                   PERFORM UNKNOWN-FILE-HANDLING
           END-EVALUATE.
       END DECLARATIVES.
       
       MAIN-PROGRAM.
           DISPLAY 'Main program with declarative exception handling'
           OPEN INPUT INPUT-FILE
           READ INPUT-FILE
           CLOSE INPUT-FILE

USE statements provide automatic exception handling for file operations. They are declared in the DECLARATIVES section and automatically invoked when file exceptions occur.

2. Multiple File Exception Handling

Multiple file exception handling allows different exception handlers for different files, providing specialized handling for various file types and operations.

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       PROCEDURE DIVISION.
       DECLARATIVES.
       INPUT-FILE-ERROR-SECTION.
           USE AFTER EXCEPTION PROCEDURE ON INPUT-FILE.
       INPUT-FILE-HANDLER.
           DISPLAY 'Input file exception'
           MOVE 'INPUT-ERROR' TO EXCEPTION-TYPE
           PERFORM LOG-EXCEPTION.
       
       OUTPUT-FILE-ERROR-SECTION.
           USE AFTER EXCEPTION PROCEDURE ON OUTPUT-FILE.
       OUTPUT-FILE-HANDLER.
           DISPLAY 'Output file exception'
           MOVE 'OUTPUT-ERROR' TO EXCEPTION-TYPE
           PERFORM LOG-EXCEPTION.
       
       WORK-FILE-ERROR-SECTION.
           USE AFTER EXCEPTION PROCEDURE ON WORK-FILE.
       WORK-FILE-HANDLER.
           DISPLAY 'Work file exception'
           MOVE 'WORK-ERROR' TO EXCEPTION-TYPE
           PERFORM LOG-EXCEPTION.
       END DECLARATIVES.

Multiple file exception handlers provide specialized handling for different file types. Each file can have its own exception handling logic tailored to its specific requirements.

Exception Recovery

1. Automatic Recovery

Automatic recovery procedures attempt to resolve exceptions automatically without user intervention, using predefined recovery strategies and fallback mechanisms.

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       PROCEDURE DIVISION.
       AUTOMATIC-RECOVERY.
           DISPLAY 'Implementing automatic recovery'
           
           PERFORM PRIMARY-OPERATION
           ON EXCEPTION
               DISPLAY 'Primary operation failed, attempting recovery'
               PERFORM RECOVERY-ATTEMPT-1
               
               IF RECOVERY-STATUS = 'FAILED'
                   PERFORM RECOVERY-ATTEMPT-2
                   
                   IF RECOVERY-STATUS = 'FAILED'
                       PERFORM RECOVERY-ATTEMPT-3
                   END-IF
               END-IF
           END-PERFORM.
       
       RECOVERY-ATTEMPT-1.
           DISPLAY 'Recovery attempt 1: Retry operation'
           PERFORM RETRY-OPERATION
           IF OPERATION-SUCCESSFUL
               MOVE 'SUCCESS' TO RECOVERY-STATUS
           ELSE
               MOVE 'FAILED' TO RECOVERY-STATUS
           END-IF.
       
       RECOVERY-ATTEMPT-2.
           DISPLAY 'Recovery attempt 2: Use alternative method'
           PERFORM ALTERNATIVE-OPERATION
           IF OPERATION-SUCCESSFUL
               MOVE 'SUCCESS' TO RECOVERY-STATUS
           ELSE
               MOVE 'FAILED' TO RECOVERY-STATUS
           END-IF.

Automatic recovery provides multiple recovery attempts with different strategies. Each attempt uses a different approach to resolve the exception condition.

2. Manual Recovery Procedures

Manual recovery procedures require user intervention or operator action to resolve exceptional conditions that cannot be handled automatically.

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       PROCEDURE DIVISION.
       MANUAL-RECOVERY.
           DISPLAY 'Manual recovery required'
           
           PERFORM OPERATION-WITH-MANUAL-RECOVERY
           ON EXCEPTION
               DISPLAY 'Exception requires manual intervention'
               PERFORM REQUEST-OPERATOR-ASSISTANCE
               PERFORM WAIT-FOR-OPERATOR-RESPONSE
               
               IF OPERATOR-RESPONSE = 'RETRY'
                   PERFORM RETRY-OPERATION
               ELSE
                   IF OPERATOR-RESPONSE = 'SKIP'
                       PERFORM SKIP-OPERATION
                   ELSE
                       IF OPERATOR-RESPONSE = 'ABORT'
                           PERFORM ABORT-PROCESSING
                       END-IF
                   END-IF
               END-IF
           END-PERFORM.
       
       REQUEST-OPERATOR-ASSISTANCE.
           DISPLAY 'OPERATOR ASSISTANCE REQUIRED'
           DISPLAY 'Exception: ' EXCEPTION-TYPE
           DISPLAY 'Please choose: RETRY, SKIP, or ABORT'
           ACCEPT OPERATOR-RESPONSE

Manual recovery procedures involve operator interaction to resolve exceptions that require human judgment or intervention. These procedures provide options for retry, skip, or abort.

Exception Logging and Monitoring

1. Exception Logging

Exception logging captures detailed information about exceptions for analysis, debugging, and system monitoring. This helps identify patterns and improve system reliability.

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       PROCEDURE DIVISION.
       LOG-EXCEPTION.
           DISPLAY 'Logging exception information'
           
           *> Capture exception details
           MOVE FUNCTION CURRENT-DATE TO EXCEPTION-TIMESTAMP
           MOVE PROGRAM-NAME TO EXCEPTION-PROGRAM
           MOVE EXCEPTION-TYPE TO EXCEPTION-CATEGORY
           MOVE EXCEPTION-MESSAGE TO EXCEPTION-DESCRIPTION
           MOVE FILE-STATUS TO EXCEPTION-STATUS
           
           *> Write to exception log
           WRITE EXCEPTION-LOG-RECORD
           
           *> Update exception statistics
           ADD 1 TO TOTAL-EXCEPTION-COUNT
           ADD 1 TO DAILY-EXCEPTION-COUNT
           
           DISPLAY 'Exception logged: ' EXCEPTION-CATEGORY ' - ' EXCEPTION-DESCRIPTION.
       
       GENERATE-EXCEPTION-REPORT.
           DISPLAY 'Generating exception report'
           DISPLAY 'Total exceptions: ' TOTAL-EXCEPTION-COUNT
           DISPLAY 'Daily exceptions: ' DAILY-EXCEPTION-COUNT
           DISPLAY 'Last exception: ' EXCEPTION-CATEGORY ' - ' EXCEPTION-DESCRIPTION

Exception logging captures comprehensive information about exceptions including timestamps, program names, exception types, and detailed descriptions for analysis and monitoring.

2. Exception Monitoring

Exception monitoring tracks exception patterns, frequencies, and trends to identify potential system issues and improve reliability through proactive measures.

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       PROCEDURE DIVISION.
       MONITOR-EXCEPTIONS.
           DISPLAY 'Monitoring exception patterns'
           
           *> Check exception thresholds
           IF DAILY-EXCEPTION-COUNT > MAX-DAILY-EXCEPTIONS
               DISPLAY 'WARNING: Daily exception threshold exceeded'
               PERFORM ALERT-ADMINISTRATOR
           END-IF
           
           *> Check exception patterns
           IF CONSECUTIVE-EXCEPTIONS > MAX-CONSECUTIVE-EXCEPTIONS
               DISPLAY 'WARNING: Too many consecutive exceptions'
               PERFORM INVESTIGATE-PATTERN
           END-IF
           
           *> Update monitoring statistics
           PERFORM UPDATE-MONITORING-STATS.
       
       ALERT-ADMINISTRATOR.
           DISPLAY 'ALERT: Exception threshold exceeded'
           DISPLAY 'Daily count: ' DAILY-EXCEPTION-COUNT
           DISPLAY 'Maximum allowed: ' MAX-DAILY-EXCEPTIONS
           DISPLAY 'Administrator notification sent'

Exception monitoring tracks exception patterns and thresholds to identify potential system issues. Alerts are generated when exception levels exceed acceptable limits.

Best Practices for Exception Handling

Common Exception Handling Patterns

Error Handling

Learn error handling

Debugging

Master debugging techniques

Robust Programming

Learn robust programming

File Status

Master file status handling

Error Recovery

Learn error recovery

Program Testing

Master program testing