Cobol Modernization

Phase 2: Testing Harness Setup

Create reusable testing infrastructure before implementing the conversion:

1. Create a state reset script - Automate restoring original data files:

   # Example: reset_state.sh
   cp data/ACCOUNTS.DAT.orig data/ACCOUNTS.DAT
   cp data/BOOKS.DAT.orig data/BOOKS.DAT
   cp data/TRANSACTIONS.DAT.orig data/TRANSACTIONS.DAT
   

2. Create a comparison script - Automate output comparison:

   # Example: compare_outputs.sh
   diff data/ACCOUNTS_PYTHON.DAT data/ACCOUNTS_COBOL.DAT
   diff data/BOOKS_PYTHON.DAT data/BOOKS_COBOL.DAT
   diff data/TRANSACTIONS_PYTHON.DAT data/TRANSACTIONS_COBOL.DAT
   

3. Preserve original COBOL outputs - Run the COBOL program first and save outputs as reference baselines before any conversion work.

Phase 3: Implementation

When writing the modern language equivalent:

1. Match COBOL data handling exactly:

   • Use fixed-width string formatting, not variable-length
   • Implement proper padding (spaces for alphanumeric, zeros for numeric)
   • Handle COBOL's implicit decimal points in numeric fields
   • Match COBOL's truncation behavior for oversized values

2. Verify file writes immediately - After writing code files, read them back to confirm complete content was saved correctly before testing.

3. Use consistent naming - Avoid creating excessive temporary files. Use a clear naming scheme:

   • *_COBOL.DAT for COBOL program outputs
   • *_PYTHON.DAT for Python program outputs
   • Clean up between test iterations

Phase 4: Systematic Testing

Test all code paths, not just the happy path:

1. Create a test matrix covering all validation scenarios:

   • Valid transactions (success case)
   • Non-existent primary entities (buyer, seller, book, etc.)
   • Ownership/permission validation failures
   • Insufficient balance/resource conditions
   • Boundary conditions (zero balance, maximum values)

2. Test each scenario independently:

   • Reset state before each test
   • Run both COBOL and modern implementation
   • Compare outputs byte-for-byte using diff

3. Document test results - Track which scenarios passed and any discrepancies found.

Common Pitfalls

Data Format Issues

• Fixed-width fields: COBOL uses fixed-width fields padded with spaces or zeros. Modern languages default to variable-length strings.
• Numeric formatting: COBOL's PIC 9(4)V99 means 4 digits, implied decimal, 2 decimal places - stored as 6 characters with no decimal point.
• Record terminators: COBOL fixed-length records may not use line terminators. Verify whether newlines are expected.

Testing Mistakes

• Incomplete edge case coverage: Testing only success and one failure case leaves validation paths untested.
• Not verifying written code: Tool responses may be truncated. Always read back written files to confirm completeness.
• State pollution: Running tests without resetting state causes cascading failures.

Process Inefficiencies

• Repeating commands: Create shell scripts for operations performed more than twice.
• Cluttered workspace: Create a consistent file naming scheme and clean up temporary files.
• Unresolved discrepancies: If data formats don't match expectations, investigate fully before proceeding.

Verification Checklist

Before declaring the modernization complete:

• All required output files are generated
• All data files match COBOL output byte-for-byte (diff returns no output)
• All validation paths have been tested (success + each failure type)
• Boundary conditions have been verified
• No temporary or debug files remain
• Code has been read back to verify complete and correct content