Modernizing COBOL-Based Billing Logic Without a Big-Bang Rewrite
Key Takeaways
- Safe COBOL-to-Java modernization in regulated billing systems happens one algorithm at a time inside a runtime that can host both languages side by side, not through a scheduled cutover of the whole system.
- We did parallel runs that fed the same production input to both the COBOL and Java versions of an algorithm, then did a field-by-field diff of the outputs; that was the real gate for going live, not code review or staging-environment testing, which we also did.
- When choosing which algorithms to convert first, we weighed billing impact, batch volume, and calculation complexity together. We started with moderate-risk candidates to prove the validation and rollback process before moving on to the highest-volume and highest-impact routines.
- COBOL’s packed decimal math and round-off rules don’t behave the same as Java’s numeric types, which led to silent calculation differences at that boundary, which in turn we only caught by...
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