COBOL modernization that keeps business behavior intact

COBOL systems are hard to change because the knowledge is in the code, not in documents. Business rules sit inside paragraphs written decades ago, spread across copybooks, JCL job streams and CICS transactions, with data held in fixed-width records and packed decimal fields. The people who wrote it have often left.

The safe starting point is understanding. We inventory programs, copybooks, jobs and files, find what is dead, and trace how data flows from input to output. Then we capture current behavior with characterization tests built from real inputs and outputs, so any later change can be compared with what the system does today.

Only then do we decide what to do with each program. Some are translated, some rewritten, some wrapped behind an API, and some kept. The wider context is covered on our mainframe modernization page.

Translation tools shorten the work but do not remove the need for testing. Any difference in rounding, truncation or sort order can change a financial or regulatory result, which is why parallel comparison is central.

• Tests built from past data only cover cases that happened. Rare year-end or exception paths need extra attention.
• Source code is sometimes missing or out of sync with what runs. We check this early.
• Cost, duration and savings depend on the estate and cannot be promised in advance.

If the COBOL works, changes rarely and the platform is supported, retaining it and exposing it through APIs can be the sensible choice. See legacy application modernization for how we choose between options.