COBOL and mainframe modernisation glossary.

Business rule extraction

Recovering the business policies a legacy system enforces, such as fees, interest and eligibility, from its code into statements people can review.

CICS

IBM's transaction server for online mainframe applications, such as screens and real-time requests, called from COBOL with EXEC CICS.

COBOL

A business programming language from 1959 that still runs much of the world's banking, insurance and government processing, mostly on mainframes.

COMP-3 (packed decimal)

A COBOL storage format holding two decimal digits per byte with the sign in the last half-byte. It stores exact decimals and maps to BigDecimal, not double.

Copybook

A reusable COBOL source file, pulled in with COPY, that usually defines a shared record layout used by many programs.

DB2

IBM's relational database on the mainframe, accessed from COBOL with embedded EXEC SQL statements.

EBCDIC

The mainframe character encoding. Its sort order differs from ASCII and Unicode, which can change results after migration.

Functional equivalence

The property that a migrated system produces the same outputs and side effects as the original for the same inputs.

Golden master testing

Testing against outputs captured from the existing system for a fixed set of inputs, so any change in behaviour shows up as a difference.

JCL

Job Control Language: the scripts that run mainframe batch jobs, naming the programs to execute and the datasets they read and write.

Mainframe

A large, highly reliable computer, such as IBM Z, used for high-volume transaction processing in banks, insurers and governments.

Mainframe modernisation

Moving mainframe applications, usually COBOL, to modern languages, platforms or cloud while keeping their behaviour the same. Also spelled modernization.

Paragraph

A named block of COBOL procedure code, run with PERFORM or reached by falling through from the paragraph above it.

Parallel run

Running the legacy and new systems side by side on the same live inputs, typically through a period-end, and reconciling their results before cutover.

PERFORM THRU

A COBOL statement that runs every paragraph from one name through another in source order, a common source of migration bugs.

PIC clause

The PICTURE clause that fixes a COBOL field's type, number of digits, decimal places and sign, for example PIC S9(7)V99.

REDEFINES

A COBOL clause that lets two data layouts share the same bytes of storage, so one memory area can be read in different ways.

Replay testing

Recording real inputs and outputs of the legacy system and re-running them against migrated code on every change, comparing results field by field.

Strangler fig pattern

Replacing a legacy system piece by piece, routing more traffic to new components over time until the old system can be retired.

VSAM

A mainframe file system for keyed, sequential and relative record access, widely used by COBOL batch and online programs.

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