How to modernise an IBM i (AS/400) system by wrapping it, not replacing it

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Many businesses still rely on decades-old AS/400 (now IBM i) systems to run core operations such as production, warehousing, and invoicing, and the instinct to scrap and rebuild them is usually the wrong call. The real challenges are isolation from modern systems, a shrinking pool of RPG programmers, and outdated security posture — none of which require a full rewrite to fix. A safer approach is to expose the existing RPG programs and DB2 database through modern APIs, allowing external applications to communicate with the legacy system without altering its internal logic. This method, known as the 'strangler fig' pattern, lets a modern layer be built gradually around the AS/400, delivering incremental value while keeping the core system running uninterrupted. Rewriting from scratch risks discarding years of refined business logic and reintroducing long-solved bugs, making the wrap-and-extend strategy both less risky and more practical.

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