The IBM i installed base runs on some of the most battle-tested business logic in enterprise computing. Manufacturing operations that have been running on RPG for 20 or 30 years have, embedded in those programs, an extraordinary amount of accumulated business knowledge — order management rules, pricing logic, inventory allocation routines, and shop floor control that reflects decades of operational refinement.
The modernization question for most of these shops isn’t “how do we replace this logic” — it’s “how do we expose it to modern interfaces without breaking something we can’t afford to break.”
Open Access Handlers and REST API exposure are the answer more often than a rewrite.
Why Rewrites Fail
The history of IBM i modernization is littered with failed rewrites. The pattern is consistent: a company decides the green-screen ERP is too old, funds a multi-year replacement project, and discovers two years in that the replacement doesn’t handle the edge cases the RPG programs handle — the special pricing logic for a specific customer category, the allocation rules that took years to tune, the order sequencing logic that nobody fully documented because it was “in the programs.”
Rewrites fail not because modern platforms can’t handle the logic, but because the logic isn’t fully understood by the people specifying the replacement. RPG programs are, in many cases, the most accurate specification of business requirements available.
The Alternative: Exposing Logic, Not Replacing It
IBM i provides two primary mechanisms for exposing existing RPG business logic to modern interfaces without modifying the core programs:
Open Access Handlers
Open Access is a capability introduced in IBM i 7.1 that allows RPG programs to interact with non-traditional data sources — web services, JSON, XML — using the same file I/O model they’ve always used. An Open Access handler intercepts RPG read/write operations and routes them to or from a modern data source.
For modernization, Open Access allows you to take an existing RPG program that reads from a display file (the green screen) and redirect its input/output to a web interface — without changing the RPG program itself. The business logic runs exactly as it always has; the user interaction layer is modernized independently.
REST API Exposure via Integrated Web Services
IBM i’s Integrated Web Services server allows you to wrap any program, service program, or stored procedure in a REST or SOAP interface with minimal configuration. An RPG service program that calculates pricing, checks inventory availability, or validates an order can be called from a web application, mobile app, or third-party system as an HTTP endpoint.
This approach is particularly powerful for manufacturers building customer portals, mobile shop floor applications, or integration with cloud-based systems. The existing RPG logic becomes the authoritative business layer; the new interface is a consumer of that logic via API.
A practical example: A manufacturer with RPG-based pricing logic that accounts for customer contracts, volume tiers, and material surcharges can expose that logic as a REST API. Their new e-commerce platform calls the API at quote time, getting the same pricing calculation that their internal users see — without anyone rebuilding the pricing engine in the new platform.
What Modern Looks Like in Practice
IBM i environments successfully modernized with this approach typically have:
- A web-based or browser-neutral front end for internal users — replacing 5250 emulator sessions with a responsive web application that calls RPG business logic via API
- Mobile-capable interfaces for shop floor, warehouse, and field use — native or web apps that surface specific IBM i functions relevant to mobile users
- REST APIs for integration — allowing ERP functions to be called from e-commerce platforms, customer portals, EDI systems, and third-party applications
- Modern reporting and analytics — connecting IBM i DB2 data to BI platforms like Power BI or Tableau without moving the data off the system
Crucially, the underlying RPG programs — the ones running the business — are unchanged or minimally modified. The investment in those programs is protected; the user experience and integration capability are brought forward.
Getting Started
The most practical starting point is to identify one high-value, high-friction business process where modernizing the interface would have clear business impact — a customer-facing quote request process, a shop floor order status inquiry, or a mobile receiving application.
Build that one use case using Open Access or REST API exposure. Validate the approach, measure the impact, and build from there. The all-or-nothing modernization project is usually the wrong approach; an incremental strategy that proves value on each step is far more likely to succeed.
Modernizing Your IBM i Applications?
Paragon’s IBM i development team specializes in open access, REST API exposure, and front-end modernization strategies that protect your core business logic investment.
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