RPG IV Modernization: Moving iSeries Business Logic to Modern React UIs

Your RPG IV (ILE) business logic is a goldmine, but your 5250 "green screen" interface is a graveyard for user productivity. For decades, the IBM i (AS/400) has powered the world’s most critical financial, manufacturing, and logistics systems. However, as the workforce shifts toward a generation that has never seen a command line, the friction between legacy UIs and modern expectations has reached a breaking point.

The challenge isn't the reliability of the DB2 database or the efficiency of RPG fixed-format logic; it’s the massive "documentation debt" and the astronomical cost of manual UI conversion. Industry experts recommend a "decoupled" approach where the robust backend remains intact while the frontend is completely reimagined. Yet, traditional manual rewrites are where projects go to die.

TL;DR: Modernizing RPG IV systems doesn't require a high-risk "big bang" rewrite of your entire codebase. By using Replay, enterprises can leverage Visual Reverse Engineering to convert recorded legacy workflows into documented React components and Design Systems. This reduces the average screen modernization time from 40 hours to just 4 hours, bypassing the 70% failure rate typical of manual legacy migrations.

The High Cost of the "Green Screen" Glass Ceiling#

The global technical debt has ballooned to a staggering $3.6 trillion. A significant portion of this is locked within IBM i systems where the business logic is sound, but the interface is inaccessible to modern web architectures. When considering modernization moving iseries business logic to the web, architects often face a grim reality: 67% of these legacy systems lack any form of up-to-date documentation.

According to Replay’s analysis, the average enterprise rewrite timeline spans 18 to 24 months. During this period, the business is often frozen, unable to ship new features because the engineering team is buried in "feature parity" hell. Manual translation of RPG subfiles, indicators (03 to exit!), and function keys into React state management is error-prone and soul-crushing for developers.

Visual Reverse Engineering is the process of capturing real-time user interactions with legacy software and using AI-driven analysis to generate modern code equivalents, including component structures, state logic, and design tokens.

Why Traditional Modernization Moving iSeries Business Logic Fails#

Most IBM i modernization projects fail not because of the code, but because of the "Logic Gap." RPG IV is a positional or free-form language that handles data and display (DDS) in a tightly coupled manner. When you try to peel these apart manually, you realize that the "source of truth" for how a business process works isn't in a document—it’s in the muscle memory of a user who has used the system for 30 years.

1. •Documentation Rot: When the original RPG developers retire, the "why" behind specific screen flows disappears.
2. •The 70% Failure Rate: 70% of legacy rewrites fail or exceed their original timeline because the scope of "hidden" business rules in the UI is underestimated.
3. •Manual Toil: It takes an average of 40 hours per screen to manually document, design, and code a React equivalent of a complex RPG subfile.

Replay changes this dynamic by focusing on the visual output. If a user records a workflow in their 5250 emulator, Replay's AI Automation Suite identifies the patterns—the grids, the input fields, the navigation logic—and generates a production-ready React component library.

The Strategy: Decoupling UI from RPG Logic#

To succeed in modernization moving iseries business operations to a React-based architecture, you must adopt a "Service-Oriented" mindset. This involves wrapping your RPG programs (using PCML or REST APIs via Integrated Web Services for i) and connecting them to a modern React frontend.

Comparison: Manual Rewrite vs. Replay Visual Reverse Engineering#

Implementing the React Layer#

Once Replay has generated your components and flows, the next step is connecting your React frontend to the iSeries backend. Below is an example of how a modernized "Subfile" (a common RPG list display) is structured in React using TypeScript.

Code Example: Modernized RPG Subfile Component#

In the legacy world, a subfile handles pagination and selection via indicators. In React, we move this to an asynchronous data-fetching pattern.

typescript
import React, { useState, useEffect } from 'react';
import { DataTable, Button, Spinner } from '@your-org/replay-design-system';

interface CustomerRecord {
  id: string;
  name: string;
  balance: number;
  status: 'Active' | 'Inactive';
}

const CustomerSubfile: React.FC = () => {
  const [data, setData] = useState<CustomerRecord[]>([]);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    // Calling the RESTified RPG program
    fetch('/api/pgm/CUSTLIST')
      .then(res => res.json())
      .then(data => {
        setData(data);
        setLoading(false);
      });
  }, []);

  if (loading) return <Spinner />;

  return (
    <div className="p-6">
      <h2 className="text-2xl font-bold mb-4">Customer Master File</h2>
      <DataTable 
        columns={[
          { header: 'ID', accessor: 'id' },
          { header: 'Customer Name', accessor: 'name' },
          { header: 'Current Balance', accessor: 'balance', cell: (val) => `$${val}` },
          { header: 'Status', accessor: 'status' }
        ]}
        data={data}
        onRowClick={(row) => console.log('Equivalent to Option 5 (Display)', row)}
      />
      <div className="mt-4 flex gap-2">
        <Button variant="primary">F3 - Exit</Button>
        <Button variant="secondary">F6 - Add New</Button>
      </div>
    </div>
  );
};

export default CustomerSubfile;

For more on how to structure these transitions, see our guide on Legacy Modernization Strategies.

Bridging the Gap with Visual Reverse Engineering#

The core of the Replay platform is its ability to turn a video recording into a "Blueprint." When an enterprise user records a session of them navigating through an iSeries inventory management system, Replay’s AI analyzes the screen transitions.

It identifies that "Screen 10" is a search filter, "Screen 20" is a results grid, and "Screen 30" is a detail view. It then maps these to a centralized Flows architecture. This is critical for modernization moving iseries business logic because it ensures that no "hidden" screens are missed during the migration.

Handling "The Indicator Problem"#

RPG uses indicators (*IN01 - *IN99) to control screen attributes (e.g., making a field read-only or changing its color). In a modern React UI, this is handled through state and props. Replay’s AI Automation Suite detects these visual changes in the recording and suggests the appropriate state variables in the generated TypeScript code.

typescript
// Generated by Replay AI Automation Suite
// Maps to Legacy Indicator *IN35 (Error on Field)

import React from 'react';
import { TextField } from '@your-org/replay-design-system';

interface OrderInputProps {
  orderAmount: number;
  hasError: boolean; // Replaces *IN35 logic
}

export const OrderInput: React.FC<OrderInputProps> = ({ orderAmount, hasError }) => {
  return (
    <TextField
      label="Enter Order Amount"
      value={orderAmount}
      error={hasError}
      helperText={hasError ? "Amount exceeds credit limit" : ""}
      variant="outlined"
    />
  );
};

Security and Compliance in Regulated Industries#

Many iSeries systems reside in highly regulated sectors like banking, healthcare, and government. Moving these to the web requires more than just pretty buttons; it requires enterprise-grade security.

Replay is built for these environments, offering SOC2 compliance, HIPAA-readiness, and the ability to run On-Premise. This ensures that while you are modernization moving iseries business interfaces to React, your sensitive data never leaves your controlled network.

Industry experts recommend that any modernization tool must support local deployment to satisfy data residency requirements. This is why Replay offers a "Blueprint" export that allows developers to take the generated code and run it in their own air-gapped CI/CD pipelines.

The Roadmap to Success#

If you are tasked with modernization moving iseries business workflows to a modern stack, follow this phased approach:

1. •Inventory & Record: Use Replay to record the top 20% of workflows that account for 80% of user time.
2. •Generate the Design System: Let the Replay Library create a unified React component library based on your legacy patterns, but styled for the 21st century.
3. •API Enablement: Use tools like MuleSoft, IBM i Integrated Web Services, or Eradani to expose your RPG IV procedures as REST endpoints.
4. •Assemble & Deploy: Use the Replay Blueprints (the visual editor) to wire the generated React components to your new APIs.
5. •Iterate: Move away from the "Big Bang" approach. Deploy one module at a time—starting with "Read-Only" dashboards before moving into complex CRUD (Create, Read, Update, Delete) operations.

For a deeper look at this process, check out our article on UI Reverse Engineering.

Overcoming the "Skill Gap"#

One of the biggest hurdles in modernization moving iseries business systems is the talent gap. RPG developers often struggle with React's functional paradigm, and React developers are often baffled by the IBM i's EBCDIC character encoding and library list system.

Replay acts as the "Universal Translator." By providing documented, clean, and readable React code from the start, it allows RPG developers to see how their business logic translates visually into modern code. Conversely, it gives React developers a clear map (Blueprints) of how the legacy system functions without requiring them to learn how to navigate a green screen.

Frequently Asked Questions#

Does modernization moving iseries business logic require changing the underlying RPG code?#

Not necessarily. Most modern strategies involve "wrapping" existing RPG IV programs in APIs. Replay focuses on the frontend modernization, allowing you to keep your stable backend logic while providing a modern React interface. This minimizes risk and avoids the 70% failure rate associated with complete backend rewrites.

How does Replay handle complex subfiles and multi-page lists?#

Replay's Visual Reverse Engineering captures the behavior of subfiles across multiple interactions. It recognizes patterns like "Page Down" for pagination and "Option 2" for editing. It then generates React components that utilize modern patterns like infinite scroll or paginated DataTables, while maintaining the underlying business flow documented in the Replay Flows tool.

Is the code generated by Replay maintainable by my current team?#

Yes. Replay generates standard, human-readable TypeScript and React code. It doesn't use proprietary runtimes or "black box" libraries. The output is a standard React project that can be managed in Git and integrated into any standard CI/CD pipeline, ensuring your team has full ownership of the modernized UI.

Can Replay help with SOC2 or HIPAA compliance during modernization?#

Absolutely. Replay is built for regulated environments. It offers On-Premise deployment options, meaning your source code and data recordings never leave your infrastructure. The platform is designed to meet the rigorous security standards of Financial Services, Healthcare, and Government sectors.

What is the average time savings when using Replay?#

According to Replay's analysis, enterprises save an average of 70% in modernization time. A project that would typically take 18 months of manual coding and documentation can often be completed in a matter of weeks by using visual recordings to bootstrap the component library and architecture.

Ready to modernize without rewriting? Book a pilot with Replay