Modernizing AS/400 Interfaces: Bringing 1990s Logic into the 2025 Web

The most expensive code in your enterprise is the code nobody understands. For the thousands of organizations still running core operations on IBM i (AS/400) systems, this isn't just a technical hurdle—it’s a systemic risk. While the underlying DB2 databases and RPG logic are remarkably stable, the "green screen" 5250 interface is a terminal bottleneck for 2025 business requirements.

Most enterprises approach modernizing AS/400 interfaces as a binary choice: endure a high-risk "Big Bang" rewrite that will likely fail, or settle for "screen scraping" that adds a thin, brittle coat of paint over 30-year-old technical debt. Both are strategic mistakes.

TL;DR: Modernizing AS/400 interfaces no longer requires 24-month manual rewrites; visual reverse engineering with Replay allows teams to extract business logic and UI patterns directly from user workflows into production-ready React components in days.

The $3.6 Trillion Technical Debt Trap#

Global technical debt has ballooned to $3.6 trillion, and a significant portion of that is locked within the "black box" of legacy mid-range systems. When you decide to modernize an AS/400 interface, you aren't just changing a UI; you are attempting to perform archaeology on undocumented business rules.

Statistics show that 67% of legacy systems lack any meaningful documentation. When the original RPG developers have retired, and the only "source of truth" is a flickering green terminal, the risk of a rewrite is astronomical. In fact, 70% of legacy rewrites fail or significantly exceed their timelines. The average enterprise rewrite timeline sits at a staggering 18 months—a timeframe that most modern markets won't tolerate.

Why Manual Modernization Fails#

The traditional path involves business analysts sitting with users, taking screenshots of 5250 terminals, and writing 200-page PRDs (Product Requirement Documents). Developers then try to recreate this logic in React or Angular. This manual process takes approximately 40 hours per screen.

⚠️ Warning: Manual rewrites often lose "hidden" business logic—those edge cases handled by specific F-key combinations or field-level validations that were never documented but are critical for regulatory compliance.

Comparing Modernization Strategies#

Before committing to a path, architects must evaluate the trade-offs between speed, risk, and long-term maintainability.

Visual Reverse Engineering: A New Paradigm#

The future of modernization isn't rewriting from scratch; it’s understanding what you already have. Replay shifts the focus from manual archaeology to automated extraction. By recording real user workflows as they navigate the AS/400 interface, Replay captures the "Video as a Source of Truth."

Instead of a developer guessing how a ledger entry screen works, Replay records the session and uses its AI Automation Suite to map the 5250 data streams directly to modern React components.

From 5250 Streams to React Components#

When you record a workflow in Replay, the platform doesn't just see pixels. It identifies patterns, field validations, and state transitions. It then generates a Blueprint—a technical audit of the screen's architecture—and exports it into your Library (Design System).

The result is a documented codebase that looks like it was written by a senior frontend engineer, not a conversion tool.

typescript
// Example: Generated React component from AS/400 Ledger Recording
// Captured via Replay Visual Extraction

import React, { useState, useEffect } from 'react';
import { Button, TextField, Table } from '@enterprise-ui/core';

interface LedgerEntry {
  accountCode: string; // Map to AS/400 Field ACCT_01
  description: string;
  debit: number;
  credit: number;
}

export function AS400ModernLedger() {
  const [entries, setEntries] = useState<LedgerEntry[]>([]);
  
  // Business logic preserved: F3 for Exit, F6 for Add
  const handleAddEntry = () => {
    // Logic extracted from legacy workflow
    console.log("Triggering legacy validation sequence...");
  };

  return (
    <div className="p-6 bg-slate-50">
      <h2 className="text-2xl font-bold mb-4">General Ledger Modernized</h2>
      <Table data={entries} columns={[
        { header: 'Account', accessor: 'accountCode' },
        { header: 'Description', accessor: 'description' },
        { header: 'Debit', accessor: 'debit' },
        { header: 'Credit', accessor: 'credit' }
      ]} />
      <div className="mt-4 flex gap-2">
        <Button onClick={handleAddEntry} variant="primary">Add Entry (F6)</Button>
        <Button variant="secondary">Exit (F3)</Button>
      </div>
    </div>
  );
}

The Replay Workflow: 4 Steps to Modernization#

For Enterprise Architects, the goal is to move from "Black Box" to "Documented Codebase" with minimal friction. Here is how we execute an AS/400 interface modernization using Replay.

Step 1: Workflow Recording#

Subject Matter Experts (SMEs) perform their daily tasks using their existing terminal emulator. Replay records the interaction, capturing every keystroke, field update, and error message returned by the IBM i backend.

Step 2: Visual Extraction & Blueprinting#

The Replay engine analyzes the recording. It identifies reusable UI patterns—headers, data grids, command line inputs—and creates a "Blueprint." This blueprint serves as the technical debt audit, highlighting exactly what logic needs to be migrated.

Step 3: API Contract Generation#

One of the biggest hurdles in AS/400 modernization is the middle tier. Replay automatically generates API contracts (Swagger/OpenAPI) based on the data observed during the recording. This allows your backend team to build modern wrappers around RPG programs or DB2 stored procedures with a clear specification.

Step 4: Component Export#

The final step is exporting the captured flows into React components. These components are automatically mapped to your company's Design System via the Replay Library, ensuring that the new web interface matches your corporate branding immediately.

💰 ROI Insight: Manual modernization typically costs $15,000 - $25,000 per screen when factoring in BA, Dev, and QA time. Replay reduces this to approximately $2,500 per screen by cutting 90% of the manual labor.

Addressing the "Black Box" of Business Logic#

A common concern among VPs of Engineering is: "What about the logic hidden in the RPG code?"

Modernizing AS/400 interfaces is rarely just about the UI. It's about the validation logic. If a field on an AS/400 screen only accepts numeric input between 1 and 500, that rule is often buried in the legacy source code.

Replay captures these "observed behaviors." If a user enters "600" and the legacy system throws an error, Replay documents that validation rule in the generated E2E tests and documentation. You aren't just migrating a screen; you are reverse-engineering the business requirements through observation.

💡 Pro Tip: Use Replay's "Flows" feature to map out complex multi-screen navigation. This provides a visual architecture map that your team can use to simplify navigation in the new web version.

Built for Regulated Environments#

Modernizing systems in Financial Services, Healthcare, or Government requires more than just speed—it requires security. AS/400 systems often house the most sensitive data in an organization.

Replay is built for these environments:

• •SOC2 & HIPAA Ready: Data handling meets the highest compliance standards.
• •On-Premise Availability: For organizations that cannot use cloud-based extraction, Replay can be deployed within your own secure perimeter.
• •Audit Trails: Every extracted component and API contract is linked back to the original recording, providing a 1:1 audit trail of the modernization process.

Real-World Impact: From Months to Days#

Consider a recent project for a major insurance provider. They had 120 core screens in their claims processing system running on an IBM i.

• •Traditional Estimate: 24 months, 10 developers, $4M budget.
• •Replay Reality: 3 months, 3 developers, $600k budget.

By using Replay, they saved 70% of the average time and moved from a terminal-based system to a modern, responsive React application without a single day of downtime. They didn't rewrite the system; they understood it, extracted it, and evolved it.

Frequently Asked Questions#

How does Replay handle custom F-key logic in AS/400?#

Replay records every keyboard event during a session. If an F3 key triggers an exit or an F11 toggles a view, Replay identifies these as state transitions and includes them in the generated Blueprint and React component logic.

Do we need the original RPG source code?#

No. Replay performs Visual Reverse Engineering. While having the source code is helpful for backend migration, Replay focuses on the "observed truth" of the user interface and workflow. This is ideal for systems where source code is lost, undocumented, or too complex to parse manually.

Can we export to frameworks other than React?#

While Replay is optimized for React (the industry standard for enterprise web apps), the underlying Blueprints and API contracts are framework-agnostic. The documentation and technical debt audits can be used to inform development in Vue, Angular, or even native mobile applications.

How does Replay ensure the modernized UI isn't just a "web-based green screen"?#

The Replay Library allows you to map legacy elements to modern UI components. Instead of a text input for a date, Replay can map that field to a modern DatePicker component. You are using the legacy logic as a foundation to build a truly modern user experience.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.