Modernizing AS/400 ERPs: Moving from Green Screens to Cloud-Native React Without Downtime

The AS/400 is the cockroach of the enterprise: it refuses to die because it is too critical to kill. For decades, IBM i systems have powered the backbones of global logistics, insurance, and manufacturing. But as the "Green Screen" talent pool hits retirement age and the $3.6 trillion global technical debt mountain grows, the risk of doing nothing has finally eclipsed the risk of modernization.

The traditional path—the "Big Bang" rewrite—is a suicide mission. Statistics show that 70% of legacy rewrites fail or significantly exceed their timelines. When you are modernizing 400 ERPs, you aren't just changing a UI; you are attempting to untangle decades of undocumented RPG logic, hidden dependencies, and proprietary database triggers.

The future isn't rewriting from scratch—it's understanding what you already have through Visual Reverse Engineering.

TL;DR: Modernizing AS/400 ERPs no longer requires 24-month high-risk rewrites; by using Replay to record user workflows, enterprises can extract business logic and generate production-ready React components in weeks, reducing modernization timelines by 70%.

The Archaeology Problem: Why Manual Modernization Fails#

Most enterprise architects approach modernizing 400 ERPs like an archaeological dig. They assign senior developers to "read the code," only to find that 67% of these legacy systems lack any meaningful documentation. You’re not just reading RPG; you’re reading the intentions of a developer who left the company in 1994.

This manual "code archaeology" consumes roughly 40 hours per screen. In a standard ERP with 500+ screens, you are looking at years of manual labor before a single line of modern code is even written.

The Cost of the "Wait and See" Approach#

⚠️ Warning: Attempting a Big Bang rewrite of a core ERP without a "source of truth" for existing business logic usually results in "Feature Parity Gap," where the new system fails to handle the 5% of edge cases that represent 90% of your business value.

From 5250 Protocol to React: The Replay Methodology#

The breakthrough in modernizing 400 ERPs lies in treating the user interface as the source of truth. Every time a power user navigates a 5250 terminal screen, they are executing a precise sequence of business logic.

Replay captures these workflows. By recording the "Green Screen" sessions, Replay’s AI Automation Suite performs visual reverse engineering to identify data fields, validation rules, and navigational flows. It then maps these to modern React components and API contracts.

Step 1: Workflow Mapping (The Recording)#

Instead of diving into RPG source code, you record the actual business process. Whether it’s "Order Entry" or "Inventory Adjustment," the recording captures the state changes and data requirements.

Step 2: Component Extraction#

Replay analyzes the recording and generates documented React components. This isn't just a "skin" over the old system; it's a clean-room implementation of the UI logic.

typescript
// Example: Generated React Component from an AS/400 'Order Entry' Screen
// This component was extracted via Replay Visual Reverse Engineering
import React, { useState, useEffect } from 'react';
import { Button, Input, Table, Alert } from '@/components/ui-library'; 

export function ModernOrderEntry({ legacyId }: { legacyId: string }) {
  const [orderData, setOrderData] = useState<any>(null);
  const [loading, setLoading] = useState(true);

  // Business logic preserved: AS/400 validation for Region Code 04
  const validateRegion = (code: string) => {
    return code.startsWith('04') ? 'International Shipping Required' : null;
  };

  return (
    <div className="p-6 bg-white rounded-lg shadow-md">
      <h2 className="text-2xl font-bold mb-4">ERP Order Management</h2>
      <div className="grid grid-cols-2 gap-4">
        <Input 
          label="Customer ID" 
          defaultValue={orderData?.CUST_ID} 
          onChange={(e) => {/* ... */}}
        />
        <Input 
          label="Region Code" 
          error={validateRegion(orderData?.REGION)}
          defaultValue={orderData?.REGION}
        />
      </div>
      {/* Replay-generated table based on legacy subfile patterns */}
      <Table data={orderData?.lineItems} />
      <Button onClick={() => handleSave()}>Sync to IBM i</Button>
    </div>
  );
}

💡 Pro Tip: Focus on "High-Velocity Workflows" first. Identify the 20% of screens that users spend 80% of their time in. Modernizing these provides the fastest ROI.

Bridging the Gap: API Contracts and Data Integrity#

The biggest hurdle in modernizing 400 ERPs is the backend. You cannot simply shut down the DB2 database. Replay generates API contracts (OpenAPI/Swagger) that define how the new React frontend should communicate with the legacy backend, often via a middleware layer like MuleSoft, IBM i Access, or custom REST wrappers.

Automated API Contract Generation#

Replay doesn't just give you a UI; it gives you the blueprint for the integration. By observing the data flowing in and out of the legacy screens, it identifies the required parameters for your new microservices.

yaml
# Generated OpenAPI Spec for AS/400 Inventory Service
openapi: 3.0.0
info:
  title: Legacy ERP Inventory Bridge
  version: 1.0.0
paths:
  /inventory/{partId}:
    get:
      summary: Extracted from 'INV_INQ_01' screen
      parameters:
        - name: partId
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: Successful retrieval
          content:
            application/json:
              schema:
                type: object
                properties:
                  warehouse_qty: {type: integer}
                  bin_location: {type: string}
                  last_audit_date: {type: string, format: date}

💰 ROI Insight: Manual documentation of a single complex ERP module typically takes 200+ hours. Replay reduces this to under 20 hours, representing a 90% reduction in "discovery" costs.

Technical Debt Audit: Knowing What Not to Move#

Not every part of your 400 ERP deserves to live in the cloud. Over 30 years, these systems accumulate "dead code"—features for product lines that no longer exist or tax laws that have been repealed.

Replay’s Technical Debt Audit feature identifies which screens and workflows are actually being used by your staff. If a screen hasn't been accessed in the last 12 months, it shouldn't be part of your modernization roadmap. This "pruning" alone can reduce the scope of a modernization project by 30-40%.

The Modernization Workflow#

1. •Assessment: Run Replay's audit to identify active vs. dormant workflows.
2. •Recording: Subject matter experts (SMEs) record standard and edge-case paths through the legacy system.
3. •Extraction: Replay generates React components, CSS (matching your modern design system), and documentation.
4. •Refinement: Developers use the Replay Blueprint Editor to tweak the generated code and connect it to real APIs.
5. •Deployment: Deploy the new React modules side-by-side with the legacy system (the Strangler Fig pattern) to ensure zero downtime.

📝 Note: For regulated industries like Healthcare or Financial Services, Replay offers on-premise deployment and is HIPAA-ready, ensuring that sensitive data used during the recording process never leaves your secure perimeter.

Case Study: Manufacturing ERP Transformation#

A global Tier-1 automotive supplier was stuck on a highly customized AS/400 ERP. They faced a critical talent shortage; only two developers remained who understood the RPG logic for their proprietary Just-In-Time (JIT) sequencing.

The Challenge:

• •1,200 screens.
• •Zero documentation.
• •18-month estimate for manual rewrite.

The Replay Solution: By using Replay, they recorded the top 50 most critical workflows. Within 3 weeks, Replay had generated the core React components for their production floor dashboard.

The Results:

• •Time Savings: Project completed in 4 months instead of 18.
• •Accuracy: 100% of business rules (including complex JIT calculations) were preserved because they were extracted from live execution.
• •Cost: Saved $1.2M in developer hours.

Frequently Asked Questions#

How does Replay handle complex "Subfiles" in AS/400?#

AS/400 subfiles (the legacy version of data tables) are notoriously difficult to modernize. Replay recognizes subfile patterns during recording and automatically maps them to modern, paginated, and sortable React Table components, preserving the underlying data structure.

Do we need to modify our RPG code?#

No. Replay is non-invasive. It observes the presentation layer and the data stream. This allows you to modernize the user experience and document the system without touching a single line of RPG until you are ready to migrate the backend logic to microservices.

Can Replay generate E2E tests?#

Yes. Because Replay records the actual user journey, it can automatically generate Playwright or Cypress E2E tests that mirror the legacy workflow. This ensures that your new React application behaves exactly like the old system, providing a "safety net" for your developers.

What about data security during recording?#

Replay is built for regulated environments. We offer SOC2 compliance, HIPAA-ready configurations, and the ability to run the entire extraction engine on-premise. Sensitive data can be masked during the recording process so it never enters the AI training or extraction loops.

The Future is Understanding, Not Rewriting#

The era of the multi-year, multi-million dollar "Big Bang" failure is over. Modernizing 400 ERPs is no longer a choice between staying in the dark ages or risking the business on a rewrite.

By using Visual Reverse Engineering, you turn your legacy "black box" into a documented, modern codebase. You stop guessing what your system does and start seeing it in high definition.

Archaeology is for museums. Replay is for the enterprise.

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