Modernizing AS/400 Inventory Systems: Bringing Green Screens into the 2025 Web Era

Your inventory system is 35 years old, runs on a machine the size of a refrigerator, and the only developer who understands the underlying RPG logic is three years past retirement age. This is the reality for thousands of enterprises in manufacturing, logistics, and retail. The AS/400 (now IBM i) is a workhorse, but its "green screen" 5250 interface has become a bottleneck for innovation, talent acquisition, and real-time data visibility.

The traditional response to this problem is the "Big Bang" rewrite. You hire a consultancy, spend $5M, and 24 months later, you have a failed project that only captures 60% of the original system's undocumented business logic. With a 70% failure rate for legacy rewrites, the "burn it down" approach is no longer a viable strategy for the modern CTO.

TL;DR: Modernizing AS/400 inventory systems doesn't require a risky multi-year rewrite; Replay’s Visual Reverse Engineering extracts undocumented business logic from user workflows, reducing modernization timelines from years to weeks.

The AS/400 Modernization Paradox#

The AS/400 remains in production because it is incredibly stable and the DB2 database at its core is a masterpiece of relational integrity. However, the technical debt is staggering. Global technical debt has reached $3.6 trillion, and a significant portion of that is locked in legacy terminal systems that lack documentation.

When you attempt to modernize these systems manually, your architects spend months in "software archaeology." They interview users, try to find 20-year-old spec docs that don't exist, and attempt to read spaghetti COBOL or RPG code.

The Cost of Manual Extraction#

Why Modernizing AS/400 Inventory Systems Fails#

Most inventory modernization projects fail because they underestimate the "hidden logic." In an AS/400 system, business rules are often buried in the display file (DSPF) logic or triggered by specific function keys (F1-F24) that aren't documented anywhere.

When a warehouse clerk hits

to "Partial Receive" an order, a cascade of legacy triggers occurs. If your new React frontend doesn't perfectly replicate that state machine, your inventory counts will drift, leading to catastrophic supply chain failures.

⚠️ Warning: Never attempt to modernize a legacy inventory system by reading the database schema alone. The true business logic lives in the interaction between the user and the 5250 terminal.

From Black Box to Documented Codebase#

Replay changes the paradigm by using Video as the Source of Truth. Instead of reading code, Replay records a real user performing their daily workflows—like a "Stock Adjustment" or "Cycle Count"—and uses Visual Reverse Engineering to extract the underlying architecture.

Step 1: Record the Workflow#

A subject matter expert (SME) performs a standard inventory task in the existing green screen. Replay captures the screen transitions, data inputs, and terminal state changes.

Step 2: Extraction and Mapping#

Replay’s AI Automation Suite analyzes the recording. It identifies that a specific field on the

screen maps to a "Quantity on Hand" variable and recognizes the validation logic that prevents negative inventory.

Step 3: Generating Modern Components#

Replay doesn't just give you a screenshot; it generates functional React components and API contracts.

typescript
// Example: Generated React Component from AS/400 Inventory Screen
// Source: Green Screen INV_ADJ_04
import React, { useState } from 'react';
import { Button, Input, Card, Alert } from '@/components/ui/library';

export const InventoryAdjustment = ({ partNumber, currentWarehouse }) => {
  const [adjustmentQty, setAdjustmentQty] = useState(0);
  const [reasonCode, setReasonCode] = useState('');
  const [error, setError] = useState<string | null>(null);

  // Business logic preserved from legacy F9 validation trigger
  const handleValidate = (value: number) => {
    if (value > 9999) {
      setError("Adjustment exceeds maximum allowable threshold for terminal 04");
      return false;
    }
    return true;
  };

  return (
    <Card title={`Adjust Stock: ${partNumber}`}>
      <Input 
        label="Adjustment Quantity"
        type="number"
        onChange={(e) => setAdjustmentQty(Number(e.target.value))}
      />
      {/* Reason codes extracted from legacy subfile lookup */}
      <Select label="Reason Code" options={['DMG', 'RET', 'CYC']} />
      
      <Button onClick={() => handleValidate(adjustmentQty)}>
        Submit Adjustment (Legacy F10)
      </Button>
      {error && <Alert type="error">{error}</Alert>}
    </Card>
  );
};

Bridging the Gap: API Contracts and E2E Tests#

One of the greatest risks in modernizing AS/400 inventory is the break between the new UI and the legacy backend. Replay mitigates this by generating OpenAPI (Swagger) contracts based on the data observed during the recording.

💡 Pro Tip: Use the generated API contracts to build a "Strangler Fig" facade. This allows you to swap out the legacy backend for a modern microservice later without changing the UI.

Generated API Contract Example#

yaml
# Generated by Replay - AS/400 Stock Update Service
openapi: 3.0.0
info:
  title: Inventory Management API
  version: 1.0.0
paths:
  /inventory/adjust:
    post:
      summary: Extracted from INV001 Screen Flow
      requestBody:
        content:
          application/json:
            schema:
              type: object
              properties:
                part_id: {type: string, example: "XP-99"}
                warehouse_loc: {type: string, example: "WH-01"}
                qty_change: {type: integer}
                action_key: {type: string, description: "Maps to legacy F10"}

The "Library" and "Blueprints" Advantage#

For enterprise architects, consistency is the primary concern. When modernizing hundreds of inventory screens, you cannot afford "CSS drift" or fragmented logic.

1. •The Library: Replay allows you to import your corporate Design System. When the platform extracts a green screen, it automatically maps legacy elements (like subfiles) to your modern React data tables.
2. •Flows: Replay visualizes the entire architecture of your legacy system. You can see how a user moves from "Receiving" to "Put-away," identifying redundant steps that can be optimized in the new web version.
3. •Blueprints: This is the visual editor where architects can refine the extracted logic, ensuring that 30 years of edge cases are preserved in the transition.

💰 ROI Insight: By automating the documentation and component generation, enterprises typically see a 70% reduction in labor costs. On a $2M modernization budget, that’s $1.4M returned to the bottom line.

Built for Regulated Environments#

Modernizing AS/400 inventory systems often happens in highly regulated sectors—Healthcare (pharmaceutical tracking), Government (defense logistics), and Financial Services.

Replay is built for these environments:

• •SOC2 & HIPAA Ready: Your data and workflows remain secure.
• •On-Premise Availability: For systems that cannot touch the public cloud, Replay can be deployed within your own VPC or data center.
• •Technical Debt Audit: Replay provides a comprehensive report of what was extracted and what remains, giving auditors a clear paper trail of the modernization process.

The Future Isn't Rewriting—It's Understanding#

The "Big Bang" rewrite is a relic of the 2010s. In 2025, the goal is to leverage the stability of the AS/400 while providing the agility of a React-based web ecosystem. By using Replay to visually reverse engineer your workflows, you move from a "black box" state to a fully documented, modern codebase in weeks, not years.

You don't need to find a 70-year-old RPG developer to explain how the inventory valuation works. You just need to record the process, let Replay extract the truth, and start building the future.

Frequently Asked Questions#

How does Replay handle complex RPG/COBOL logic that isn't visible on the screen?#

Replay captures the state transitions and data outputs. While it doesn't "read" the source code files directly, it observes the results of the logic (e.g., how a total is calculated or when an error message appears). This "Black Box" testing approach ensures the new system behaves exactly like the old one, regardless of how messy the underlying code is.

Can we modernize incrementally?#

Yes. This is the recommended approach. Using Replay, you can modernize one high-value flow (like "Real-time Stock Lookup") while keeping the rest of the system on the green screen. This "Strangler Fig" approach reduces risk and provides immediate ROI.

What if our AS/400 system has zero documentation?#

That is exactly what Replay was built for. Since 67% of legacy systems lack documentation, Replay creates the documentation for you by observing the system in action. The video recording becomes your new "functional specification."

How long does a typical pilot take?#

A pilot usually takes 5-10 business days. During this time, we typically extract 3-5 complex screens and generate a fully functional React prototype connected to a mock API.

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