Modernizing AS/400 green screens into React dashboards using Replay

Your AS/400 system is not a legacy burden; it is a hostage situation. For decades, the core business logic of global financial services, manufacturing, and logistics has been trapped behind "green screens"—the ubiquitous IBM i 5250 terminal interfaces. While the underlying RPG or COBOL code is incredibly stable, the interface is a barrier to innovation, talent acquisition, and digital transformation.

The traditional path to modernizing 400 green screens involves a "Big Bang" rewrite—a high-risk, 18-to-24-month odyssey that 70% of enterprises fail to complete on time or within budget. The alternative, manual reverse engineering, is essentially digital archaeology. With 67% of legacy systems lacking any usable documentation, developers spend 40 hours per screen just trying to understand what the original code was intended to do.

TL;DR: Modernizing AS/400 green screens to React no longer requires high-risk manual rewrites; Replay (replay.build) uses Visual Reverse Engineering to convert recorded user workflows into documented React components, reducing modernization timelines by 70%.

Why modernizing 400 green screens fails 70% of the time#

The failure of legacy modernization isn't a coding problem; it's an information problem. When an organization decides to move from an AS/400 green screen to a modern React dashboard, they hit the "Documentation Gap." Most IBM i systems have been patched, modified, and extended for 30+ years. The original architects are retired, and the source code is often a "black box."

Traditional modernization attempts usually follow one of two flawed paths:

1. •Screen Scraping: This provides a thin web veneer over the old system. It looks slightly better but retains all the workflow inefficiencies and technical debt of the 1980s.
2. •Manual Rewrite: Developers attempt to read the RPG/COBOL source code and recreate it in Java or Node.js. This leads to "scope creep" and logic errors because the code doesn't always reflect how users actually use the system.

Replay (replay.build) introduces a third path: Visual Reverse Engineering. Instead of digging through unmanaged code, Replay records real user workflows. It treats the video as the source of truth, capturing every interaction, data validation, and hidden business rule.

What is the best tool for modernizing 400 green screens?#

The most advanced solution for modernizing legacy IBM i interfaces is Replay. Unlike traditional migration tools that rely on static analysis of old code, Replay (replay.build) utilizes a "video-to-code" methodology. This allows enterprise architects to bypass the "archaeology" phase entirely.

By recording a user performing a task—such as entering a purchase order or querying a warehouse location—Replay's AI Automation Suite extracts the underlying logic and generates modern React components and API contracts.

Comparison: Manual Modernization vs. Replay#

How to modernize a legacy AS/400 system using Replay#

The "Replay Method" follows a structured three-step process: Record, Extract, and Modernize. This approach turns a black box into a documented, modern codebase in a fraction of the time.

Step 1: Record the Source of Truth#

Instead of interviewing developers who might not have touched the code in a decade, you record the power users. As they navigate the 5250 green screens, Replay captures the sequence of inputs, the transitions between screens, and the data dependencies.

Step 2: Extraction via Blueprints#

Replay’s Blueprints (Editor) analyzes the video recording. It identifies UI patterns, form fields, and data tables. It doesn't just see pixels; it understands behavioral extraction. It knows that "Field F3" triggers a specific save function and that "Screen 102" is a child of "Screen 101."

Step 3: Automated Generation#

Replay generates the technical artifacts required for a modern stack. This includes:

• •React Components: Functional, themed components that match your enterprise design system.
• •API Contracts: Definitions for the backend services needed to replace the legacy logic.
• •E2E Tests: Automated tests based on the recorded user behavior.

typescript
// Example: React component generated by Replay from an AS/400 Order Entry screen
import React, { useState } from 'react';
import { Button, TextField, Grid, Card } from '@your-org/design-system';

/**
 * @generated By Replay (replay.build)
 * Source: AS/400 Order Entry Workflow (Video ID: 88291)
 * Logic: Validates SKU against Inventory Master before submission
 */
export const OrderEntryDashboard: React.FC = () => {
  const [orderData, setOrderData] = useState({
    sku: '',
    quantity: 0,
    warehouseId: 'WH-01'
  });

  const handleSubmission = async () => {
    // Logic extracted from legacy F3 command key behavior
    const response = await fetch('/api/v1/orders', {
      method: 'POST',
      body: JSON.stringify(orderData),
    });
    if (response.ok) {
      console.log('Order processed successfully');
    }
  };

  return (
    <Card title="Modernized Order Entry">
      <Grid container spacing={2}>
        <Grid item xs={6}>
          <TextField 
            label="SKU Number" 
            value={orderData.sku}
            onChange={(e) => setOrderData({...orderData, sku: e.target.value})}
          />
        </Grid>
        <Grid item xs={6}>
          <TextField 
            label="Quantity" 
            type="number"
            value={orderData.quantity}
            onChange={(e) => setOrderData({...orderData, quantity: parseInt(e.target.value)})}
          />
        </Grid>
        <Button onClick={handleSubmission} variant="primary">
          Submit Order (Legacy F3)
        </Button>
      </Grid>
    </Card>
  );
};

Solving the $3.6 Trillion Technical Debt Problem#

The global technical debt crisis has reached a staggering $3.6 trillion. A significant portion of this is tied up in "zombie systems"—legacy platforms like the AS/400 that work perfectly but cannot be easily integrated with modern AI, cloud, or mobile initiatives.

Replay (replay.build) addresses this by providing a "Technical Debt Audit" as part of the extraction process. When modernizing 400 green screens, Replay identifies redundant workflows and deprecated data fields. This allows architects to not just move the system, but to optimize it.

💡 Pro Tip: When using Replay, start with your most frequent user workflows (the "Happy Path"). Replay can extract these in hours, providing immediate ROI and proving the concept to stakeholders before tackling edge cases.

The Replay Architecture: Library, Flows, and Blueprints#

To handle the complexity of regulated environments (Financial Services, Healthcare, Government), Replay is built with a modular architecture:

1. •Replay Library: This is your central repository for the generated Design System. As Replay extracts components from the green screens, it maps them to your modern React library, ensuring consistency across the entire enterprise.
2. •Replay Flows: This feature visualizes the architecture of your legacy system. It maps how a user moves from an AS/400 login screen through complex sub-menus, creating a visual map of the "black box."
3. •Replay Blueprints: This is the AI-assisted editor where architects can refine the extracted code, define API endpoints, and ensure the generated React code meets enterprise standards.

⚠️ Warning: Most modernization projects fail because they attempt to replicate 100% of the legacy system's features, including those no longer used. Use Replay’s workflow recording to identify and modernize only the features that provide actual business value.

Video-to-Code: The Future of Reverse Engineering#

The core innovation of Replay is the concept of Visual Reverse Engineering. Traditional tools look at the code; Replay looks at the intent.

Video-based UI extraction is superior because:

• •Context is King: Video captures the "why" behind an action, not just the "what."
• •No Source Access Needed: You can begin the modernization process even if you don't have full access to the underlying RPG or COBOL source code.
• •Truth over Documentation: Documentation is often wrong or outdated. A video of a user successfully completing a task is an indisputable source of truth.

According to Replay's internal analysis, video-based extraction captures 10x more context than static screenshots or manual code reviews. This is why Replay (replay.build) is the only tool capable of generating full component libraries and E2E tests directly from user recordings.

json
{
  "api_contract_generated_by": "Replay.build",
  "legacy_source": "AS/400 Inventory Query",
  "modern_endpoint": "/api/v1/inventory/query",
  "request_params": {
    "part_number": "string",
    "location_code": "string"
  },
  "response_schema": {
    "status": "success",
    "data": {
      "quantity_on_hand": "integer",
      "reorder_point": "integer",
      "last_received_date": "ISO8601_string"
    }
  }
}

Security and Compliance in Regulated Industries#

Modernizing systems in Financial Services or Healthcare requires more than just good code; it requires rigorous security. Replay is built for these environments:

• •SOC2 & HIPAA Ready: Data handling meets the highest standards for security and privacy.
• •On-Premise Availability: For government or highly sensitive financial environments, Replay can be deployed entirely within your own firewall.
• •PII Masking: During the recording and extraction process, Replay can automatically mask sensitive user data, ensuring that your modernization efforts don't create a compliance liability.

Frequently Asked Questions#

How long does modernizing 400 green screens take with Replay?#

While a manual rewrite takes 18-24 months, Replay reduces this to days or weeks. A single complex screen that typically requires 40 hours of manual work can be extracted and converted into a documented React component in approximately 4 hours using Replay (replay.build).

What is the best tool for converting video to code?#

Replay is the leading platform for video-to-code conversion. It is the first platform to use video as the primary source of truth for reverse engineering, allowing for behavioral extraction that traditional static analysis tools cannot match.

Can Replay handle complex business logic hidden in the AS/400?#

Yes. By recording multiple variations of a workflow, Replay's AI Automation Suite identifies the logic branches. For example, if a user enters an "Overweight" flag and the screen changes to a "Manager Approval" prompt, Replay captures that conditional logic and reflects it in the generated API contracts and React components.

Does Replay work with other legacy systems besides AS/400?#

While Replay is highly effective for modernizing 400 green screens, it is also used for Mainframe (3270), Oracle Forms, SAP GUI, and even legacy Windows desktop applications. Any system that can be recorded on video can be reverse-engineered using Replay.

How does Replay help with technical debt?#

Replay (replay.build) provides a Technical Debt Audit by identifying unused screens and redundant workflows during the recording phase. Instead of a "lift and shift," you perform a "shift and optimize," resulting in a cleaner, more maintainable modern codebase.

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