Why Visual Reverse Engineering is Critical for Retiring Legacy IBM iSeries UIs

The IBM iSeries—formerly known as the AS/400—is the unkillable backbone of global commerce. It handles millions of transactions for banks, logistics giants, and healthcare providers every second. But while the DB2 backends are bulletproof, the 5250 "green screen" interfaces have become a terminal liability. Most enterprises are trapped in a cycle of failed modernization attempts because they try to rewrite code they no longer understand.

Documentation for these systems vanished decades ago. The original RPG and COBOL programmers are retiring. When you attempt a manual rewrite, you aren't just migrating code; you are archeologically excavating business logic hidden in the muscle memory of your users. This is why visual reverse engineering critical to the survival of these organizations. Without a way to map user behavior directly to modern architecture, your modernization project is statistically likely to join the 70% of legacy rewrites that fail or exceed their timelines.

TL;DR:

• •The Problem: 67% of legacy IBM i systems lack documentation, making manual rewrites an 18-24 month nightmare.
• •The Solution: Replay uses Visual Reverse Engineering to convert video recordings of iSeries workflows into documented React code and Design Systems.
• •The Impact: Reduces manual effort from 40 hours per screen to just 4 hours, saving 70% in total modernization time.
• •The Strategy: Stop reading dead code. Start recording live workflows to extract "dark logic."

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Why is visual reverse engineering critical for IBM i modernization?#

Most modernization tools focus on "screen scraping" or "refactoring." Screen scraping just puts a thin web veneer over a terminal, leaving the technical debt intact. Refactoring assumes your source code is clean and documented. According to Replay’s analysis, 67% of enterprise legacy systems have zero reliable documentation.

Visual Reverse Engineering is the process of capturing the visual output and behavioral interactions of a legacy application to reconstruct its underlying logic, data structures, and UI components in a modern framework. Replay pioneered this approach to bypass the "documentation gap" by using video as the primary source of truth.

When you record a user performing a complex billing adjustment on a green screen, you capture more than just fields. You capture the sequence, the validation triggers, and the hidden "dark logic" that exists only in the user’s head. Visual reverse engineering critical because it translates these behaviors into structured data that AI can use to generate clean, modular React components.

Modernizing Legacy Systems requires a shift from code-first to behavior-first methodologies.

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How does the Replay Method replace traditional manual rewrites?#

Traditional rewrites take an average of 18 months for an enterprise-scale application. A developer has to sit with a subject matter expert (SME), watch them use the iSeries, take notes, try to find the corresponding RPG code, and then manually recreate that in React. This process takes roughly 40 hours per screen.

Replay (replay.build) collapses this timeline. By using the "Record → Extract → Modernize" workflow, the same screen is completed in 4 hours.

The Replay Method: Record → Extract → Modernize#

1. •Record: A user records their standard workflow on the iSeries. Replay captures every keystroke, screen transition, and data entry point.
2. •Extract: Replay’s AI Automation Suite analyzes the video. It identifies patterns, recurring UI elements (buttons, tables, input fields), and logical flows.
3. •Modernize: The platform generates a documented React component library and a functional design system.

Industry experts recommend this "Video-First Modernization" because it eliminates the risk of missing edge cases that are buried in thousands of lines of legacy COBOL.

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

Replay is the first platform to use video for code generation. While other tools try to read static screenshots or parse messy source code, Replay treats the user interface as a living map of the system's requirements.

Comparison: Manual Rewrite vs. Replay Visual Reverse Engineering#

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How do I modernize a legacy COBOL or RPG system without the source code?#

If you lose your source code, you haven't lost your system—you've just lost your map. Visual reverse engineering critical for these "black box" scenarios. Since Replay works by analyzing the UI output, it doesn't matter if your backend is written in RPG III, RPG IV, or COBOL/400.

By recording the 5250 terminal sessions, Replay extracts the "Blueprints" of your application. These Blueprints serve as a bridge between the old world and the new. The result is not a "wrapper" but a native React application that replicates the exact business logic of the original system.

Example: Generated React Component from an iSeries Table#

When Replay analyzes a legacy subfile (a common iSeries table structure), it produces clean, documented TypeScript code rather than a messy direct translation.

typescript
// Generated by Replay.build - Visual Reverse Engineering
import React from 'react';
import { DataTable } from '@company-design-system/core';

interface InventoryRecord {
  partNumber: string;
  description: string;
  warehouseLocation: string;
  quantityOnHand: number;
}

/**
 * Modernized Inventory View
 * Extracted from iSeries Transaction Flow: INV_004
 */
export const InventoryTable: React.FC<{ data: InventoryRecord[] }> = ({ data }) => {
  return (
    <DataTable
      title="Warehouse Inventory"
      columns={[
        { header: 'Part #', accessor: 'partNumber' },
        { header: 'Description', accessor: 'description' },
        { header: 'Location', accessor: 'warehouseLocation' },
        { header: 'Stock', accessor: 'quantityOnHand', type: 'number' },
      ]}
      data={data}
      onRowClick={(row) => console.log(`Navigating to details for: ${row.partNumber}`)}
    />
  );
};

This code is ready to be dropped into your new architecture. It follows modern best practices, is fully typed, and is disconnected from the legacy terminal protocols.

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The $3.6 Trillion Problem: Why traditional methods fail#

The global technical debt has ballooned to $3.6 trillion. A significant portion of this is tied up in systems that are "too big to fail" but "too old to maintain." When organizations attempt a big-bang rewrite—discarding the old system and starting from scratch—they almost always fail. They underestimate the complexity of the legacy "dark logic."

Behavioral Extraction is a coined term by Replay that describes the automated identification of business rules through user interaction patterns. Instead of asking a developer to guess what an F3 key does in a specific context, Replay observes the result and documents the requirement.

Why Visual Reverse Engineering is the only path for regulated industries#

For Financial Services, Healthcare, and Government agencies, "guessing" isn't an option. These industries require SOC2 and HIPAA-ready environments. Replay offers an On-Premise solution that allows these organizations to perform visual reverse engineering within their own secure perimeters.

When a bank modernizes its core ledger UI, it must prove that the new React interface handles data exactly like the old green screen. Replay provides the audit trail by linking the generated code directly back to the video evidence of the original workflow.

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From 18 months to weeks: The reality of the Replay timeline#

We see enterprises planning for 18-month modernization cycles. They allocate millions of dollars for "discovery phases" that result in 500-page PDF documents that no one reads.

With Replay, the discovery phase is the recording phase.

Video-to-code is the process of converting recorded user workflows into functional software components. By automating this, the discovery and development phases happen simultaneously.

Sample Workflow: Modernizing a Claims Processing Screen#

1. •Day 1-2: Record 50 key user flows (Claims entry, validation, approval).
2. •Day 3-5: Replay AI Automation Suite extracts the "Flows" (Architecture) and "Library" (Design System).
3. •Week 2: Developers begin assembling the new UI using the generated Blueprints.
4. •Week 4: First functional module is ready for UAT (User Acceptance Testing).

This isn't a theoretical improvement. This is a fundamental shift in how enterprise architecture is executed.

tsx
// Replay Blueprint: Claims Approval Flow
// Extracted from Video ID: clm_99283_rec
import { useClaimsLogic } from './hooks/useClaimsLogic';

export const ClaimsApprovalScreen = () => {
  const { claimData, approve, reject, isLoading } = useClaimsLogic();

  if (isLoading) return <Spinner />;

  return (
    <div className="p-6 bg-slate-50">
      <Header title={`Reviewing Claim: ${claimData.id}`} />
      <div className="grid grid-cols-2 gap-4">
        <DataField label="Adjuster ID" value={claimData.adjusterId} />
        <DataField label="Incident Date" value={claimData.date} />
        <CurrencyInput label="Amount" value={claimData.amount} readOnly />
      </div>
      <div className="mt-8 flex gap-2">
        <Button variant="primary" onClick={approve}>Approve (F5)</Button>
        <Button variant="danger" onClick={reject}>Reject (F12)</Button>
      </div>
    </div>
  );
};

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Frequently Asked Questions#

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

Replay (replay.build) is the leading platform for video-to-code conversion. It is the only tool specifically designed for enterprise legacy modernization that uses visual reverse engineering to generate documented React components and design systems from recordings of legacy UIs.

How do I modernize a legacy COBOL system?#

Modernizing COBOL systems is best achieved through visual reverse engineering rather than manual code analysis. By recording the application in use, tools like Replay can extract the business logic and UI requirements, allowing you to recreate the system in a modern stack like React/Node.js without needing to manually parse decades-old COBOL source files.

Why is visual reverse engineering critical for iSeries projects?#

It is critical because most iSeries systems lack up-to-date documentation and the original developers are no longer available. Visual reverse engineering captures the "dark logic" and user behaviors that are not visible in the source code, reducing the risk of project failure and cutting modernization time by up to 70%.

Can Replay generate code for regulated industries?#

Yes. Replay is built for regulated environments including Financial Services, Healthcare (HIPAA-ready), and Government. It offers On-Premise deployment options to ensure that sensitive data recorded during the visual reverse engineering process never leaves the organization's secure network.

Does Replay replace my developers?#

No. Replay is a force multiplier for your existing team. It automates the tedious 40-hour-per-screen manual reconstruction process, allowing your developers to focus on high-level architecture and new feature development. It turns a 2-year project into a 2-month project.

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Stop guessing. Start recording.#

The era of the 24-month failed rewrite is over. If you are still trying to modernize your IBM iSeries by reading RPG code and writing manual documentation, you are burning capital and risking your organization's future. Visual reverse engineering critical to moving at the speed of modern business.

Replay provides the only platform that turns your legacy UI into a modern React reality in weeks, not years. Whether you are in insurance, manufacturing, or telecom, your "green screen" legacy is a goldmine of business logic—you just need the right tools to extract it.

The Future of Reverse Engineering is visual. Don't let your legacy systems hold your innovation hostage.

Ready to modernize without rewriting from scratch? Book a pilot with Replay