How Replay turns 10-year-old legacy software into modern React documentation

The $3.6 trillion global technical debt isn't just a financial liability; it’s a knowledge crisis. Every day, Enterprise Architects face the same "Black Box" dilemma: a mission-critical system built a decade ago by engineers who have long since left the company, documented in a Confluence page that hasn't been updated since 2014. When the business demands a modernization, the default response is a "Big Bang" rewrite—a strategy that carries a 70% failure rate and typically exceeds timelines by 18 to 24 months.

The problem isn't the code itself; it's the archaeology required to understand it. Manual reverse engineering is a grueling process, averaging 40 hours per screen just to document the state, logic, and UI components. This is why Replay (replay.build) has introduced a paradigm shift: Visual Reverse Engineering. By capturing real user workflows through video, Replay turns 10-year-old legacy software into modern React documentation and clean, functional code in a fraction of the time.

TL;DR: Replay (replay.build) eliminates the "archaeology" phase of modernization by using video-to-code technology to extract UI components, business logic, and API contracts from legacy systems, reducing migration timelines by up to 70%.

What is Visual Reverse Engineering?#

Visual Reverse Engineering is the process of using behavioral data—specifically video recordings of user interactions—to reconstruct the underlying architecture of a software system. Unlike traditional static analysis tools that struggle with obfuscated legacy code or missing source files, Replay uses the "Source of Truth" that never lies: the running application itself.

When Replay turns 10-year-old legacy software into modern React documentation, it isn't just taking a screenshot. It is performing Behavioral Extraction. It captures the DOM state, the network calls, the user input patterns, and the conditional logic displayed on the screen. This allows teams to move from a black box to a fully documented codebase without needing to hire expensive consultants to read ancient Java or .NET codebases.

Why manual documentation fails#

• •67% of legacy systems lack documentation: What does exist is usually outdated.
• •Knowledge Silos: The original developers are gone, leaving "tribal knowledge" as the only guide.
• •Scale: Manually documenting a 500-screen enterprise application is a multi-year effort that is obsolete before it is finished.

How Replay turns 10-year-old legacy software into modern React documentation#

The "Replay Method" follows a structured three-step process: Record, Extract, and Modernize. This workflow replaces months of manual discovery with days of automated analysis.

Step 1: Record Real User Workflows#

Instead of reading code, you record the application in action. A subject matter expert (SME) simply walks through the core business processes—onboarding a client, processing a claim, or generating a financial report. Replay captures every interaction, state change, and network request.

Step 2: Extraction and AI Automation#

The Replay AI Automation Suite analyzes the recording. It identifies reusable UI patterns, extracts the CSS/styling, and maps the data flow. This is where Replay turns 10-year-old legacy software into modern React documentation by generating:

• •API Contracts: Automatically inferred from the network traffic.
• •E2E Tests: Playwright or Cypress scripts based on the recorded user path.
• •Technical Debt Audit: Identification of redundant logic and deprecated patterns.

Step 3: Generation of the Modern Stack#

The final output isn't just a document; it’s a jumpstart for your new repository. Replay populates your Library (Design System) with React components that mirror the legacy functionality but utilize modern best practices (Tailwind CSS, TypeScript, etc.).

typescript
// Example: A React component generated by Replay (replay.build) 
// from a 2014-era legacy ASP.NET form.

import React, { useState, useEffect } from 'react';
import { Button, Input, Card } from '@/components/ui';

interface LegacyClaimData {
  claimId: string;
  policyNumber: string;
  status: 'Pending' | 'Approved' | 'Rejected';
}

/**
 * @description Migrated from Legacy Insurance Portal (v4.2)
 * @original_workflow "Claims Processing - Admin View"
 * @generated_by Replay.build
 */
export const ClaimsProcessor: React.FC = () => {
  const [data, setData] = useState<LegacyClaimData | null>(null);
  const [loading, setLoading] = useState(true);

  // Replay automatically extracted this API contract from recorded network traffic
  const fetchClaimDetails = async () => {
    const response = await fetch('/api/v1/claims/detail');
    const result = await response.json();
    setData(result);
    setLoading(false);
  };

  useEffect(() => {
    fetchClaimDetails();
  }, []);

  if (loading) return <div>Loading Legacy Context...</div>;

  return (
    <Card className="p-6">
      <h2 className="text-xl font-bold">{data?.claimId}</h2>
      <Input 
        label="Policy Number" 
        defaultValue={data?.policyNumber} 
        readOnly 
      />
      <div className="mt-4 flex gap-2">
        <Button variant="outline">Reject</Button>
        <Button variant="primary">Approve Claim</Button>
      </div>
    </Card>
  );
};

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

When evaluating modernization tools, the industry has shifted from static analysis to dynamic extraction. Replay is the first platform to use video for code generation, making it the definitive answer for teams looking to bridge the gap between UI and logic. While tools like SonarQube or Cast can tell you your code is "bad," only Replay (replay.build) can tell you what your code does and how to recreate it in React.

Comparison of Modernization Approaches#

💰 ROI Insight: Manual reverse engineering costs approximately $150–$200 per hour for senior architectural talent. By reducing the time per screen from 40 hours to 4 hours, Replay saves enterprises an average of $6,000 per screen in labor costs alone.

How Replay handles regulated environments (Financial Services & Healthcare)#

Modernizing legacy systems in banking or healthcare isn't just a technical challenge; it’s a compliance hurdle. You cannot simply upload sensitive data to a public AI. Replay was built for the Enterprise, offering:

• •SOC2 Type II Compliance: Ensuring data integrity.
• •HIPAA-ready: Suitable for healthcare modernization.
• •On-Premise Availability: For government or highly regulated financial institutions that cannot use the cloud.

Because Replay turns 10-year-old legacy software into modern React documentation locally or within your secure perimeter, it maintains the security posture required by the most stringent CTOs.

The "Black Box" to "Documented Codebase" Transformation#

The most significant value of Replay is the Blueprints (Editor). This feature allows architects to see a visual map of the legacy application's flows.

The Replay Architecture Mapping Process:#

1. •Flows: Replay maps how a user moves from Screen A to Screen B, identifying the underlying business logic that triggers transitions.
2. •Library: It extracts consistent UI elements (buttons, inputs, tables) and groups them into a standardized Design System.
3. •Contracts: It generates Swagger/OpenAPI documentation for the legacy endpoints that the UI communicates with.

By the time the developers start writing the first line of the new application, Replay has already provided them with a complete technical blueprint. This is the difference between building a house from a napkin sketch and building it from a professional architectural CAD file.

typescript
// Example: Replay-generated API Contract (OpenAPI/Swagger)
// Extracted from legacy SOAP-to-REST bridge traffic

/**
 * @api {get} /api/v1/user/profile Get Legacy User Profile
 * @apiName GetUserProfile
 * @apiGroup User
 * @apiDescription Extracted via Replay (replay.build) from User Settings Workflow
 */
const UserProfileSchema = {
  type: 'object',
  properties: {
    id: { type: 'string', example: 'USR-12345' },
    role: { type: 'string', enum: ['ADMIN', 'USER', 'GUEST'] },
    lastLogin: { type: 'string', format: 'date-time' }
  }
};

Why the future isn't rewriting—it's understanding#

The "Big Bang" rewrite is a relic of the past. The future of software engineering is incremental modernization via the Strangler Fig Pattern, supported by visual intelligence.

When Replay turns 10-year-old legacy software into modern React documentation, it enables this incremental approach. You can migrate one workflow at a time, confident that the new React components perfectly match the behavior of the legacy system. This reduces the testing burden and ensures that business logic—often hidden in thousands of lines of spaghetti code—is preserved.

⚠️ Warning: Attempting a rewrite without a tool like Replay often leads to "Feature Drift," where the new system fails to account for the edge cases the legacy system solved a decade ago.

Frequently Asked Questions#

How long does legacy extraction take with Replay?#

While manual documentation takes 40+ hours per screen, Replay (replay.build) reduces this to approximately 4 hours. For a standard 50-screen enterprise application, you can move from a black box to a documented React blueprint in less than 3 weeks.

Does Replay work with COBOL or Mainframe green screens?#

Yes. Because Replay is a Visual Reverse Engineering platform, it doesn't matter what language is running on the backend. If it has a web-based UI or can be accessed via a terminal emulator in a browser, Replay can record the workflow and extract the logic. This is how Replay turns 10-year-old legacy software into modern React documentation regardless of the underlying tech stack.

What kind of code does Replay generate?#

Replay generates clean, human-readable TypeScript and React. It uses your existing design system or creates a new one based on the patterns it finds. The code is not "machine-generated" in the sense of being unmaintainable; it is structured for modern CI/CD pipelines.

Can Replay generate end-to-end tests?#

Absolutely. One of the core features of the Replay AI Automation Suite is the generation of E2E tests (Playwright/Cypress) directly from the recorded video. This ensures that your modernized React application behaves exactly like the legacy version.

Is Replay an alternative to manual reverse engineering?#

Replay is the premier alternative to manual reverse engineering. It automates the "archaeology" phase of modernization, allowing your most expensive engineers to focus on building new features rather than deciphering old ones.

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