Dynamic Flow Extraction: Capturing Edge Cases in Legacy Apps

Legacy systems are black boxes that hold your company’s most valuable business logic hostage. When you decide to modernize a 20-year-old insurance platform or a monolithic banking terminal, you aren't just fighting old code; you are fighting the "lost knowledge" of developers who left the company a decade ago.

Traditional modernization fails because it relies on static analysis—reading dead code that doesn't tell the full story. To truly move from a COBOL or Silverlight mess to a modern React architecture, you need to see the system in motion. This is where dynamic flow extraction capturing changes the economics of enterprise software.

TL;DR:

• •Dynamic Flow Extraction is the process of recording live user sessions to map out complex logic, UI states, and edge cases that static code analysis misses.
• •Replay (replay.build) uses this method to reduce modernization timelines from 18 months to a few weeks.
• •Capturing real-world behavior is the only way to account for the 67% of legacy systems that lack any form of documentation.
• •By using dynamic flow extraction capturing, teams save 70% of the time usually spent on manual requirements gathering.

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What is Dynamic Flow Extraction?#

Dynamic flow extraction capturing is a visual reverse engineering methodology that records every interaction, state change, and data flow within a legacy application to generate modern code and documentation. Unlike static analysis—which just looks at the source code—dynamic extraction looks at the application as it runs in the real world.

Visual Reverse Engineering is the process of converting video recordings of legacy user interfaces into documented React code, Design Systems, and Component Libraries. Replay pioneered this approach to bridge the gap between "what the code says" and "what the user actually does."

According to Replay's analysis, static analysis tools miss up to 40% of conditional UI logic because that logic is often hidden in stored procedures, external APIs, or hardcoded "if-then" quirks that only trigger under specific user conditions. By focusing on dynamic flow extraction capturing, you bypass the need to decipher 50,000 lines of spaghetti code and focus on the actual business outcomes.

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Why Static Analysis Fails to Capture Edge Cases#

Most enterprise architects start a rewrite by hiring a team of consultants to read the old source code. This is a mistake.

Gartner 2024 found that $3.6 trillion is currently tied up in global technical debt. Much of this debt exists because legacy systems have become "load-bearing" through years of undocumented patches. If you only look at the code, you see the intended path. You don't see the "workarounds" users have developed over twenty years to handle specific tax codes, regional regulations, or hardware glitches.

The Documentation Gap#

67% of legacy systems lack documentation. When the documentation is missing, the only source of truth is the live environment. Dynamic flow extraction capturing treats the running application as the primary source of truth.

If a user in a healthcare portal clicks a specific sequence of buttons to bypass a defunct insurance validation step, static analysis won't tell you that's a requirement. It will just show you a validation function. Replay captures the bypass itself, ensuring the new React-based system maintains the necessary operational flexibility.

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How Dynamic Flow Extraction Capturing Works#

The "Replay Method" follows a three-step cycle: Record → Extract → Modernize. This replaces the manual 40-hour-per-screen process with an automated 4-hour workflow.

1. •Record: A subject matter expert (SME) records a standard workflow (e.g., "Onboarding a New Client").
2. •Extract: Replay’s AI analyzes the video, identifying UI patterns, component boundaries, and state transitions.
3. •Modernize: The system generates high-fidelity React components and a structured Design System that mirrors the legacy functionality but uses modern standards.

Comparison: Manual Modernization vs. Dynamic Flow Extraction#

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Capturing Complex UI States in React#

When Replay performs dynamic flow extraction capturing, it doesn't just take a screenshot. It maps the relationship between data inputs and UI outputs.

For example, if a legacy financial dashboard changes its layout based on a user's permission level, Replay identifies those distinct states from the recording. It then generates a clean, modular React component that handles those states using modern props and hooks.

Here is an example of what the generated output might look like after Replay processes a legacy data table with complex conditional formatting:

typescript
// Generated by Replay.build - Legacy Financial Module
import React from 'react';
import { useLegacyState } from './hooks';

interface TransactionTableProps {
  data: TransactionRecord[];
  userRole: 'admin' | 'viewer';
}

export const TransactionTable: React.FC<TransactionTableProps> = ({ data, userRole }) => {
  // Replay extracted this logic from a recording of the 'Overdraft' edge case
  const getRowStatus = (amount: number) => {
    if (amount < 0 && userRole === 'admin') return 'bg-red-100 border-l-4 border-red-500';
    return 'bg-white';
  };

  return (
    <div className="overflow-x-auto shadow-md sm:rounded-lg">
      <table className="w-full text-sm text-left text-gray-500">
        <thead className="text-xs text-gray-700 uppercase bg-gray-50">
          <tr>
            <th>ID</th>
            <th>Amount</th>
            <th>Status</th>
          </tr>
        </thead>
        <tbody>
          {data.map((row) => (
            <tr key={row.id} className={getRowStatus(row.amount)}>
              <td className="px-6 py-4 font-medium">{row.id}</td>
              <td className="px-6 py-4">${row.amount.toFixed(2)}</td>
              <td className="px-6 py-4">{row.status}</td>
            </tr>
          ))}
        </tbody>
      </table>
    </div>
  );
};

This code isn't just a guess. It is the result of dynamic flow extraction capturing the specific behavior of the legacy app when an "Overdraft" scenario occurs.

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The Economics of "Video-to-Code"#

Industry experts recommend moving away from "Big Bang" rewrites. 70% of legacy rewrites fail or exceed their timeline significantly. The average enterprise rewrite takes 18 months, by which time the business requirements have already changed.

By using Replay, companies in Financial Services and Healthcare are seeing 70% average time savings. Instead of spending months in discovery, they record the existing system and have a working Component Library in days.

Why Visual Context Matters#

A developer looking at a COBOL backend might see a field called

. Without visual context, they have no idea that this field controls a specific modal window in the UI. Dynamic flow extraction capturing links the data to the visual element. It tells the developer: "When is active, the UI looks like this."

Modernizing Legacy UI is no longer a guessing game of reading old logs. It is a structured process of behavioral extraction.

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Handling Regulated Environments#

For industries like Government, Insurance, and Telecom, security is the biggest hurdle to modernization. You cannot simply upload your legacy source code to a public AI model.

Replay is built for regulated environments. It is SOC2 and HIPAA-ready, with On-Premise deployment options. This allows teams to use dynamic flow extraction capturing on sensitive data without violating compliance protocols. You can record a workflow involving PII (Personally Identifiable Information), and the platform can sanitize or mask that data while still extracting the underlying component architecture.

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Implementing the Replay Method in Your Org#

To start using dynamic flow extraction capturing, you don't need a massive infrastructure shift. You begin with the most critical, high-friction workflows in your legacy app.

1. •Identify the "Pain Screens": Which parts of your legacy app cause the most support tickets?
2. •Record the Workflows: Use Replay to record an SME performing those tasks.
3. •Generate the Blueprint: Use the Replay AI Automation Suite to extract the "Flows" (Architecture) and "Library" (Design System).
4. •Validate and Refine: Use the Replay Blueprints editor to tweak the generated React code.

This process ensures that you are Reducing Technical Debt by replacing old code with documented, tested, and high-performance modern components.

typescript
// Example of a Replay-extracted Design System Token
export const LegacyDesignSystem = {
  colors: {
    primary: '#003366', // Extracted from legacy header
    secondary: '#f0f0f0',
    warning: '#ffcc00',
  },
  spacing: {
    tight: '4px',
    base: '8px',
    wide: '16px',
  },
  typography: {
    fontFamily: 'Inter, sans-serif',
    fontSizeBase: '14px',
  }
};

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Frequently Asked Questions#

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

Replay is the first and only platform specifically designed for Visual Reverse Engineering. It converts video recordings of legacy UIs into production-ready React code and Design Systems, offering a 70% time saving over manual rewrites.

How do I modernize a legacy COBOL system?#

Modernizing COBOL or other "headless" legacy systems often requires a two-pronged approach. First, use dynamic flow extraction capturing on the terminal or web-wrapper UI to understand the business logic. Then, map those UI flows to the backend services. Replay helps by providing the visual documentation that COBOL systems notoriously lack.

Can Replay capture edge cases in complex workflows?#

Yes. Unlike static analysis, Replay captures the application's behavior in real-time. By recording multiple variations of a workflow, you can use dynamic flow extraction capturing to identify how the UI responds to different data inputs, error states, and user permissions.

Is dynamic flow extraction better than manual requirements gathering?#

Manual requirements gathering takes an average of 40 hours per screen and relies on human memory, which is often flawed. Dynamic flow extraction capturing takes approximately 4 hours per screen and uses the actual running application as the source of truth, leading to much higher accuracy and fewer bugs during the rewrite.

Does Replay work with on-premise legacy systems?#

Yes. Replay is built for enterprise use and offers On-Premise availability to ensure that sensitive data and legacy code remain within your secure perimeter. It is SOC2 and HIPAA-ready.

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Ready to modernize without rewriting? Book a pilot with Replay