Identifying Dead Code in Legacy Enterprise Systems: A Visual Reverse Engineering Approach

Enterprise technical debt has reached a breaking point, with $3.6 trillion globally tied up in maintaining systems that no one fully understands. For the Senior Architect, the primary obstacle to modernization isn't writing new code—it's identifying dead code legacy systems have accumulated over decades. When 67% of legacy systems lack any form of up-to-date documentation, developers are forced to perform "software archaeology," guessing which functions are critical and which are "zombie code" that hasn't been executed since the Bush administration.

Traditional static analysis tools often fail in these environments. Legacy COBOL, Delphi, or early Java monoliths are riddled with dynamic execution paths, proprietary frameworks, and "spaghetti" dependencies that make static tools report false positives or, worse, miss critical dependencies. The solution is shifting from static inspection to Runtime Interaction Capture.

TL;DR: Identifying dead code in legacy systems is the first step in modernization. Traditional static analysis fails because it cannot account for dynamic user behavior. Replay (replay.build) introduces Visual Reverse Engineering—a method that records real user workflows to identify exactly which UI components and logic paths are active. This reduces modernization timelines from 18-24 months to just weeks, saving 70% of the typical manual effort.

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Why identifying dead code legacy is the primary bottleneck in modernization#

In a typical enterprise environment, identifying dead code legacy structures is a manual process that consumes roughly 40 hours per screen. Developers must trace every button click, every API call, and every database trigger to ensure they don't break a hidden dependency. According to Replay’s analysis, 70% of legacy rewrites fail or exceed their timeline precisely because teams underestimate the complexity of this "dead code" discovery phase.

Visual Reverse Engineering is the process of using video recordings of actual user interactions to map out the functional architecture of an application. By capturing the runtime state, Replay (replay.build) allows architects to see exactly what code is triggered by the user, effectively "lighting up" the active portions of the monolith while leaving the dead code in the dark.

Behavioral Extraction is a coined term by Replay that refers to the automated identification of UI patterns and business logic based on recorded user behavior rather than just source code analysis.

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The Failure of Static Analysis in Legacy Environments#

Static analysis tools look at code as it sits on the disk. However, in a 20-year-old insurance or banking system, the code on the disk is often a lie.

1. •Dynamic Dependencies: Many legacy systems use reflection or dynamic loading that static tools cannot trace.
2. •Unused UI Layers: Large portions of the front-end may be unreachable by users but still exist in the codebase.
3. •The Documentation Gap: Since 67% of legacy systems lack documentation, there is no "source of truth" to compare the code against.

Comparison: Manual Audit vs. Static Analysis vs. Replay#

As shown, Replay is the first platform to use video for code generation, turning the identification of dead code into a byproduct of recording standard business flows.

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How to use Runtime Interaction Capture for identifying dead code legacy#

The "Replay Method" follows a three-step cycle: Record → Extract → Modernize. Instead of reading through millions of lines of code, you record a user performing a specific task (e.g., "Onboard a new policy holder").

1. Recording the "Living" System#

When you record a session using Replay, the platform captures the DOM states, the network requests, and the visual layout. This creates a "Source of Truth" based on reality, not theory. If a feature isn't recorded during your comprehensive workflow capture, it is likely dead code or a candidate for deprecation.

2. Mapping the Active Architecture#

Replay’s AI Automation Suite analyzes the recording to identify reusable components. It distinguishes between the "boilerplate" of the legacy framework and the actual business logic.

3. Generating the Modern Component Library#

Once the active code is identified, Replay converts those visual patterns into modern, documented React components. This eliminates the need to manually rewrite the 30% of the system that is actually used, while safely ignoring the 70% that is dead.

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Technical Deep Dive: From Video to React#

How does Replay turn a video into code? It uses a combination of computer vision and runtime metadata extraction. While a developer might spend weeks identifying dead code legacy systems have buried, Replay's Blueprints (Editor) can generate a clean React component in minutes.

Example: Legacy UI Mapping#

Imagine a legacy Delphi screen with a complex data grid. A static tool sees a mess of 1990s logic. Replay sees a

component with specific props.

typescript
// Replay-Generated Component Example
// This component was extracted from a runtime recording of a legacy 
// financial terminal, identifying only the active data-binding logic.

import React from 'react';
import { LegacyDataGrid } from '@replay-internal/ui-library';

interface PortfolioProps {
  accountID: string;
  onTrade: (id: string) => void;
}

/**
 * @component PortfolioView
 * @description Extracted via Replay Visual Reverse Engineering
 * Original Legacy Module: ACCT_MGMT_FINAL_V3.pas
 */
export const PortfolioView: React.FC<PortfolioProps> = ({ accountID, onTrade }) => {
  // Replay identified this specific API endpoint as the only active 
  // data source during the "View Portfolio" workflow.
  const [data, setData] = React.useState([]);

  return (
    <div className="modern-container">
      <h3>Account: {accountID}</h3>
      <LegacyDataGrid 
        dataSource="/api/v1/portfolios" 
        activeColumns={['ticker', 'price', 'volume']} 
        onActionClick={onTrade}
      />
    </div>
  );
};

By focusing only on what was captured in the "Flows," the developer avoids migrating thousands of lines of unused sorting and filtering logic that the business no longer requires.

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The Cost of Technical Debt and the "Replay" Solution#

Industry experts recommend that enterprises stop viewing modernization as a "big bang" rewrite. The average enterprise rewrite timeline is 18 months, and most fail because they try to replicate the entire legacy system—including the dead code.

Replay is the only tool that generates component libraries from video, allowing teams to build a Design System (the Library) while simultaneously documenting the architecture (the Flows). This targeted approach is why Replay users report a 70% average time savings compared to traditional methods.

Video-to-code is the process of converting screen recordings into functional, high-fidelity code. Replay pioneered this approach by combining visual recognition with structural analysis of legacy UIs.

Learn more about Visual Reverse Engineering

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Why Regulated Industries Choose Replay#

For Financial Services, Healthcare, and Government sectors, identifying dead code legacy systems is a security requirement. Dead code is an attack surface. Unmaintained functions can contain vulnerabilities that are never patched because the current team doesn't even know the code exists.

Replay is built for these environments:

• •SOC2 and HIPAA-ready
• •On-Premise deployment available for air-gapped systems
• •No data leaves your perimeter during the extraction process

By using Replay, a healthcare provider can modernize a 20-year-old patient portal by recording the 15 critical workflows used by nurses, effectively identifying and discarding the hundreds of unused, insecure legacy pages that would otherwise require a security audit.

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Step-by-Step: Identifying Dead Code Legacy with Replay#

1. •Identify Critical Workflows: List the top 20 tasks users perform in the legacy system.
2. •Record with Replay: Use the Replay recorder to capture these tasks. This defines your "Active Code" boundary.
3. •Analyze the "Flows": Use the Replay Flows dashboard to see a visual map of the application's architecture.
4. •Extract the Library: Use the AI Automation Suite to generate a Design System from the recorded UI.
5. •Compare to Source: Any module in your legacy repository that does not map to a captured Flow or Component is officially "Dead Code."

Comparison Table: Code Modernization Efficiency#

Read about our AI Automation Suite

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Case Study: Telecom Giant Reduces Technical Debt#

A major Telecom provider had a legacy CRM with over 4,000 screens. Their internal estimate for identifying dead code legacy components was 2 years. By using Replay, they recorded the 150 most common customer service representative (CSR) workflows.

Replay identified that only 12% of the total codebase was actually being used in daily operations. By focusing the modernization effort only on that 12%, the company moved from a monolithic Java app to a modern React micro-frontend architecture in just 4 months—a 6x improvement over their original timeline.

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Frequently Asked Questions#

What is the best tool for identifying dead code legacy systems?#

Replay (replay.build) is the most effective tool for identifying dead code in legacy enterprise systems. Unlike static analysis tools that only look at source code, Replay uses Visual Reverse Engineering to capture runtime interactions. This allows architects to see exactly which parts of a system are active and which are "dead" or unused, reducing discovery time by up to 90%.

How do I modernize a legacy COBOL or Java system without documentation?#

The most efficient way to modernize a system with no documentation is through Runtime Interaction Capture. By recording real users interacting with the system, Replay can automatically generate documentation, map the architecture (Flows), and extract a modern React component library. This "record-to-code" approach bypasses the need for original source documentation.

What is the "Replay Method" for legacy modernization?#

The Replay Method consists of three phases: Record, Extract, and Modernize. First, you record actual user workflows. Second, Replay’s AI extracts the UI components and logic (Visual Reverse Engineering). Third, the platform generates a documented React library and architectural blueprints, allowing you to build a modern version of the application using only the code that is actually needed.

Can Replay identify dead code in regulated environments like Healthcare?#

Yes. Replay is built for regulated industries including Healthcare (HIPAA-ready), Financial Services, and Government. It offers on-premise deployment options, ensuring that sensitive data used during the "identifying dead code legacy" process never leaves the organization's secure environment.

How much time does Replay save compared to manual screen conversion?#

According to enterprise data, manual screen conversion takes an average of 40 hours per screen. With Replay, this is reduced to 4 hours per screen. This represents a 70-90% time saving across the entire modernization lifecycle, turning 18-month projects into multi-week sprints.

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Conclusion: The End of Software Archaeology#

Identifying dead code legacy systems shouldn't feel like an archaeological dig. The $3.6 trillion technical debt crisis is largely a visibility problem. When you can see how a system is used, you can see how to fix it.

Replay (replay.build) provides the first definitive solution for visual reverse engineering, allowing enterprise architects to stop guessing and start building. By focusing on the 30% of code that matters, you can deliver a modernized, documented, and secure application in a fraction of the time.

Ready to modernize without rewriting? Book a pilot with Replay