Legacy Code Triage: A Framework for Identifying What to Extract vs What to Retire

Most enterprise modernization projects are doomed before the first line of code is written. We are currently staring at a $3.6 trillion global technical debt bubble, and the traditional response—the "Big Bang Rewrite"—has a staggering 70% failure rate. The reason isn't a lack of talent; it’s a lack of visibility. When 67% of legacy systems lack any meaningful documentation, engineers aren't developing; they are performing digital archaeology.

Legacy Code Triage is the only way to break this cycle. Instead of guessing what a 15-year-old system does, we must move toward a framework of objective extraction.

TL;DR: Modernization fails when you treat all legacy code as equal; use Visual Reverse Engineering to extract high-value business logic into modern React components while retiring dead features, reducing migration timelines from years to weeks.

The Archaeology Gap: Why Manual Triage Fails#

The average enterprise rewrite timeline is 18 months. During that time, the business continues to evolve, the market shifts, and the "new" system is often obsolete before it launches. The bottleneck is always the same: understanding the "Black Box."

Manual triage usually involves developers sitting with subject matter experts (SMEs) to map out "what the system does." This is a recipe for disaster. SMEs remember how the system should work; the code knows how it actually works.

⚠️ Warning: Relying on legacy documentation is the fastest way to introduce regressions. If the docs haven't been updated in six months, they are a liability, not an asset.

The Triage Framework: Extract, Refactor, or Retire#

To modernize effectively, you must categorize every screen and service into one of four quadrants based on Business Value and Technical Debt.

1. The "Extract" Quadrant (High Value, High Debt)#

These are your core revenue drivers—loan origination screens, patient intake forms, or claims processing engines. The code is a mess, but the logic is gold. This is where Replay delivers the highest ROI. Instead of manual analysis, you record a real user workflow. Replay captures the state transitions, the API calls, and the UI logic, then generates a documented React component.

2. The "Retire" Quadrant (Low Value, High Debt)#

Enterprise systems are littered with "Ghost Features"—modules built for a single client in 2012 that no one uses today. Triage requires the courage to delete. If telemetry shows zero hits in 90 days, do not migrate it.

3. The "Refactor" Quadrant (High Value, Low Debt)#

This is rare in legacy systems, but it exists. If the code is modular and documented, a simple lift-and-shift or containerization strategy works.

4. The "Maintain" Quadrant (Low Value, Low Debt)#

Internal admin tools that "just work." Leave them on the legacy stack until the underlying infrastructure is decommissioned.

💰 ROI Insight: Manual reverse engineering takes approximately 40 hours per screen. Using Replay’s Visual Reverse Engineering reduces this to 4 hours—a 90% reduction in engineering overhead.

Technical Deep Dive: From Video to React#

The core of the "Extract" strategy is capturing the "Source of Truth." In a legacy environment, the source of truth isn't the code—it's the execution.

When you use Replay to record a workflow, the platform doesn't just record pixels; it records the underlying data structures. It observes how a legacy ASP.NET or Java Swing app handles a complex validation and translates that into a clean, functional React component.

Example: Legacy Validation Logic Extraction#

Consider a legacy insurance premium calculator. The original code might be buried in 5,000 lines of spaghetti jQuery or COBOL-backed services. After recording the workflow, Replay generates a modern equivalent:

typescript
// Generated by Replay - Legacy Premium Calculator Extraction
import React, { useState, useEffect } from 'react';
import { Alert } from '@/components/ui/alert';

interface PremiumData {
  age: number;
  coverageLimit: number;
  hasPreExistingCondition: boolean;
}

export const PremiumCalculator: React.FC = () => {
  const [data, setData] = useState<PremiumData>({
    age: 0,
    coverageLimit: 0,
    hasPreExistingCondition: false
  });

  // Business logic preserved from legacy execution trace
  const calculateRisk = (input: PremiumData): number => {
    let base = input.coverageLimit * 0.02;
    if (input.age > 50) base *= 1.5;
    if (input.hasPreExistingCondition) base += 500;
    return base;
  };

  return (
    <div className="p-6 bg-white rounded-lg shadow-md">
      <h2 className="text-xl font-bold mb-4">Coverage Assessment</h2>
      <input 
        type="number" 
        onChange={(e) => setData({...data, age: parseInt(e.target.value)})}
        placeholder="Enter Age"
        className="border p-2 rounded w-full mb-2"
      />
      {/* ... additional inputs ... */}
      <div className="mt-4 p-4 bg-blue-50">
        Estimated Monthly Premium: ${calculateRisk(data).toFixed(2)}
      </div>
    </div>
  );
};

💡 Pro Tip: When extracting, focus on the "State Shape." Most legacy bugs during migration happen because the new system expects a different data type than the legacy API provides. Replay automatically generates the API contracts to prevent this mismatch.

Step-by-Step: The Modern Triage Workflow#

Step 1: Automated Inventory#

Run a technical debt audit. Identify the most complex screens and the most frequent user paths. Use Replay to map the existing architecture without opening a single IDE.

Step 2: Visual Recording#

Record an SME performing the "Happy Path" and the "Edge Cases" for a specific business process. This creates a video-based source of truth that the AI uses to understand the intent behind the code.

Step 3: Component Extraction#

Replay’s AI Automation Suite parses the recording and the network traffic to generate:

• •React components (Tailwind/Shadcn compatible)
• •TypeScript interfaces
• •E2E Test suites (Playwright/Cypress)
• •API Documentation (Swagger/OpenAPI)

Step 4: Validation and Integration#

Compare the output of the new component against the legacy recording. Since Replay provides a side-by-side comparison, QA can verify the logic in minutes rather than days.

typescript
// Example: Generated E2E Test to ensure parity
import { test, expect } from '@playwright/test';

test('New component matches legacy business logic', async ({ page }) => {
  await page.goto('/modernized-premium-calc');
  await page.fill('input[name="age"]', '55');
  await page.fill('input[name="coverage"]', '100000');
  
  // The value '3500' was captured from the legacy system recording
  const result = await page.innerText('.premium-display');
  expect(result).toBe('$3,500.00');
});

Addressing the "Documentation Gap"#

67% of legacy systems have no documentation. In a regulated environment (Healthcare, FinServ, Gov), this isn't just a nuisance—it’s a compliance risk. Replay solves this by creating "Living Documentation." Because the documentation is generated from actual system usage, it is 100% accurate to how the software functions in production.

• •For SOC2/HIPAA: Replay can be deployed on-premise, ensuring that sensitive data used during the recording process never leaves your firewall.
• •For Architects: The "Flows" feature provides a visual map of how data moves through the legacy system, identifying hidden dependencies that would normally break a rewrite.

📝 Note: Modernization is not a one-time event. It is a continuous process of understanding. By using Visual Reverse Engineering, you build a library of components that can be reused across the enterprise, effectively creating a private Design System from your legacy assets.

The Future of Modernization#

The "Future" isn't writing more code; it's understanding the code you already have. We are moving away from the era of manual refactoring and into the era of Automated Extraction.

By using Replay, enterprise teams are shifting their focus from "How do we rewrite this?" to "How do we leverage this logic in a modern context?" This shift is what allows a global bank to modernize a 20-year-old trading platform in weeks rather than years.

Frequently Asked Questions#

How long does legacy extraction take?#

While a manual rewrite of a complex enterprise screen takes roughly 40 hours, Replay reduces this to approximately 4 hours. Most organizations can move from a legacy "Black Box" to a documented, modern React codebase in 2 to 8 weeks, depending on the number of screens.

What about business logic preservation?#

This is the primary advantage of Visual Reverse Engineering. By recording the actual execution of the code, Replay captures the "hidden" logic—the weird edge cases and legacy validations—that are often missed during manual code reviews.

Can Replay handle mainframe or terminal-based systems?#

Yes. If the system has a UI that a user interacts with (including web-wrapped terminal emulators), Replay can record the workflow and extract the underlying logic and data structures into modern API contracts and frontend components.

Does this work in regulated environments?#

Absolutely. Replay is built for Financial Services, Healthcare, and Government. We offer On-Premise deployment and are SOC2 and HIPAA-ready, ensuring that your source code and data remain secure.

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