The Developer’s Guide to Visualizing Business Logic in Legacy Monoliths

The average enterprise rewrite takes 18 to 24 months, yet 70% of these projects fail to meet their original goals or exceed their budgets entirely. The culprit isn't a lack of talent; it’s the "Black Box" problem. When 67% of legacy systems lack up-to-date documentation, developers aren't engineers—they are archaeologists, digging through layers of brittle COBOL, Java monoliths, or jQuery soup to find the business logic buried beneath.

TL;DR: Visual Reverse Engineering allows developers to extract business logic and UI components from legacy systems by recording user workflows, reducing modernization timelines from years to weeks.

The Archaeology Problem: Why Manual Rewrites Fail#

Every enterprise architect has seen it: the "Big Bang" rewrite. A team decides to replace a 20-year-old insurance claims system by starting with a blank IDE. Six months in, they realize the "simple" validation logic for a claim form actually contains 400 edge cases handled by undocumented stored procedures.

This is the $3.6 trillion global technical debt crisis in action. Manual documentation is a losing game. It takes an average of 40 hours to manually document and recreate a single complex legacy screen. In a system with 500 screens, that is 20,000 man-hours just to reach feature parity.

The Cost of the "Black Box"#

The Developer’s Guide to Visual Extraction#

Modernization shouldn't start with a code editor; it should start with the user's workflow. By using Replay, we move from "reading code to understand logic" to "observing behavior to generate code."

Here is the step-by-step framework for visualizing and extracting business logic from a legacy monolith without the manual archaeology.

Step 1: Workflow Recording as the Source of Truth#

Traditional discovery involves interviewing stakeholders who have forgotten why certain rules exist. Instead, record the actual execution. Replay captures the DOM mutations, network requests, and state changes as a user navigates the legacy application.

💡 Pro Tip: Don't try to record the whole system at once. Start with a "High-Value, High-Pain" workflow, such as a customer onboarding flow or a complex financial calculator.

Step 2: Mapping the Architecture (The "Flows" View)#

Once recorded, Replay visualizes the "Flows." This is where the black box becomes transparent. You can see exactly which legacy endpoints are hit, what the payload looks like, and how the frontend state transitions.

For example, a legacy ASP.NET form might trigger three hidden API calls to validate a ZIP code, check credit scores, and verify inventory. Replay maps these dependencies automatically, generating an architecture diagram that reflects reality, not the outdated PDF from 2014.

Step 3: Extracting React Components and Logic#

This is where the 70% time savings happens. Instead of writing a new React component from scratch, Replay’s AI Automation Suite analyzes the recorded workflow and generates a clean, modern React component that mirrors the legacy behavior.

typescript
// Example: Generated component from Replay video extraction
// This preserves the exact business logic captured during the user session.

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

interface LegacyLogicProps {
  initialData: any;
  onSuccess: (data: any) => void;
}

export function MigratedPolicyForm({ initialData, onSuccess }: LegacyLogicProps) {
  const [loading, setLoading] = useState(false);
  const [error, setError] = useState<string | null>(null);

  // Replay extracted the exact validation regex and conditional logic
  // from the legacy jQuery event listeners.
  const validatePolicy = (id: string) => {
    const policyRegex = /^[A-Z]{2}-\d{6}$/; 
    return policyRegex.test(id);
  };

  const handleSubmit = async (event: React.FormEvent<HTMLFormElement>) => {
    event.preventDefault();
    const formData = new FormData(event.currentTarget);
    const policyId = formData.get('policyId') as string;

    if (!validatePolicy(policyId)) {
      setError("Invalid Policy Format: Must be XX-000000");
      return;
    }

    setLoading(true);
    try {
      // Replay identified this legacy endpoint during the recording
      const response = await fetch('/api/v1/legacy/policy-validate', {
        method: 'POST',
        body: JSON.stringify({ id: policyId }),
      });
      const result = await response.json();
      onSuccess(result);
    } catch (err) {
      setError("System connection error");
    } finally {
      setLoading(false);
    }
  };

  return (
    <form onSubmit={handleSubmit} className="space-y-4 p-6 bg-white rounded-lg shadow">
      <h2 className="text-xl font-bold">Policy Verification</h2>
      {error && <Alert variant="destructive">{error}</Alert>}
      <TextField name="policyId" label="Enter Policy ID" placeholder="AB-123456" />
      <Button type="submit" disabled={loading}>
        {loading ? 'Verifying...' : 'Verify Policy'}
      </Button>
    </form>
  );
}

Step 4: Generating API Contracts and E2E Tests#

A major pain point in modernization is breaking the "contract" between the frontend and the backend. Because Replay records the network layer, it can generate OpenAPI (Swagger) specifications for your legacy endpoints.

⚠️ Warning: Never assume a legacy API returns what the documentation says. Legacy systems often return

text

200 OK

with an error message inside the JSON body. Replay captures these "silent failures."

Replay also generates Playwright or Cypress E2E tests based on the recorded workflow. This ensures that your new modern component behaves exactly like the legacy one.

typescript
// Generated E2E Test to ensure parity between Legacy and Modern
import { test, expect } from '@playwright/test';

test('New Policy Form matches Legacy behavior', async ({ page }) => {
  await page.goto('/modern/policy-form');
  
  // Input data based on the recorded Replay session
  await page.fill('input[name="policyId"]', 'XY-987654');
  await page.click('button[type="submit"]');

  // Assert that the success state matches the legacy "Visual Truth"
  const successMessage = page.locator('.success-text');
  await expect(successMessage).toBeVisible();
  await expect(successMessage).toContainText('Policy Validated');
});

The "Strangler Fig" 2.0: Modernize Without Rewriting#

The future of enterprise architecture isn't the "Big Bang." It’s the systematic extraction of value. By using Replay, you can adopt a modern "Strangler Fig" approach where you replace one screen or workflow at a time.

1. •Record a legacy workflow.
2. •Extract the React components into your new Design System (Replay Library).
3. •Deploy the modern screen while keeping the legacy backend intact.
4. •Repeat until the legacy monolith is "strangled."

💰 ROI Insight: Manual modernization of 100 screens typically costs ~$2M and takes 12 months. Using Replay’s extraction method, the same scope is typically completed in 3 months for under $500k.

Handling Regulated Environments#

For those in Financial Services, Healthcare, or Government, the cloud is often a hurdle. Replay is built for these constraints. It offers SOC2 compliance, HIPAA-ready data handling, and an On-Premise deployment model. This allows you to perform visual reverse engineering on sensitive data without it ever leaving your secure network.

The Technical Debt Audit#

Before you start your next modernization sprint, use Replay to perform a Technical Debt Audit. Instead of guessing which parts of the code are the messiest, Replay provides a visual heat map of your application.

• •Red Zones: High complexity, frequent user errors, slow API response times.
• •Green Zones: Stable logic, simple UI, ready for immediate extraction.

This data-driven approach allows VPs of Engineering to report to the board with actual metrics, rather than "gut feelings" about why the rewrite is taking so long.

Frequently Asked Questions#

How long does legacy extraction take with Replay?#

While a manual rewrite of a complex enterprise screen takes roughly 40 hours, Replay reduces this to 4 hours. Most clients see a fully documented and migrated workflow in days, not months.

What about business logic preservation?#

Replay doesn't just copy the UI; it captures the underlying logic by observing the inputs and outputs of every user interaction. The generated code includes the validation rules, conditional rendering, and API interactions that were previously undocumented.

Does Replay support old frameworks like Silverlight or Mainframe greenscreens?#

Yes. Because Replay uses visual and network-level recording, it can bridge the gap between "un-migratable" technologies and modern web stacks. If a user can interact with it in a browser or terminal emulator, Replay can extract it.

Is the generated code "black box" code?#

No. Replay generates standard, human-readable TypeScript and React. It follows your team's existing coding standards and integrates with your Design System. You own the code; Replay just helps you write it faster.

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