How to Handle Edge Cases in Legacy Migrations Using Visual Replay Data

The reason 70% of legacy rewrites fail isn't the "happy path"—it's the 15-year-old edge case hidden in a undocumented backend or a spaghetti-code frontend. When you embark on a "Big Bang" rewrite, you aren't just writing new code; you are performing digital archaeology on a $3.6 trillion global technical debt pile. Without a way to see how users actually trigger obscure business logic, you are flying blind.

TL;DR: Legacy migration edge cases are best handled by capturing real-world user workflows as "Visual Replay" data, which Replay then transforms into documented React components and API contracts, reducing manual discovery time by 90%.

The Archaeology Problem: Why Manual Discovery Fails#

Most enterprise systems—especially in Financial Services and Healthcare—suffer from a documentation gap. Statistics show that 67% of legacy systems lack any form of up-to-date documentation. When a VP of Engineering asks a team to modernize a 20-year-old insurance claims portal, the developers spend 40 hours per screen just trying to understand the conditional logic buried in the legacy stack.

Manual discovery relies on:

1. •Reading obfuscated code.
2. •Interviewing users who have "forgotten" why they click certain buttons.
3. •Guessing at API payloads that haven't been updated since 2012.

This is why the average enterprise rewrite takes 18-24 months. You are paying senior engineers to be historians, not architects.

How to Handle Edge Cases with Visual Reverse Engineering#

Visual Reverse Engineering changes the "Source of Truth" from the code to the user's interaction. By recording real user workflows, Replay captures every state change, every API call, and every obscure validation rule that only triggers for a specific subset of users in a regulated environment.

Step 1: Record the "Black Box"#

Instead of guessing how a complex financial form handles a non-resident tax status, you record a user performing that specific task. Replay captures the DOM mutations, the network requests, and the internal state transitions.

💡 Pro Tip: Don't try to record everything at once. Start with the "Critical Path" (the 20% of features used 80% of the time) and then move to the high-risk edge cases identified in your Technical Debt Audit.

Step 2: Extract the Logic into Blueprints#

Once recorded, Replay doesn't just give you a video; it gives you a Blueprint. This is a structured representation of the legacy component's behavior. It identifies the "if-then-else" logic that handles those pesky edge cases.

Step 3: Generate Modern React Components#

Replay's AI Automation Suite takes the Blueprint and generates a clean, documented React component. It doesn't just copy the old code; it recreates the behavior using modern patterns (hooks, functional components, TypeScript).

typescript
// Example: Generated component from Replay extraction
// This component preserves a legacy edge case discovered during recording:
// The "specialHandling" flag only triggers when the user is from a specific region.

import React, { useState, useEffect } from 'react';
import { ModernInput, Alert } from '@enterprise-ds/core';

interface LegacyMigrationProps {
  userId: string;
  regionCode: string; // Discovered as a critical edge case dependency
}

export function ClaimsAdjustmentForm({ userId, regionCode }: LegacyMigrationProps) {
  const [isSpecialHandling, setIsSpecialHandling] = useState(false);

  // Replay identified this logic from a legacy jQuery 'change' listener
  useEffect(() => {
    if (regionCode === 'ZA-404' || regionCode === 'EU-99') {
      setIsSpecialHandling(true);
    }
  }, [regionCode]);

  return (
    <div className="p-6 space-y-4">
      <h2 className="text-xl font-bold">Adjust Claim</h2>
      {isSpecialHandling && (
        <Alert type="warning">
          Note: This region requires manual compliance review (Regulation 402.b).
        </Alert>
      )}
      <ModernInput 
        label="Adjustment Amount" 
        placeholder="0.00"
        onValidate={(val) => val < 10000} // Validation extracted from legacy XHR interceptor
      />
    </div>
  );
}

Preserving API Contracts and E2E Tests#

One of the biggest pain points in modernization is breaking the contract between the frontend and the backend. When you "handle" an edge case, you must ensure the data sent to the legacy backend remains identical in structure, even if the UI is modern.

Replay automatically generates API contracts based on the visual data. If the legacy system expects a strangely formatted date string or a specific header for an edge case, Replay documents it.

⚠️ Warning: Never assume a legacy API follows REST conventions. We often find "POST" requests that act like "GETs" or status codes that are always 200 OK even when the body contains an error string. Replay catches these "silent failures" during the extraction phase.

Automated E2E Test Generation#

To ensure the edge case is truly handled, Replay generates Playwright or Cypress tests based on the original recording. This creates a "Golden Path" that the new component must pass.

typescript
// Generated Playwright test to verify the edge case discovered via Replay
import { test, expect } from '@playwright/test';

test('Verify Regional Compliance Edge Case', async ({ page }) => {
  // Setup: Navigate to the migrated screen
  await page.goto('/claims/adjust?region=ZA-404');

  // Assert: The specific warning discovered in the legacy system appears
  const alert = page.locator('text=Regulation 402.b');
  await expect(alert).toBeVisible();

  // Assert: The submit button is disabled until compliance check is clicked
  const submitBtn = page.locator('button[type="submit"]');
  await expect(submitBtn).toBeDisabled();
});

The "Replay" Workflow: From Months to Days#

How do we get from a 40-hour manual screen rewrite to a 4-hour automated extraction? By removing the "Discovery Tax."

Step 1: Assessment & Recording#

Use the Replay Library to catalog your existing screens. A developer or QA engineer records the workflow. For a complex insurance underwriting screen, this takes about 15 minutes.

Step 2: Blueprinting#

Replay analyzes the recording. It identifies:

• •Input fields and their validation logic.
• •Hidden state transitions (e.g., fields that appear only when a checkbox is clicked).
• •Network dependencies.
• •CSS/Styling patterns for the Design System.

Step 3: Generation & Refinement#

Using the Blueprints Editor, the architect reviews the extracted logic. You can see exactly which "edge cases" were captured. You then click "Generate," and Replay outputs the React code, the CSS modules, and the TypeScript interfaces.

Step 4: Verification#

Run the generated E2E tests against the new component. If the tests pass, you have successfully modernized the screen while preserving 100% of the business logic.

💰 ROI Insight: In a recent project for a Tier 1 Bank, Replay reduced the migration of a 50-screen internal tool from an estimated 14 months to just 6 weeks. The team saved over $800,000 in developer salaries alone.

Handling Regulated Environments#

For industries like Government or Healthcare, data privacy is paramount. Replay is built for these high-stakes environments:

• •SOC2 & HIPAA Ready: Data is handled with the highest security standards.
• •On-Premise Available: For systems that cannot touch the public cloud, Replay can run entirely within your VPC.
• •PII Masking: Visual recordings can automatically mask sensitive user data before processing, ensuring compliance with GDPR and CCPA.

Frequently Asked Questions#

How does Replay handle business logic that isn't visible in the UI?#

Replay monitors the "bridge" between the UI and the backend. While it cannot see inside a stored procedure in a database, it captures the inputs sent to that procedure and the outputs returned. By observing these patterns across multiple replays, Replay can document the API contract required to keep that backend logic functioning during and after the migration.

What if the legacy code is too messy to understand?#

That is the beauty of Visual Reverse Engineering. Replay doesn't care how messy the legacy code is. It cares about what the code does in the browser. By treating the legacy system as a "black box" and observing the DOM and Network layers, we bypass the need to ever read the original spaghetti code.

Can Replay help with building a Design System?#

Yes. The Library feature in Replay identifies recurring UI patterns across different screens. It can suggest common components (Buttons, Modals, Inputs) that should be abstracted into your modern Design System, ensuring consistency across the newly modernized application.

How long does legacy extraction actually take?#

On average, a single complex screen that would take a senior developer 40-60 hours to manually document and rewrite can be processed in Replay in under 4 hours. This includes recording, blueprint generation, and final code refinement.

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