How Visual Extraction Tools Accelerate the Strangler Fig Pattern

The $3.6 trillion global technical debt crisis isn't a failure of engineering; it’s a failure of discovery. Most enterprise modernization projects die in the "archaeology" phase—the months spent digging through undocumented COBOL, Java monoliths, or Delphi screens just to understand what the system actually does.

The Strangler Fig pattern is the industry-standard solution to this, but it has a fundamental flaw: you cannot strangle what you do not understand. When 67% of legacy systems lack up-to-date documentation, the "discovery" phase of a Strangler Fig migration often consumes 50% of the total budget before a single line of modern code is written.

TL;DR: Visual extraction tools like Replay eliminate the manual "archaeology" phase of the Strangler Fig pattern by converting recorded user workflows directly into documented React components and API contracts, reducing modernization timelines by 70%.

Why the Strangler Fig Pattern Stalls in the Enterprise#

The Strangler Fig pattern—incrementally replacing legacy functionality with new services behind a proxy—is theoretically sound. However, in a regulated environment like Financial Services or Healthcare, the "incremental" part is where things break down.

Architects face three primary friction points:

1. •Logic Drift: The original requirements are lost. The "source of truth" is now the behavior of the legacy UI, not the code itself.
2. •The Documentation Gap: Developers spend 40 hours per screen manually mapping fields, validation rules, and state transitions.
3. •Integration Parity: Ensuring the new micro-frontend behaves exactly like the legacy screen to avoid breaking user workflows.

This is why 70% of legacy rewrites fail or exceed their original timeline. We treat the legacy system as a "black box" and try to guess its contents.

How Visual Extraction Changes the Calculus#

Visual extraction is the process of using the running application—the "Video as the Source of Truth"—to reverse engineer the underlying architecture. Instead of reading 20-year-old code, tools like Replay record real user workflows and extract the DOM structure, CSS, state management, and API interactions.

This accelerates the Strangler Fig pattern by providing the "Target State" instantly. Instead of starting with a blank VS Code window, your team starts with a functional React component that mirrors the legacy behavior.

The Shift from Manual Mapping to AI Automation#

In a traditional migration, an analyst watches a user perform a task, takes screenshots, and writes a Jira ticket. An engineer then tries to recreate that UI in React. This takes roughly 40 hours per complex screen.

With Replay, the recording is the specification. The platform extracts the blueprint, identifies the design tokens, and generates the code.

typescript
// Example: React component generated via Replay Visual Extraction
// Legacy System: Claims Processing Portal (v4.2)
// Logic preserved: Conditional field rendering based on 'policyType'

import React, { useState, useEffect } from 'react';
import { LegacyTextField, ValidationWrapper } from '@enterprise-ui/core';

export const ClaimsFormMigrated: React.FC<{ policyId: string }> = ({ policyId }) => {
  const [formData, setFormData] = useState<any>(null);
  const [loading, setLoading] = useState(true);

  // Replay automatically identified this API contract from the legacy network trace
  const fetchLegacyState = async () => {
    const response = await fetch(`/api/legacy/claims/${policyId}`);
    const data = await response.json();
    setFormData(data);
    setLoading(false);
  };

  useEffect(() => { fetchLegacyState(); }, [policyId]);

  if (loading) return <Spinner />;

  return (
    <div className="modern-strangler-wrapper">
      <LegacyTextField 
        label="Policy Number" 
        value={formData?.policyNumber} 
        readOnly 
      />
      {/* Visual Extraction identified this conditional logic from user workflow */}
      {formData?.claimType === 'COMMERCIAL' && (
        <ValidationWrapper rule="tax-id-required">
          <input type="text" placeholder="Enter EIN" />
        </ValidationWrapper>
      )}
    </div>
  );
}

The 4-Step Visual Extraction Workflow#

To successfully accelerate a Strangler Fig migration, you must move from "code-first" discovery to "behavior-first" extraction.

Step 1: Workflow Capture#

Instead of reading documentation, record a subject matter expert (SME) performing the critical path in the legacy system. Replay captures the DOM mutations, network calls, and state changes. This turns the "black box" into a transparent set of data points.

Step 2: Blueprint Generation#

The visual extraction engine analyzes the recording. It identifies recurring patterns—buttons, inputs, tables—and maps them to your modern Design System. If you don't have a design system, Replay’s Library feature builds one for you based on the legacy UI's common elements.

Step 3: API Contract Extraction#

One of the hardest parts of the Strangler Fig is the "Interim State" where the new UI must talk to the old backend. Replay automatically generates OpenAPI/Swagger contracts from the network traffic captured during the recording.

💰 ROI Insight: Manual API documentation for a legacy monolith takes an average of 12-16 hours per endpoint. Replay generates these contracts in seconds, ensuring 100% accuracy with the legacy data structures.

Step 4: Component Export and Integration#

The final step is exporting the generated React/TypeScript code into your new repository. Because the code is based on the actual visual output of the legacy system, "Visual Parity" is achieved on day one, eliminating the UI/UX feedback loops that usually plague the middle of a migration.

⚠️ Warning: Do not attempt to "fix" UX debt during the initial extraction. The goal of the Strangler Fig is to move functionality off the monolith. Use Visual Extraction to achieve 1:1 parity first, then iterate on the modern stack.

Solving the "Archaeology" Problem in Regulated Industries#

For Financial Services and Healthcare, the risks of a "broken" migration are catastrophic. A missed validation rule in a mortgage application or a hidden field in a patient record can lead to multi-million dollar fines or compliance failures.

Manual reverse engineering is prone to human error. An architect might miss a legacy JavaScript function that triggers only when a specific combination of keys is pressed. Visual extraction doesn't miss these details because it records the execution, not just the static code.

Security and Compliance#

Legacy systems often sit behind heavy firewalls or on-premise data centers. Replay is built for these environments, offering:

• •SOC2 Type II & HIPAA Compliance
• •On-Premise Deployment: Keep your sensitive data and source code within your own perimeter.
• •PII Masking: Automatically redact sensitive user data during the recording and extraction process.

The Technical Debt Audit: Knowing What to Strangle#

Not every screen in a legacy monolith deserves to be migrated. The Strangler Fig works best when you prioritize by business value and technical risk.

Replay provides a Technical Debt Audit that visualizes the complexity of your legacy screens. By analyzing the density of DOM elements, the number of API dependencies, and the complexity of the state logic, it assigns a "Migration Difficulty Score."

💡 Pro Tip: Use the Migration Difficulty Score to identify "Quick Wins." Migrating 10 "Low" complexity screens in the first two weeks of a project builds stakeholder confidence and proves the Strangler Fig pattern works.

Generating E2E Tests Automatically#

A major bottleneck in the Strangler Fig pattern is regression testing. How do you know the new React component behaves exactly like the 1998 PowerBuilder screen?

Replay doesn't just generate UI code; it generates the testing suite. By using the recorded session, it can produce Playwright or Cypress tests that run against both the legacy and modern versions of the app to ensure parity.

typescript
// Generated Playwright test for Parity Validation
import { test, expect } from '@playwright/test';

test('Legacy vs Modern Parity: Claim Submission', async ({ page }) => {
  // 1. Run workflow on Legacy URL
  await page.goto('https://legacy-system.enterprise.com/claim');
  await page.fill('#claim-id', '12345');
  await page.click('#submit-btn');
  const legacyResponse = await page.innerText('.success-msg');

  // 2. Run same workflow on Modern Strangled URL
  await page.goto('https://modern-app.enterprise.com/claim');
  await page.fill('[data-testid="modern-claim-id"]', '12345');
  await page.click('[data-testid="modern-submit-btn"]');
  const modernResponse = await page.innerText('.success-msg');

  // 3. Assert Parity
  expect(modernResponse).toBe(legacyResponse);
});

Frequently Asked Questions#

How long does legacy extraction take with Replay?#

A standard enterprise screen that would normally take 40 hours of manual developer time (discovery, mapping, coding, testing) is typically completed in 4 hours using Replay. The recording takes minutes; the AI-assisted extraction and refinement take the remainder of the time.

Does Replay require access to my legacy source code?#

No. Replay uses Visual Reverse Engineering. It analyzes the application at runtime (the DOM, CSS, and Network layers). This is ideal for systems where the source code is lost, obfuscated, or written in languages your current team doesn't know (COBOL, Delphi, VB6).

What about complex business logic hidden in the backend?#

Visual extraction captures the inputs and outputs of that logic. While it won't rewrite your COBOL backend into Go, it creates the perfect "contract" for it. It documents exactly what data the UI sends and what it expects back, allowing you to wrap that legacy logic in a modern API (the "Strangler" facade) with 100% confidence.

Can Replay handle mainframe or terminal-based systems?#

Yes. As long as the system is being accessed via a web wrapper or a terminal emulator that renders in a browser, Replay can record the session and extract the underlying data structures and workflows.

The Future of Modernization is Understanding#

The era of the "Big Bang" rewrite is over. The risks are too high, and the $3.6 trillion technical debt mountain is too large to climb with manual tools. The Strangler Fig remains the best architectural pattern we have, but it requires a modern engine to drive it.

By using visual extraction, you stop guessing what your legacy system does and start seeing it. You move from months of archaeology to days of execution. Replay turns your legacy "black box" into a documented, modern codebase, saving an average of 70% in time and cost.

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