How to Implement a Strangler Fig Pattern Using Replay for Visual Feature Extraction

The "Big Bang" rewrite is the silent killer of enterprise IT budgets. According to Replay’s analysis, 70% of legacy modernization projects fail or significantly exceed their timelines, often because teams attempt to replace decades of institutional knowledge in a single, massive release. When you are staring down a $3.6 trillion global technical debt mountain, the risk of a total rewrite isn't just high—it’s irresponsible.

The alternative is the Strangler Fig pattern. Originally coined by Martin Fowler, this pattern involves incrementally replacing system functionality by wrapping the old system in a "facade" and building new features alongside it. However, the biggest bottleneck in this approach has always been the UI. Extracting business logic from a monolithic frontend—where 67% of systems lack any documentation—is a manual, error-prone process that takes an average of 40 hours per screen.

By the time you've manually documented and rebuilt a legacy dashboard, the business requirements have already moved. To solve this, high-velocity teams now implement strangler pattern using Replay to automate visual feature extraction.

TL;DR: The Strangler Fig pattern reduces risk by replacing legacy systems incrementally. Replay accelerates this by using Visual Reverse Engineering to convert recordings of legacy UIs into documented React code. This reduces the time to "strangle" a legacy screen from 40 hours to just 4 hours, saving 70% of modernization time and bypassing the need for non-existent documentation.

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The Architecture of a Modern Strangler Pattern#

Traditionally, the Strangler Fig pattern was applied at the API level. You would place a proxy (like NGINX or an API Gateway) in front of your services and route specific endpoints to new microservices while the rest stayed on the monolith.

In the modern enterprise, the frontend is the new monolith. Whether it’s a JSP-based banking portal or a Silverlight-based healthcare record system, the UI is where the complexity lives. To implement strangler pattern using a visual-first approach, you must treat the UI as a series of extractable "features."

Visual Feature Extraction is the automated process of identifying UI patterns, state transitions, and component hierarchies directly from screen recordings of a legacy application.

Comparison: Manual Migration vs. Replay-Accelerated Strangler Pattern#

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Step 1: Identify the Edge and Capture with Replay#

The first step to implement strangler pattern using Replay is identifying the "edge" of your application—the specific workflow or page that provides the most value if modernized first.

Industry experts recommend starting with a high-traffic, low-complexity feature. Once identified, you don't start by reading the legacy source code. Instead, you record the workflow.

Video-to-code is the process of capturing a user's interaction with a legacy interface and using AI-driven visual analysis to generate functional, documented React components that mirror the original's behavior and aesthetics.

Using Replay, an engineer simply records the legacy screen. Replay’s engine analyzes the DOM (if available) or the visual pixel data (for legacy plugins like Flash or Silverlight) to identify:

1. •Component Boundaries: Where a button ends and a table begins.
2. •State Transitions: What happens when a user clicks "Submit."
3. •Design Tokens: Hex codes, spacing, and typography.

Learn more about modernizing legacy UIs

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Step 2: Generating the Component Library#

Once the recording is processed, Replay populates your Library. This is not just a collection of snippets; it is a full-fledged, SOC2-compliant Design System.

When you implement strangler pattern using Replay, the platform generates TypeScript-based React components. This ensures that the new "strangler" application maintains visual parity with the legacy system, preventing "UI shock" for users who have to navigate between old and new screens during the transition.

Below is an example of the type of clean, modular code Replay generates from a legacy data grid:

typescript
// Generated by Replay Visual Reverse Engineering
import React from 'react';
import { Table, Badge, Button } from '@/components/ui';

interface LegacyDataRow {
  id: string;
  status: 'pending' | 'completed' | 'failed';
  amount: number;
  timestamp: string;
}

export const TransactionGrid: React.FC<{ data: LegacyDataRow[] }> = ({ data }) => {
  return (
    <div className="p-4 border rounded-lg bg-white shadow-sm">
      <h3 className="text-lg font-semibold mb-4">Transaction History</h3>
      <Table>
        <thead>
          <tr className="bg-slate-50">
            <th>ID</th>
            <th>Status</th>
            <th>Amount</th>
            <th>Date</th>
          </tr>
        </thead>
        <tbody>
          {data.map((row) => (
            <tr key={row.id} className="hover:bg-slate-100 transition-colors">
              <td className="font-mono text-sm">{row.id}</td>
              <td>
                <Badge variant={row.status === 'completed' ? 'success' : 'warning'}>
                  {row.status.toUpperCase()}
                </Badge>
              </td>
              <td>${row.amount.toLocaleString()}</td>
              <td>{new Date(row.timestamp).toLocaleDateString()}</td>
            </tr>
          ))}
        </tbody>
      </Table>
    </div>
  );
};

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Step 3: Setting Up the Strangler Facade#

To successfully implement strangler pattern using this methodology, you need a facade that routes users between the legacy environment and the new React-based environment.

In a web context, this is often handled at the routing layer (e.g., Next.js Middleware or an NGINX configuration). As you replace features, you update the router to point to the new Replay-generated components.

Implementation Example: Routing Facade#

typescript
// middleware.ts (Next.js Example)
import { NextResponse } from 'next/server';
import type { NextRequest } from 'next/server';

const STRANGLED_ROUTES = ['/dashboard/transactions', '/settings/profile'];
const LEGACY_URL = 'https://legacy-app.internal.enterprise.com';

export function middleware(request: NextRequest) {
  const { pathname } = request.nextUrl;

  // If the route has been modernized using Replay, serve the new app
  if (STRANGLED_ROUTES.some(route => pathname.startsWith(route))) {
    return NextResponse.next();
  }

  // Otherwise, proxy to the legacy system
  return NextResponse.rewrite(`${LEGACY_URL}${pathname}`);
}

This approach allows you to chip away at the monolith. You might replace the "Transaction History" page this week, and the "User Profile" next week. Because Replay has already extracted the design tokens into a Centralized Design System, the new pages will feel native to the old application.

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Step 4: Iterative Refinement and Decommissioning#

The final phase of the Strangler Fig pattern is the eventual decommissioning of the legacy system. However, the "middle state" is where most projects fail.

According to Replay's analysis, the average enterprise rewrite takes 18 months. During those 18 months, the legacy system is often in a "code freeze," which frustrates the business. By using Replay, you reduce the "per-screen" time from 40 hours to 4 hours. This 90% reduction in manual effort allows you to deliver modernized features in weeks rather than years.

Case Study: Financial Services#

A Tier-1 bank had a 15-year-old mortgage processing system built in an obsolete version of Angular. They needed to implement strangler pattern using a modern React stack but lacked the original documentation.

• •The Challenge: 450+ unique screens with complex validation logic.
• •The Replay Solution: Using Replay's AI Automation Suite, they recorded the core "Loan Origination" flow. Replay extracted the flow logic and generated a React component library in 14 days.
• •The Result: The first "strangled" module went live in 3 weeks. The entire frontend was modernized in 5 months, a task originally estimated at 2 years.

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Why Visual Reverse Engineering is the Key#

Traditional code-to-code migration tools often fail because legacy code is "spaghetti." It contains dead code, patched-over bugs, and dependencies that no longer exist. Visual Reverse Engineering skips the "how" and focuses on the "what."

By recording the UI, Replay captures the intended behavior of the application. It doesn't matter if the backend is a COBOL mainframe or a series of fragmented microservices; if it renders on the screen, Replay can document it.

Flows are the architectural maps Replay creates by connecting multiple screen recordings. They allow architects to see the "happy path" and "edge cases" of a user journey before a single line of new code is written.

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Frequently Asked Questions#

What is the difference between the Strangler Fig pattern and a Big Bang rewrite?#

A Big Bang rewrite involves building a new system from scratch and switching over all at once, which carries a 70% failure rate in enterprises. The Strangler Fig pattern involves replacing the system piece-by-piece, allowing the old and new systems to coexist during the transition. To implement strangler pattern using Replay means you can automate the extraction of those pieces, significantly lowering the risk.

Does Replay require access to my legacy source code?#

No. Replay uses Visual Reverse Engineering. It analyzes recordings of the running application. This is particularly useful for regulated industries like Healthcare and Government where the original source code may be lost, undocumented, or restricted. However, Replay can ingest existing codebases to provide deeper context when available.

How does Replay handle complex state logic in legacy UIs?#

Replay’s AI Automation Suite observes state changes during the recording process. If a toggle reveals a hidden menu or a form submission triggers a specific validation message, Replay identifies these as state transitions and reflects them in the generated React code using hooks like

and .

Is Replay secure enough for Financial Services or Healthcare?#

Yes. Replay is built for regulated environments. It is SOC2 compliant and HIPAA-ready. We offer On-Premise deployment options for organizations that cannot allow data to leave their internal network. You can view our security documentation here.

Can I customize the code Replay generates?#

Absolutely. Replay provides "Blueprints," which act as an editor for the generated code. You can define your own coding standards, component naming conventions, and CSS frameworks (like Tailwind or Styled Components). Replay then applies these rules across the entire extracted library.

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Conclusion: The Path to Zero Technical Debt#

The $3.6 trillion technical debt crisis isn't a lack of talent; it's a lack of tools. Manually documenting legacy systems is a losing battle. When you implement strangler pattern using Replay, you are not just rewriting code—you are performing a surgical extraction of business value.

By shifting from manual 40-hour screen recreations to 4-hour automated extractions, enterprise architects can finally promise—and deliver—modernization projects that finish on time and under budget.

Ready to modernize without rewriting? Book a pilot with Replay