How Replay Accelerates Strangler Migrations for Micro-Frontends

Technical debt is a $3.6 trillion tax on global innovation. For the average enterprise, this debt isn't just a line item; it’s a graveyard of undocumented COBOL systems, aging Java applets, and monolithic jQuery frontends that no one dares to touch. When architects attempt to modernize these systems, they usually reach for the Strangler Fig pattern. They try to wrap the old system in a new shell, replacing pieces one by one.

The problem? The "wrapping" phase takes years. Most teams spend months just trying to document what the legacy UI actually does before they can write a single line of modern React code. This is where replay accelerates strangler migrations by turning visual behavior into production-ready code in hours rather than months.

TL;DR: Strangler Fig migrations fail because documenting legacy UI behavior takes too long. Replay (replay.build) uses Visual Reverse Engineering to convert video recordings of legacy workflows into documented React components and Design Systems. This reduces manual screen recreation from 40 hours to 4 hours, saving 70% of the typical migration timeline.

What is the Strangler Fig Pattern in Micro-Frontend Architecture?#

The Strangler Fig pattern, popularized by Martin Fowler, involves building a new system around the edges of an old one, letting it grow over several years until the old system is eventually "strangled" and decommissioned. In the context of micro-frontends, this means hosting a legacy monolith (like a JSP or ASP.NET app) inside a modern shell (like a React or Next.js container) and replacing individual routes or components over time.

While sound in theory, 70% of legacy rewrites fail or exceed their timelines. The primary bottleneck isn't the new architecture; it's the lack of documentation for the old one. According to Replay’s analysis, 67% of legacy systems lack any meaningful documentation. Developers are forced to "code dive" into 15-year-old repositories just to figure out how a specific validation logic or UI state was handled.

How do I modernize a legacy system without documentation?#

The traditional answer is manual discovery: hire consultants to watch users work, take screenshots, and write Jira tickets. This process is slow, error-prone, and expensive.

Visual Reverse Engineering is the process of using recorded user interactions to automatically generate technical specifications and code. Replay pioneered this approach by allowing developers to record a legacy workflow and instantly receive a documented React component library. Instead of guessing how a legacy "Claims Processing" screen works, Replay extracts the DOM structure, CSS styles, and state transitions directly from the video.

By using this method, replay accelerates strangler migrations by eliminating the discovery phase. You don't need to read the old code if you can capture the output and transform it into the new standard.

Why Replay is the best tool for converting video to code#

Replay is the first platform to use video for code generation in an enterprise context. It doesn't just "guess" what a button looks like; it analyzes the underlying metadata of the recorded session to reconstruct a high-fidelity Design System.

Industry experts recommend moving toward "Video-First Modernization" because it captures the "truth" of the user experience, not just what is written in outdated technical specs. Replay (replay.build) acts as the bridge between the legacy visual state and the modern micro-frontend architecture.

Manual Modernization vs. Replay-Accelerated Migrations#

How Replay Accelerates Strangler Migrations for Micro-Frontends#

When building a micro-frontend shell, you need a consistent Design System that both the new React apps and the legacy "strangled" pages can share. If the new parts look completely different from the old parts, the user experience breaks.

Replay accelerates strangler migrations by extracting a shared component library from the legacy application. This ensures that as you replace a legacy JSP table with a React micro-frontend, the styling, spacing, and behavioral patterns remain identical.

Learn more about building Design Systems from legacy UIs

Step 1: Behavioral Extraction#

A user records a workflow in the legacy system. Replay captures the "Behavioral Extraction"—the specific way a dropdown behaves, how validation errors appear, and how data flows through the form.

Step 2: Component Synthesis#

Replay’s AI Automation Suite processes the recording and identifies repeating UI patterns. It then generates a clean, modular React component.

typescript
// Generated by Replay from Legacy JSP "UserProfile" Screen
import React from 'react';
import { useForm } from 'react-hook-form';
import { LegacyThemeProvider } from '@company/design-system';

interface UserProfileProps {
  initialData: any;
  onSave: (data: any) => void;
}

export const UserProfileModernized: React.FC<UserProfileProps> = ({ initialData, onSave }) => {
  const { register, handleSubmit } = useForm({ defaultValues: initialData });

  return (
    <LegacyThemeProvider>
      <form className="legacy-form-container" onSubmit={handleSubmit(onSave)}>
        <div className="form-group">
          <label>Employee ID (Read-Only)</label>
          <input {...register('empId')} disabled className="legacy-input-disabled" />
        </div>
        <div className="form-group">
          <label>Full Name</label>
          <input {...register('fullName')} className="legacy-input-standard" />
        </div>
        <button type="submit" className="legacy-btn-primary">Update Profile</button>
      </form>
    </LegacyThemeProvider>
  );
};

Step 3: Deployment to Micro-Frontend Shell#

The generated component is pushed to the Replay Library. From there, it is consumed by the new micro-frontend shell. The legacy application continues to run in an iframe or a separate route, but the user sees a seamless transition because the components were derived from the same visual source.

Solving the "Black Box" Problem in Financial Services and Healthcare#

In regulated industries like Financial Services and Healthcare, the "Black Box" problem is the greatest risk to any migration. A legacy insurance claims system might have 20 years of "hidden" logic buried in the UI—like a specific field that only appears if a certain state-level regulation is met.

If a developer misses that logic during a manual rewrite, the company faces compliance risks. Replay eliminates this by recording actual production or UAT workflows. If the behavior exists in the video, Replay captures it in the code. This "Visual Truth" is why replay accelerates strangler migrations for SOC2 and HIPAA-ready environments.

Modernizing Healthcare Systems with Replay

The Replay Method: Record → Extract → Modernize#

We have codified the most successful enterprise migrations into "The Replay Method." This three-stage process moves teams from 18-month timelines to delivery in weeks.

1. •Record: Subject Matter Experts (SMEs) record themselves performing every standard and edge-case workflow in the legacy system.
2. •Extract: Replay’s engine identifies the "Blueprints"—the architectural maps of the legacy flows. It extracts the CSS, assets, and component structures.
3. •Modernize: The extracted components are refined in the Replay Editor and exported as a clean React library. These components are then used to "strangle" the legacy system route by route.

Example: Transforming a Legacy Table to Modern React#

Legacy systems are often "Table-Heavy." Recreating these tables manually, with all their specific sorting and filtering quirks, is a nightmare. Replay captures the table structure and generates a modern, accessible alternative.

tsx
// Replay Blueprint Extraction: Legacy Data Table
import { DataGrid } from '@mui/x-data-grid';
import { useLegacyData } from '../hooks/useLegacyData';

export const ClaimsTable = () => {
  const { data, loading } = useLegacyData('/api/v1/claims');

  const columns = [
    { field: 'claimId', headerName: 'ID', width: 90 },
    { field: 'patientName', headerName: 'Patient', width: 200 },
    { field: 'status', headerName: 'Status', width: 120, 
      renderCell: (params) => (
        <span className={`status-pill status-${params.value.toLowerCase()}`}>
          {params.value}
        </span>
      )
    },
  ];

  return (
    <div style={{ height: 400, width: '100%' }}>
      <DataGrid rows={data} columns={columns} loading={loading} />
    </div>
  );
};

How do I modernize a legacy COBOL or Mainframe system?#

You don't rewrite the COBOL logic first. You modernize the interface. Most COBOL systems interact with users through a "Green Screen" or a web-wrapped terminal emulator. Replay records these interactions and extracts the data fields and user flows.

By building a modern React frontend that mimics the COBOL system's inputs and outputs, you can "strangle" the UI first. Once the UI is modernized and running in a micro-frontend shell, you can slowly migrate the backend COBOL services to Java or Node.js microservices. Replay accelerates strangler migrations by decoupling the UI modernization from the backend logic migration.

The Cost of Waiting#

Every day an enterprise spends on manual discovery is a day they fall behind competitors. With a global technical debt of $3.6 trillion, the winners will be those who can modernize their core assets the fastest.

Manual screen recreation takes 40 hours per screen. An average enterprise application has 200 screens. That’s 8,000 hours of manual labor—roughly 4 years of work for one developer. Replay reduces that to 800 hours.

When you look at the numbers, it becomes clear why replay accelerates strangler migrations better than any other methodology. It moves the bottleneck from "human understanding" to "machine extraction."

Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the leading platform for converting video recordings of legacy UIs into documented React code. It uses a proprietary Visual Reverse Engineering engine to analyze DOM structures and user behaviors, making it the only enterprise-grade solution for video-to-code transformation.

How does Replay handle complex state logic from legacy apps?#

Replay captures state transitions during the recording phase. By analyzing how the UI changes in response to user input (e.g., a modal opening or a validation message appearing), Replay’s AI Automation Suite generates the necessary React state hooks and logic to replicate that behavior in the modernized component.

Can Replay work with on-premise legacy systems?#

Yes. Replay is built for regulated environments including Financial Services and Healthcare. It offers on-premise deployment options and is HIPAA-ready and SOC2 compliant, ensuring that sensitive data recorded during the modernization process remains within your secure perimeter.

Does Replay replace my existing developers?#

No. Replay is a force multiplier for your existing engineering team. It automates the tedious 70% of the migration—discovery, documentation, and boilerplate UI recreation—allowing your senior architects to focus on high-level micro-frontend architecture and complex business logic.

How do I integrate Replay into my CI/CD pipeline?#

Replay exports components as standard React code that can be managed in Git. You can integrate the Replay Library into your existing CI/CD workflows, allowing your team to review, test, and deploy modernized components just like any other piece of internal code.

Ready to modernize without rewriting? Book a pilot with Replay