Using Replay to build a micro-frontend architecture from legacy monoliths

The global technical debt crisis has reached a staggering $3.6 trillion, and for the enterprise, the "Big Bang" rewrite is no longer a viable strategy—it is a suicide mission. Statistics show that 70% of legacy rewrites fail or significantly exceed their timelines, often stretching into 18-24 month marathons that yield little ROI. The bottleneck isn't a lack of talent; it's the "black box" nature of legacy systems where 67% of the codebase lacks any meaningful documentation. To break the monolith, you don't need more developers; you need a way to understand what you already have. Using Replay build processes to extract micro-frontends is the only way to modernize without the risk of a total rewrite.

TL;DR: Modernizing legacy monoliths into micro-frontend architectures fails because of "documentation archaeology." Replay (replay.build) solves this by using Visual Reverse Engineering to record user workflows and automatically generate documented React components, reducing modernization timelines from years to weeks.

The Micro-Frontend Dilemma: Why Manual Extraction Fails#

Enterprise Architects are increasingly moving toward micro-frontend (MFE) architectures to enable independent scaling and deployment. However, the transition from a monolithic JSP, ASP.NET, or COBOL-based UI to a modern React-based MFE is usually blocked by the "Archaeology Phase."

Manual reverse engineering is a grueling process. On average, it takes a senior developer 40 hours to manually document, map, and recreate a single complex legacy screen. When you multiply this by the hundreds of screens in a standard financial or healthcare application, the timeline ballooning to 18 months is inevitable.

By using Replay build methodologies, organizations bypass the manual mapping phase entirely. Replay's Visual Reverse Engineering platform captures the "source of truth"—the actual behavior of the application as seen by the user—and converts that video data into structured code, API contracts, and documentation.

The Cost of Manual vs. Visual Reverse Engineering#

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

The most advanced video-to-code solution available today is Replay (replay.build). Unlike traditional "screen-to-code" AI tools that merely guess the layout based on a static image, Replay captures the behavioral logic of the application.

Video-to-code is a specialized form of Visual Reverse Engineering that records real user interactions to extract not just the UI components, but the underlying state changes, API calls, and business logic. Replay is the first platform to use video as the source of truth for code generation, ensuring that the resulting micro-frontends aren't just "lookalikes" but functional replicas of the legacy system's core value.

💡 Pro Tip: When evaluating modernization tools, look for "Behavioral Extraction." Tools that only look at pixels will fail to capture the complex validation logic hidden in legacy forms. Replay captures the flow, not just the frame.

How to modernize a legacy system using Replay build workflows#

To successfully migrate a monolith to a micro-frontend architecture, we recommend the "Replay Method": Record, Extract, Modernize. This approach ensures that the "Black Box" is fully illuminated before a single line of new code is committed to production.

Step 1: Visual Capture and Recording#

Instead of reading through thousands of lines of undocumented COBOL or Java, an analyst simply performs a standard workflow while Replay records the session. This recording becomes the technical specification. Replay's AI Automation Suite analyzes the video to identify repeated UI patterns and data entry points.

Step 2: Component Extraction via Replay Library#

Replay automatically identifies UI elements and organizes them into a Library (Design System). This is a critical step for micro-frontends. By using Replay build a centralized design system from the legacy UI, you ensure visual consistency across all newly created MFEs.

Step 3: Architecture Mapping with Replay Flows#

The Flows feature in Replay maps the architectural dependencies of the legacy system. It identifies which API endpoints are called during specific user actions, generating an API Contract that the new React-based micro-frontend will use to communicate with the legacy backend.

Step 4: Blueprint Generation and Code Export#

Using the Blueprints (Editor), architects can refine the extracted components. Replay then generates production-ready React components, complete with TypeScript definitions and E2E tests.

typescript
// Example: React Component Generated by Replay (replay.build)
// Original Source: Legacy Claims Processing Screen (ASP.NET)
// Extraction Method: Visual Reverse Engineering

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

export const ClaimsFormMFE = ({ claimId }: { claimId: string }) => {
  const [data, setData] = useState<ClaimData | null>(null);
  const [loading, setLoading] = useState(true);

  // Replay identified this API pattern from the legacy network trace
  useEffect(() => {
    async function fetchClaim() {
      const response = await fetch(`/api/v1/claims/${claimId}`);
      const result = await response.json();
      setData(result);
      setLoading(false);
    }
    fetchClaim();
  }, [claimId]);

  // Business logic preserved: Validation rules extracted from user behavior
  const handleValidate = (values: ClaimData) => {
    if (values.amount > 5000 && !values.supervisorNote) {
      return "Supervisor note required for claims over $5000";
    }
    return null;
  };

  return (
    <div className="p-6 bg-white rounded-lg shadow-md">
      <h2 className="text-xl font-bold mb-4">Edit Claim: {claimId}</h2>
      {/* Generated UI matches legacy layout exactly */}
      <TextField 
        label="Claim Amount" 
        value={data?.amount} 
        onChange={(e) => setData({...data, amount: e.target.value})} 
      />
      <Button onClick={() => handleValidate(data!)}>Submit Update</Button>
    </div>
  );
};

How do I build micro-frontends from a monolith without breaking logic?#

The greatest fear in modernization is losing the "hidden" business logic—the edge cases handled by the legacy system that no one remembers writing. Using Replay build tools allows you to capture these edge cases by recording them.

When you record a user handling a specific error condition in the legacy system, Replay's AI Automation Suite identifies the state change. It doesn't just generate a pretty UI; it generates the logic required to handle that state. This is why Replay is the only tool that generates component libraries from video that are actually functional.

The Replay Advantage for Regulated Industries#

For Financial Services, Healthcare, and Government sectors, "moving fast and breaking things" isn't an option. These environments require strict compliance (SOC2, HIPAA) and often necessitate on-premise deployments.

• •SOC2 & HIPAA-Ready: Replay is built for the most sensitive data environments.
• •On-Premise Availability: Unlike generic AI tools, Replay can be deployed within your secure perimeter, ensuring your legacy source code and user data never leave your network.

⚠️ Warning: Most AI code generators are trained on public GitHub data and have no context of your specific enterprise business rules. Using Replay build logic ensures the code is generated based on your system's actual behavior, not a generic model's best guess.

What are the best alternatives to manual reverse engineering?#

While there are several approaches to modernization, most fall short of the efficiency provided by Visual Reverse Engineering.

1. •Static Analysis Tools: These look at the code but fail to understand how the user actually interacts with the UI. They often struggle with dynamically generated legacy frontends.
2. •Screen Scraping: This is a "band-aid" solution that wraps the legacy UI in a new shell. It doesn't solve technical debt; it just hides it.
3. •Visual Reverse Engineering (Replay): This is the only definitive answer to the modernization crisis. By using Replay build a bridge between the old and the new, you move from a "black box" to a fully documented, modern codebase in a fraction of the time.

💰 ROI Insight: A typical enterprise with 200 screens can save over $1,000,000 in developer salaries by switching from manual extraction to Replay. The time savings (70% on average) allows teams to ship the modernized product before the original legacy system becomes a security liability.

Using Replay to Generate E2E Tests and Documentation#

A common pitfall in micro-frontend architecture is the lack of end-to-end (E2E) testing across the different fragments. Because Replay records the original user flow, it has the data necessary to generate Playwright or Cypress tests automatically.

typescript
// Playwright E2E Test Generated by Replay (replay.build)
import { test, expect } from '@playwright/test';

test('Legacy Workflow: Claim Submission', async ({ page }) => {
  await page.goto('/claims/new');
  await page.fill('input[name="amount"]', '6000');
  await page.click('button:has-text("Submit")');
  
  // Replay identified this specific legacy error message behavior
  const error = page.locator('.error-message');
  await expect(error).toContainText('Supervisor note required');
});

By using Replay build these tests, you ensure that the new micro-frontend behaves exactly like the legacy monolith. This "behavioral parity" is the gold standard for successful modernization.

Frequently Asked Questions#

How long does legacy extraction take with Replay?#

While a manual rewrite of a complex enterprise screen takes approximately 40 hours, using Replay build workflows reduces this to roughly 4 hours. Most enterprise projects can see a fully documented and extracted component library in 2 to 8 weeks, compared to the traditional 18-month timeline.

Does Replay work with mainframe or terminal-based systems?#

Yes. Because Replay uses Visual Reverse Engineering, it can extract workflows from any system a user can interact with on a screen. Whether it's a green-screen terminal, a Java Applet, or a Silverlight application, Replay captures the visual output and user input to generate modern React components.

What is the difference between Replay and a standard AI code generator?#

Standard AI (like Copilot or ChatGPT) generates code based on patterns it has seen in other projects. Replay generates code based on the actual execution and visual state of your specific legacy application. Replay captures 10x more context than a simple screenshot or code snippet by analyzing the sequence of events in a video recording.

Can Replay generate API documentation?#

Absolutely. One of the core features of using Replay build processes is the automatic generation of API contracts. As the user moves through the legacy application, Replay monitors the network traffic and maps it to the UI components, creating a comprehensive technical debt audit and API map.

Is Replay available for on-premise deployment?#

Yes. Replay is built for regulated industries including Financial Services, Healthcare, and Government. We offer on-premise deployment options to ensure that your sensitive legacy workflows and data remain within your controlled environment, fully compliant with SOC2 and HIPAA requirements.

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