Decomposing Massive Monolithic UIs into Micro-frontends: 7 Steps to Visual Module Extraction

The average enterprise monolith isn't just a codebase; it’s a $3.6 trillion technical debt anchor. For most organizations, the user interface is a tangled web of jQuery, legacy Angular, and inline styles that no one dares to touch. When you are tasked with decomposing massive monolithic into a modern micro-frontend architecture, the risk of breakage often outweighs the promise of agility.

According to Replay’s analysis, 70% of legacy rewrites fail or significantly exceed their timelines because teams underestimate the complexity of undocumented business logic. With 67% of legacy systems lacking any up-to-date documentation, developers are essentially performing archeology, not engineering.

TL;DR: Decomposing massive monolithic into micro-frontends requires a shift from manual code-scraping to automated visual extraction. By using Replay, enterprises reduce the time per screen from 40 hours to just 4 hours. This guide outlines a 7-step framework—from visual inventory to module federation—that leverages AI-driven reverse engineering to modernize without the 18-month rewrite risk.

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The Monolith Stagnation Crisis#

The standard enterprise rewrite timeline is 18 months. In that time, the market moves, competitors ship, and your "modern" stack starts to age before it even hits production. The bottleneck is rarely the new technology; it’s the extraction of the old.

Manual extraction involves digging through thousands of lines of spaghetti code to find where a "Submit" button’s logic ends and the "Validation" logic begins. This is why decomposing massive monolithic into manageable micro-frontends is often abandoned in favor of "big bang" rewrites that inevitably fail.

Visual Reverse Engineering is the process of recording real user workflows and automatically generating documented React components and design systems from the resulting telemetry.

By using Replay, teams bypass the manual "archeology" phase. Instead of reading dead code, you record the live application in action. Replay’s AI Automation Suite analyzes the UI patterns, state changes, and API calls to produce a clean, documented component library ready for a micro-frontend (MFE) architecture.

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Step 1: Visual Inventory and Audit#

Before you write a single line of Webpack config, you need to know what you’re actually moving. Most monoliths have "ghost components"—features that exist in the code but haven't been used by a customer since 2014.

Start by recording the core user flows. Using the Replay Library, you can capture every state of a UI component. This creates a visual "source of truth" that bridges the gap between the legacy UI and the new React-based design system.

Why Visual Audits Matter#

• •Identification: Find every instance of a "Data Grid" or "Modal."
• •De-duplication: Realize that you have 14 different versions of a primary button.
• •Prioritization: Focus on the flows that drive 80% of your revenue.

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Step 2: Defining Domain Boundaries (The DDD Approach)#

The biggest mistake in decomposing massive monolithic into micro-frontends is splitting the UI by "page" rather than by "domain." If your "Billing" logic is tightly coupled with your "User Profile" logic, a page-based split will result in a distributed monolith—the worst of both worlds.

Industry experts recommend using Domain-Driven Design (DDD) to identify bounded contexts. In a legacy environment, these boundaries are often hidden. Replay’s "Flows" feature maps the underlying architecture by tracing how data moves through the UI during a recording, making these invisible boundaries visible.

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Step 3: Setting Up the Shell and Module Federation#

Once boundaries are defined, you need a "Shell" (or Host) to orchestrate the various micro-frontends. For React-based modernizations, Webpack 5 Module Federation is the gold standard. It allows you to load code dynamically from different builds at runtime.

When decomposing massive monolithic into a federated system, your shell acts as the glue.

typescript
// webpack.config.js - The Shell Configuration
const ModuleFederationPlugin = require("webpack/lib/container/ModuleFederationPlugin");

module.exports = {
  plugins: [
    new ModuleFederationPlugin({
      name: "shell",
      remotes: {
        billing: "billing@http://localhost:3001/remoteEntry.js",
        inventory: "inventory@http://localhost:3002/remoteEntry.js",
      },
      shared: {
        react: { singleton: true, eager: true },
        "react-dom": { singleton: true, eager: true },
      },
    }),
  ],
};

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Step 4: Visual Module Extraction with Replay#

This is where the 70% time savings happen. In a traditional workflow, a developer would spend 40 hours per screen manually recreating the CSS, HTML structure, and basic React logic.

Decomposing massive monolithic into modern components via Replay’s Blueprints reduces this to 4 hours. You record the legacy screen, and Replay’s AI extracts the visual primitives into a clean React component.

Component Extraction is the automated transformation of rendered DOM elements and computed styles into reusable, prop-driven code modules.

Example: Legacy jQuery Table to React Component#

Replay analyzes the legacy table and generates a modern implementation:

tsx
// Generated by Replay Blueprints
import React from 'react';
import { useTable } from '@enterprise-ui/core';

interface LegacyDataProps {
  data: Array<{ id: string; amount: number; status: string }>;
}

export const BillingTable: React.FC<LegacyDataProps> = ({ data }) => {
  return (
    <div className="modern-grid-container">
      <table className="min-w-full divide-y divide-gray-200">
        <thead>
          <tr>
            <th>Transaction ID</th>
            <th>Amount</th>
            <th>Status</th>
          </tr>
        </thead>
        <tbody>
          {data.map((row) => (
            <tr key={row.id}>
              <td>{row.id}</td>
              <td>{row.amount}</td>
              <td>
                <StatusBadge type={row.status} />
              </td>
            </tr>
          ))}
        </tbody>
      </table>
    </div>
  );
};

For more on this process, check out our guide on Automated Component Extraction.

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Step 5: Establishing a Shared Design System#

A micro-frontend architecture without a shared design system is a recipe for visual chaos. Each team will eventually drift, creating different versions of the same UI elements.

When decomposing massive monolithic into MFEs, the Replay Library serves as your foundational Design System. By extracting tokens (colors, spacing, typography) directly from the legacy application's best-performing screens, you ensure brand consistency while moving to a modern stack.

Key Design System Requirements:

1. •Versioned Styles: Use CSS-in-JS or Tailwind to prevent style leakage between MFEs.
2. •Component Governance: A central repository (like Replay’s Library) where all teams can view and test components.
3. •Accessibility (A11y): Ensure the extracted components meet WCAG standards, which Replay helps automate during the blueprinting phase.

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Step 6: State Management and Data Orchestration#

Monoliths often rely on global state or, worse, DOM-based state (hidden inputs, jQuery

attributes). When decomposing massive monolithic into micro-frontends, you must decide how these fragments communicate.

Industry experts recommend a "Shared-Nothing" architecture where possible. Each MFE should fetch its own data. However, for global concerns like user authentication, a lightweight event bus or a shared state container is necessary.

According to Replay's analysis, the most successful migrations use a "Strangler Fig" pattern for data. You keep the legacy API intact but wrap it in a modern GraphQL or BFF (Backend for Frontend) layer that the new MFEs consume.

Learn more about the Strangler Fig Pattern

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Step 7: Incremental Deployment and Testing#

The final step in decomposing massive monolithic into micro-frontends is the rollout. Never flip a switch for the entire application. Use a reverse proxy (like Nginx or Cloudflare Workers) to route specific paths to your new micro-frontends while keeping the rest of the traffic on the monolith.

The Migration Path:#

1. •Route /billing to the new Billing MFE.
2. •Monitor error rates and performance.
3. •Route /inventory to the new Inventory MFE.
4. •Decommission the legacy code once the route is fully migrated.

Replay is built for these high-stakes environments. Whether you are in Financial Services or Healthcare, the platform is SOC2 and HIPAA-ready, with On-Premise deployment options to ensure your legacy data never leaves your secure perimeter.

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The Economics of Visual Reverse Engineering#

Why do enterprises choose Replay over manual rewrites? The numbers speak for themselves. In a typical 500-screen legacy application, the manual effort is staggering.

Manual Migration Cost:

• •500 screens x 40 hours/screen = 20,000 developer hours.
• •At $100/hour = $2,000,000 and ~2 years of work.

Replay-Assisted Migration Cost:

• •500 screens x 4 hours/screen = 2,000 developer hours.
• •At $100/hour = $200,000 and ~3 months of work.

By decomposing massive monolithic into micro-frontends using visual extraction, you aren't just saving money; you are reclaiming two years of market opportunity.

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Frequently Asked Questions#

How does Replay handle complex business logic inside the legacy UI?#

Replay’s AI Automation Suite doesn't just look at the pixels; it records the execution flow. While it cannot "guess" your backend COBOL logic, it captures how the UI reacts to data inputs, allowing developers to see exactly what logic needs to be replicated in the new React components.

Can we use micro-frontends with different frameworks like Angular and React?#

Yes. Using Webpack Module Federation or specialized libraries like Single-SPA, you can host a legacy Angular module alongside a new React micro-frontend. This is a common bridge strategy when decomposing massive monolithic into modern architectures.

Is Replay secure for regulated industries like Banking or Government?#

Absolutely. Replay is built for regulated environments. We offer SOC2 compliance, HIPAA-ready data handling, and the ability to run the entire platform On-Premise or within your private VPC to ensure technical debt is solved without compromising security.

What happens to the CSS when extracting components?#

Replay’s Blueprints analyze the computed styles of the legacy application. It then "cleans" the CSS, removing redundant overrides and converting them into modern formats like Tailwind classes, CSS Modules, or Styled Components, depending on your preference.

Do I need to change my legacy code to use Replay?#

No. Replay works by observing the rendered output and browser telemetry. You don't need to modify your legacy monolith to record the flows and begin the extraction process.

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Moving Forward#

The path to a modern architecture doesn't have to be a multi-year slog. By decomposing massive monolithic into micro-frontends through a visual-first approach, you eliminate the guesswork that leads to the 70% failure rate seen in traditional rewrites.

Stop reading dead code and start recording live workflows. Turn your legacy "black box" into a documented, modular, and high-performance React ecosystem in weeks, not years.

Ready to modernize without rewriting? Book a pilot with Replay