Platform Engineering for Legacy Systems: Creating a Unified UI Layer

The $3.6 trillion global technical debt crisis isn't a maintenance problem; it’s a visibility problem. For the average enterprise, the legacy system is a black box—a mission-critical engine wrapped in a UI that no one dares to touch because the original developers retired during the Obama administration.

When leadership demands a "modern experience," the default response is usually a "Big Bang" rewrite. It’s a seductive trap. We tell ourselves that this time, with Microservices and React, we’ll get it right. Yet, statistics from the Standish Group and McKinsey tell a grimmer story: 70% of legacy rewrites fail to meet their objectives or exceed their timelines by years. The average enterprise rewrite drags on for 18 to 24 months, often resulting in a "feature parity" race that the new system never wins.

The future of modernization isn't rewriting from scratch. It is Platform Engineering: creating a unified, modern UI layer that abstracts the complexity of the legacy system without discarding the battle-tested business logic beneath it.

TL;DR: Modernizing a legacy system via platform engineering allows teams to create a unified UI layer in weeks by using visual reverse engineering to extract components and logic, bypassing the 18-month "Big Bang" rewrite failure cycle.

The Archaeology Bottleneck: Why Documentation Fails#

The primary blocker in any modernization effort isn't coding—it's archaeology. Approximately 67% of legacy systems lack up-to-date documentation. Architects are forced to spend months "spelunking" through undocumented COBOL, Java, or .NET codebases just to understand basic user workflows.

In a traditional manual migration, an engineer spends an average of 40 hours per screen just to document, design, and re-implement a single interface. This includes:

1. •Re-discovering the business rules hidden in the frontend logic.
2. •Mapping data flows to undocumented APIs.
3. •Re-creating the UI in a modern framework like React.

This is where the platform engineering approach diverges. Instead of manual archaeology, we use Replay to perform visual reverse engineering. By recording a real user workflow, Replay extracts the underlying structure, state transitions, and component hierarchy automatically. This shifts the timeline from 40 hours per screen to just 4 hours.

Strategies for UI Layer Modernization#

When building a unified UI layer over a legacy system, architects generally choose between three paths. The "Big Bang" is the highest risk, while "Video Extraction" (Visual Reverse Engineering) provides the fastest ROI.

Modernization Approach Comparison#

⚠️ Warning: Attempting a Strangler Fig pattern without automated documentation often leads to "technical debt mirroring," where the mess of the old system is simply replicated in the new one.

Creating the Unified UI Layer: A Platform Engineering Framework#

Platform engineering for legacy systems involves creating a "Golden Path" for developers to build new features on top of old data. This requires four distinct phases of extraction and abstraction.

Step 1: Visual Technical Debt Audit#

Before writing a single line of code, you must understand what you have. Most enterprises have no idea how many unique screens or edge-case workflows exist in their legacy system.

Using Replay, teams record actual user sessions. The AI Automation Suite then analyzes these recordings to generate a technical debt audit. This identifies redundant components, inconsistent state management, and hidden business logic that must be preserved.

Step 2: Component Extraction and Design System Integration#

Once workflows are recorded, the next step is extracting React components. Instead of manually coding a "ModernForm" to match an "OldForm," Replay generates documented React components directly from the recording.

typescript
// Example: React component extracted via Replay Visual Reverse Engineering
// Source: Legacy Insurance Claims Portal (circa 2008)

import React, { useState, useEffect } from 'react';
import { Button, Input, Card } from '@enterprise-ds/core';

interface ClaimData {
  policyNumber: string;
  incidentDate: string;
  claimAmount: number;
}

export const MigratedClaimForm: React.FC<{ initialData?: ClaimData }> = ({ initialData }) => {
  const [formData, setFormData] = useState<ClaimData>(initialData || {
    policyNumber: '',
    incidentDate: '',
    claimAmount: 0
  });

  // Business Logic preserved: Validation rules extracted from legacy event handlers
  const validatePolicy = (num: string) => {
    return /^[A-Z]{2}-\d{6}$/.test(num);
  };

  return (
    <Card title="Submit Insurance Claim">
      <form className="space-y-4">
        <Input 
          label="Policy Number"
          value={formData.policyNumber}
          onChange={(e) => setFormData({...formData, policyNumber: e.target.value})}
          error={!validatePolicy(formData.policyNumber) ? "Invalid Format" : undefined}
        />
        {/* Additional fields extracted from legacy DOM structure */}
        <Button variant="primary" type="submit">Update Claim</Button>
      </form>
    </Card>
  );
};

Step 3: API Contract Generation#

A unified UI layer is useless if it can't talk to the backend. In many legacy systems, the "API" is actually a series of direct database calls or tightly coupled SOAP services.

By observing the network traffic during a Replay recording, the platform generates API contracts (Swagger/OpenAPI) and E2E tests automatically. This creates a clean interface for the new UI layer to consume.

typescript
// Example: Generated API Contract from Legacy Network Interception
/**
 * @summary Extracted Contract for Legacy Claims Service
 * @version 1.0.0
 */
export interface LegacyClaimsAPI {
  /**
   * POST /api/v1/claims/validate
   * Extracted from: User "Submit" workflow
   */
  validateClaim(payload: {
    claim_id: string;
    auth_token: string;
    payload_v2: any;
  }): Promise<{ status: 'VALID' | 'INVALID'; errors: string[] }>;
}

Step 4: Deployment in Regulated Environments#

For industries like Financial Services, Healthcare, and Government, "cloud-native" isn't always an option for the core legacy system. Platform engineering teams must ensure the new UI layer meets the same compliance standards as the old one.

💰 ROI Insight: Companies using Replay's on-premise visual extraction save an average of $1.2M in labor costs during the first six months of a modernization project by eliminating manual documentation phases.

Addressing the "Black Box" Problem#

The greatest fear of any Enterprise Architect is the "Black Box"—a system that works but no one knows why. When you modernize a legacy system by rewriting it, you risk losing the 20 years of "bug fixes" that are actually undocumented business requirements.

Replay treats the video as the source of truth. By recording the legacy system in action, you capture the intended behavior of the system, not just the code. This ensures that when you move to a modern React-based architecture, you aren't just building a prettier version of the old system—you're building a fully understood and documented version.

The Role of the Library and Blueprints#

In a platform engineering context, the "Library" (Design System) and "Blueprints" (Architecture) serve as the foundation.

• •Library: As Replay extracts components, it maps them to your modern Design System. If the legacy system uses a specific "Submit" button, Replay identifies its function and replaces it with the standardized component from your new library.
• •Blueprints: These provide the structural map. They show how different "Flows" (user journeys) interact with the legacy system's state.

💡 Pro Tip: Don't try to modernize every screen at once. Use Replay to identify the top 20% of workflows that handle 80% of user traffic. Modernize those first to show immediate value to stakeholders.

Case Study: Financial Services Modernization#

A global bank had a legacy system for commercial lending built in 2004. The manual rewrite estimate was 24 months and $5M. By using Replay to record the 15 core workflows, they were able to:

1. •Extract 45 core React components in 3 weeks.
2. •Generate API contracts for undocumented middleware.
3. •Launch a modern, mobile-responsive UI layer in 3 months.
4. •Total Savings: 18 months of development time and $3.8M in budget.

Frequently Asked Questions#

How long does legacy extraction take with Replay?#

While a manual rewrite takes 18-24 months, visual extraction typically takes 2 to 8 weeks depending on the complexity of the workflows. A single complex screen can be extracted and documented in roughly 4 hours, compared to the industry average of 40 hours for manual reverse engineering.

What about business logic preservation?#

This is the core advantage of visual reverse engineering. Because Replay records the actual execution of the legacy system, it captures the logic as it happens. The generated code includes the event handlers and validation rules observed during the recording, ensuring no "hidden" business rules are lost in transition.

Does this work for on-premise, highly secure systems?#

Yes. Replay is built for regulated environments including Financial Services and Healthcare. It offers SOC2 compliance, is HIPAA-ready, and can be deployed entirely on-premise to ensure that sensitive data never leaves your secure network.

Can we use our own Design System?#

Absolutely. Replay’s Library feature is designed to map extracted legacy elements directly to your existing React-based Design System, ensuring the new UI layer is consistent with your brand’s modern standards.

What happens to the legacy system?#

The legacy system continues to run as the "engine" (the System of Record), while the new UI layer built via platform engineering acts as the "System of Engagement." This allows you to retire the legacy system piece-by-piece over time (the Strangler Fig pattern) without the risk of a single "switch-over" date.

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