How to Modernize a 20-Year-Old Monolith Without a Single Discovery Workshop

Most modernization projects die in the boardroom before a single line of code is written. We call this "Death by Discovery." When an enterprise decides to modernize a 20-year-old monolith, the immediate instinct is to hire a consulting firm for a three-month discovery phase. You spend $500k to have junior analysts interview senior developers who would rather be coding, only to produce a 200-page PDF that is obsolete the moment it's exported.

The reality is that 67% of legacy systems lack any meaningful documentation. You aren't modernizing; you're performing digital archaeology. And with a global technical debt mountain reaching $3.6 trillion, the traditional "Big Bang" rewrite is no longer a viable strategy—it’s a career risk. 70% of these rewrites fail or significantly exceed their timelines.

We need to stop asking people what the system does and start observing what the system actually is.

TL;DR: Visual Reverse Engineering replaces months of manual discovery by recording real user workflows to automatically generate documented React components, API contracts, and technical debt audits.

The Archaeology Tax: Why Manual Discovery Fails#

When you attempt to modernize a 20-year-old monolith, you are fighting against two decades of "temporary" fixes, undocumented edge cases, and business logic that exists only in the mind of a developer nearing retirement.

The manual approach to discovery costs an average of 40 hours per screen just to document the requirements and UI state. In a typical enterprise application with 500+ screens, you’re looking at 20,000 man-hours before you even open an IDE. This is the "Archaeology Tax."

By using Replay, that 40-hour-per-screen metric drops to 4 hours. We shift the focus from "interrogating the past" to "capturing the present."

From Black Box to Documented Codebase#

The fundamental problem with a 20-year-old monolith is that it's a black box. You see the input and the output, but the transformations in between are obscured by legacy frameworks (Struts, WebForms, or early Angular).

Replay changes the source of truth from "human memory" to "runtime execution." By recording a real user workflow, Replay captures the DOM state, network calls, and user interactions. It then uses an AI Automation Suite to synthesize this data into modern, clean React components.

Step 1: Record the Workflow#

Instead of a workshop, you have a subject matter expert (SME) perform their daily tasks while Replay records the session. This isn't just a video; it's a deep-packet capture of the application's front-end and back-end interaction.

Step 2: Extraction and Synthesis#

Replay's engine parses the recording to identify patterns. It recognizes a legacy table, a complex form validation, or a nested navigation structure. It doesn't just copy the HTML; it understands the intent.

Step 3: Component Generation#

The platform generates a "Library" (Design System) and "Blueprints" (Editor-ready code). Below is an example of what Replay extracts from a legacy JSP-based insurance claim form:

typescript
// Generated via Replay Visual Reverse Engineering
// Source: /legacy/claims/entry_v2.jsp
import React, { useState, useEffect } from 'react';
import { Button, TextField, Select, Grid } from '@/components/ui';
import { useClaimsStore } from '@/store/claims';

export const ClaimEntryForm = ({ claimId }: { claimId: string }) => {
  const [loading, setLoading] = useState(true);
  const { data, updateField, syncWithLegacyAPI } = useClaimsStore();

  // Replay captured these specific validation rules from the legacy runtime
  const validatePolicyFormat = (val: string) => /^[A-Z]{3}-\d{9}$/.test(val);

  return (
    <Grid container spacing={3}>
      <Grid item xs={12} md={6}>
        <TextField
          label="Policy Number"
          value={data.policyNumber}
          error={!validatePolicyFormat(data.policyNumber)}
          onChange={(e) => updateField('policyNumber', e.target.value)}
          helperText="Format: AAA-123456789"
        />
      </Grid>
      {/* Logic preserved from legacy workflow recording */}
      {data.amount > 5000 && (
        <Grid item xs={12}>
          <div className="p-4 bg-amber-50 border-l-4 border-amber-400">
            <p className="text-sm text-amber-700">
              Note: Claims over $5,000 require supervisor override (Legacy Rule 402).
            </p>
          </div>
        </Grid>
      )}
      <Button onClick={syncWithLegacyAPI}>Sync to Core Systems</Button>
    </Grid>
  );
};

💡 Pro Tip: When modernizing, don't try to fix the business logic and the tech stack at the same time. Use Replay to capture the existing logic first, then refactor once you are on a modern stack.

Bridging the API Gap#

A 20-year-old monolith rarely has a clean REST API. You're likely dealing with SOAP, XML-over-HTTP, or direct database connections. Replay monitors the network traffic during your recording sessions to generate API Contracts automatically.

If your legacy system sends a messy 400-line XML payload, Replay maps that to a clean JSON schema and generates the TypeScript interfaces.

typescript
/**
 * Generated API Contract: Claims Submission
 * Source: Captured from Legacy Network Intercept
 */
export interface LegacyClaimPayload {
  header: {
    transactionId: string;
    timestamp: string; // ISO 8601
  };
  body: {
    claimant_info: {
      first_name: string;
      last_name: string;
      tax_id_encrypted: string;
    };
    incident_details: {
      date: string;
      description: string;
      location_code: number; // Mapped from legacy lookup table
    };
  };
}

// Replay generates the modern wrapper to handle the legacy transformation
export const submitClaim = async (data: ModernClaimRequest): Promise<void> => {
  const legacyPayload = transformToLegacy(data);
  await api.post('/api/v1/claims/process', legacyPayload);
};

⚠️ Warning: Many teams fail because they underestimate the "hidden" logic in network payloads. Replay captures the actual data flying across the wire, ensuring no hidden field is missed.

The "Flows" Architecture: Visualizing the Spaghetti#

One of the biggest hurdles when you modernize a 20-year-old monolith is understanding the state machine. How does a user get from Screen A to Screen D? In legacy systems, this logic is often hardcoded in the backend or buried in thousands of lines of jQuery.

Replay's Flows feature visualizes the architecture of your legacy application based on real usage. It builds a directed graph of every route, every modal, and every conditional redirect.

1. •Map the Surface Area: Identify every unique screen and state.
2. •Identify High-Value Workflows: Don't modernize everything. Use Replay's audit to see which screens are actually used by employees.
3. •Dependency Mapping: See which components are shared across the monolith to avoid duplication in your new React library.

💰 ROI Insight: By identifying that 30% of your legacy screens are no longer used by staff, you can reduce the scope of your modernization project by nearly a third instantly.

Built for Regulated Environments#

We work with Financial Services, Healthcare, and Government agencies. We know that "the cloud" isn't always an option for sensitive legacy data.

Replay is built with a security-first mindset:

• •SOC2 & HIPAA Ready: Data handling that meets the strictest compliance standards.
• •On-Premise Deployment: Run Replay entirely within your own firewall. No data ever leaves your network.
• •PII Masking: Automatically redact sensitive user information during the recording and extraction process.

The Technical Debt Audit: Knowing What You're Leaving Behind#

Before you migrate, you need to know the cost of your debt. Replay provides a comprehensive Technical Debt Audit during the extraction process. It flags:

• •Duplicate logic across different modules.
• •Hardcoded credentials or environment-specific strings.
• •Inconsistent UI patterns that should be consolidated into your new Design System.
• •Deprecated API endpoints that are still being called.

Frequently Asked Questions#

How does Replay handle complex business logic that isn't visible in the UI?#

Replay captures the inputs, outputs, and side effects (network calls, state changes). While it cannot "see" the internal COBOL logic on a mainframe, it documents exactly what that logic expects and what it returns. This allows you to create a "black box" integration or a clean specification for a rewrite of that specific logic.

Does Replay require access to our source code?#

No. Replay performs Visual Reverse Engineering by observing the application at runtime. This is particularly valuable for 20-year-old systems where the source code might be lost, unbuildable, or obfuscated.

What is the typical time savings?#

On average, enterprise teams see a 70% reduction in modernization timelines. A project that was estimated at 18 months of manual effort can typically be completed in 4-6 months using Replay’s extraction and automation tools.

Can Replay generate End-to-End (E2E) tests?#

Yes. Because Replay records the user's intent and the system's response, it automatically generates Playwright or Cypress tests for every workflow you record. This ensures that your modernized version maintains functional parity with the legacy system.

The Future of Modernization#

The era of the 2-year rewrite is over. The risks are too high, and the talent is too expensive to waste on manual documentation. To modernize a 20-year-old monolith effectively, you must leverage automation to understand the past so you can build the future.

Stop the workshops. Start recording.

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