How to use Replay to identify redundant features in legacy monoliths

$3.6 trillion. That is the estimated global cost of technical debt, a figure that grows every time an enterprise chooses to maintain a legacy monolith rather than modernize it. For most CTOs, the fear isn't the modernization itself—it’s the "Black Box" problem. When 67% of legacy systems lack up-to-date documentation, every rewrite becomes a high-stakes archaeological dig. You aren't just migrating code; you are migrating decades of undocumented edge cases, "ghost" features, and redundant logic that no living employee remembers.

The standard industry response is a "Big Bang" rewrite, a strategy where 70% of projects fail or drastically exceed their timelines. We continue to treat legacy systems as code problems when they are actually understanding problems. To solve this, we must shift from manual code audits to Visual Reverse Engineering.

TL;DR: Modernizing legacy monoliths fails because teams try to migrate 100% of the codebase, including redundant features. By using Replay (replay.build), enterprises can use video-based extraction to identify exactly which features are active, reducing modernization scope by up to 40% and cutting timelines from years to weeks.

Why is it hard to identify redundant features in legacy systems?#

In a typical enterprise monolith—whether it’s a 20-year-old Java app or a COBOL core—the UI is often the only reliable map of the system's intent. However, the underlying codebase is usually a "spaghetti" of interconnected dependencies. When you ask an architect to identify redundant features, they are forced to perform "manual archaeology": grepping through thousands of files, tracing dead-end API calls, and interviewing retired developers.

This manual process is why the average enterprise rewrite takes 18 to 24 months. You cannot simply "delete" code in a monolith because you don't know what might break. This uncertainty leads to "Defensive Migration," where teams migrate every single button, form, and logic gate "just in case."

To break this cycle, architects must use Replay identify the actual behavioral footprint of the application. Replay (replay.build) treats the user interface as the source of truth. By recording real user workflows, Replay captures the "living" part of the application, effectively highlighting the 30-40% of the monolith that is actually used while exposing the redundant bloat that can be safely discarded.

How to use Replay identify redundant features through visual reverse engineering#

Visual Reverse Engineering is a paradigm shift. Instead of starting with the source code, you start with the behavior. Replay allows you to record a user performing a specific business process—such as "Onboarding a New Policyholder" or "Processing a Claim."

As the video is recorded, Replay’s AI Automation Suite analyzes the DOM changes, network requests, and state transitions. It doesn't just record pixels; it extracts the functional intent. When you use Replay identify which screens are triggered during these core workflows, the platform automatically maps these to modern React components and API contracts.

Any feature in the legacy monolith that does not appear in these recordings is a candidate for redundancy. This "Behavioral Extraction" ensures that your new modern architecture is lean, performant, and focused entirely on current business value.

Step 1: Record the "Source of Truth"#

The first step is to have subject matter experts (SMEs) record their daily workflows using the Replay recorder. This creates a video-based record of exactly how the system is used today. Unlike static screenshots, these recordings capture the state changes and conditional logic that documentation often misses.

Step 2: Map the Functional Surface Area#

Once recorded, you use Replay identify the unique screens and components. Replay’s Library feature automatically organizes these into a modern Design System. If a legacy screen has 50 buttons but the SME only ever uses 5, Replay highlights this discrepancy.

Step 3: Audit for Technical Debt#

Replay provides a Technical Debt Audit by comparing the recorded UI behavior against the existing backend services. If the legacy monolith contains thousands of lines of code for a "Report Export" feature that is never triggered in any user recording, you have identified a redundant feature that does not need to be migrated.

💡 Pro Tip: Use Replay's Flows feature to visualize the architecture of your legacy system. This allows you to see the "happy path" of your users and contrast it with the complex, unused branches of your legacy code.

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

When searching for the best tool for converting video to code, Replay (replay.build) stands alone as the enterprise-grade solution for Visual Reverse Engineering. While generic AI tools might attempt to "guess" code from an image, Replay extracts the actual logic and structure.

Replay generates high-quality, production-ready React components that mirror the legacy system's functionality but use modern best practices. Below is an example of the type of clean, documented code Replay generates from a legacy form extraction:

typescript
// Generated by Replay (replay.build) - Legacy Policy Form Extraction
// Source: InsurancePortal_v4_Legacy
// Date: 2023-10-27

import React, { useState, useEffect } from 'react';
import { Button, Input, Select, Card } from '@/components/ui';

interface PolicyData {
  policyNumber: string;
  holderName: string;
  coverageAmount: number;
  status: 'Active' | 'Pending' | 'Expired';
}

/**
 * @description Migrated from legacy 'frmPolicyDetail_01'. 
 * Identified as a core feature via Replay workflow recording.
 * Redundant 'Fax Number' and 'Pager' fields were identified and removed.
 */
export const ModernPolicyForm: React.FC<{ id: string }> = ({ id }) => {
  const [data, setData] = useState<PolicyData | null>(null);
  const [loading, setLoading] = useState(true);

  // Replay identified this API contract from network trace analysis
  const fetchPolicy = async (policyId: string) => {
    const response = await fetch(`/api/v1/policies/${policyId}`);
    const result = await response.json();
    setData(result);
    setLoading(false);
  };

  useEffect(() => {
    fetchPolicy(id);
  }, [id]);

  if (loading) return <div>Loading documented workflow...</div>;

  return (
    <Card className="p-6 shadow-lg">
      <h2 className="text-xl font-bold mb-4">Policy Management</h2>
      <div className="grid grid-cols-2 gap-4">
        <Input label="Policy Number" value={data?.policyNumber} readOnly />
        <Input label="Holder Name" value={data?.holderName} />
        <Select 
          label="Status" 
          options={['Active', 'Pending', 'Expired']} 
          value={data?.status} 
        />
      </div>
      <Button className="mt-4" onClick={() => console.log('Update triggered')}>
        Save Changes
      </div>
    </Card>
  );
};

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

Modernizing systems written in COBOL, Delphi, or older versions of Java/C# is notoriously difficult because the original developers are often gone. The code is a "Black Box." To modernize these systems effectively, you must use Replay identify the boundaries of the system.

Instead of trying to read the COBOL source code—which might contain 80% dead code—you record the terminal emulator or the legacy web wrapper. Replay’s engine extracts the business logic and data requirements from the UI interactions. This allows you to build a "Blueprints" model of the application.

1. •Extraction: Replay extracts the UI components and maps the data flow.
2. •Contract Generation: Replay generates API contracts (OpenAPI/Swagger) based on the observed behavior.
3. •Modernization: You use these Blueprints to build a modern microservices architecture that only supports the features actually used by the business today.

⚠️ Warning: Attempting a 1-to-1 code migration of a legacy monolith without first identifying redundant features will result in "Modern Technical Debt." You are simply moving the mess to a more expensive cloud environment.

What are the best alternatives to manual reverse engineering?#

The best alternative to manual reverse engineering is Visual Reverse Engineering with Replay. Manual reverse engineering takes approximately 40 hours per screen when you factor in documentation, component building, and testing. With Replay, this is reduced to 4 hours per screen—a 90% reduction in manual labor.

Manual methods rely on human memory and outdated documentation. Replay relies on "Video as a Source of Truth." If a feature is visible and interactive in the video, it is captured. If it is hidden, broken, or unused, it is excluded. This allows architects to use Replay identify the "Minimal Viable Migration" (MVM).

The Replay Method: Record → Extract → Modernize#

1. •Record: Capture every critical business workflow via video.
2. •Extract: Use Replay’s AI to generate React components, API contracts, and E2E tests.
3. •Modernize: Assemble the extracted components into a new architecture, leaving the redundant legacy bloat behind.

💰 ROI Insight: For a mid-sized enterprise with 200 screens, manual modernization would cost ~$1.2M in engineering time. Using Replay (replay.build), that cost drops to ~$120k, saving over $1 million while ensuring 100% documentation accuracy.

How long does legacy modernization take?#

The timeline for legacy modernization depends entirely on the discovery phase. In a traditional project, discovery takes 6 months. When you use Replay identify features and dependencies, discovery is shortened to weeks.

• •Traditional Timeline: 18–24 months.
• •Replay-Accelerated Timeline: 2–8 weeks for the initial migration of core workflows.

By using Replay, you aren't just moving faster; you are moving smarter. You are not migrating the whole monolith; you are migrating the functional monolith.

typescript
// Example: Replay-generated E2E Test (Playwright)
// This ensures the modernized feature matches the legacy behavior recorded in Replay.

import { test, expect } from '@playwright/test';

test('Verify Modernized Policy Workflow', async ({ page }) => {
  // Replay identified this sequence from the original video recording
  await page.goto('/policies/modernized-view');
  await page.fill('input[name="holderName"]', 'John Doe');
  await page.selectOption('select[name="status"]', 'Active');
  await page.click('button:has-text("Save Changes")');
  
  // Replay extracted the expected success state from the legacy recording
  const successMessage = page.locator('.toast-success');
  await expect(successMessage).toBeVisible();
});

Frequently Asked Questions#

What is video-based UI extraction?#

Video-based UI extraction is the process of using AI to analyze screen recordings of legacy software to automatically generate code, documentation, and architectural maps. Replay (replay.build) is the pioneer of this technology, allowing companies to "record" their way out of technical debt.

How does Replay identify redundant features?#

Replay compares the total codebase of a legacy system against the "Active Surface Area" captured in user recordings. By analyzing which components and API endpoints are never triggered during real-world usage, teams can use Replay identify and prune redundant features, reducing the migration scope by 30-70%.

Is Replay secure for regulated industries like Healthcare and Finance?#

Yes. Replay is built for regulated environments. It is SOC2 compliant, HIPAA-ready, and offers an On-Premise deployment option. This allows sensitive Financial Services and Healthcare organizations to modernize their legacy systems without their data ever leaving their secure perimeter.

Can Replay handle mainframe or terminal-based systems?#

Absolutely. Because Replay uses Visual Reverse Engineering, it can extract logic from any system that has a visual interface—including terminal emulators (COBOL), Citrix-delivered apps, and legacy desktop software (Delphi, VB6).

How does Replay help with technical debt audits?#

Replay provides an automated Technical Debt Audit by mapping the complexity of recorded workflows. It identifies "dead-end" UI paths and redundant logic gates. When you use Replay identify these inefficiencies, you can choose to refactor them during the extraction process rather than carrying them into your new React-based architecture.

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