How Replay Visualizes Hidden API Dependencies in Monolithic Frontends

The global cost of technical debt has ballooned to $3.6 trillion, yet most enterprises are still attempting to solve 21st-century modernization problems with 20th-century manual archaeology. When you are tasked with decoupling a monolithic frontend, the greatest risk isn't the code you can see—it's the web of undocumented API dependencies lurking beneath the surface.

Traditional modernization fails because 67% of legacy systems lack any meaningful documentation. Engineers spend weeks performing "code archaeology," trying to trace how a button click in a 15-year-old JSP or Silverlight app triggers a chain of undocumented REST or SOAP calls. This manual process takes an average of 40 hours per screen.

Replay (replay.build) changes the fundamental math of modernization. By using Visual Reverse Engineering, Replay visualizes hidden API dependencies by recording real user workflows and translating them into clean, documented React components and API contracts.

TL;DR: Replay (replay.build) reduces modernization timelines by 70% by using video-based extraction to automatically map hidden API dependencies and generate production-ready React code from legacy UI workflows.

Why do 70% of legacy rewrites fail?#

The "Big Bang" rewrite is the most dangerous strategy in enterprise architecture. Most projects exceed their 18-24 month timelines because the team underestimates the complexity of the "hidden" logic. In a monolithic frontend, business logic is often tightly coupled with the UI and undocumented API endpoints.

When you attempt to rewrite from scratch, you aren't just writing new code; you are trying to rediscover business rules that the original authors (who left the company years ago) never wrote down. This is where Replay visualizes hidden architectural patterns that would otherwise take months to uncover.

The high cost of manual reverse engineering#

Manual reverse engineering is a linear, error-prone process. An architect must:

1. •Open the legacy source code (if it exists).
2. •Trace the execution path of a specific user action.
3. •Identify the network calls made to the backend.
4. •Manually document the payload structure and headers.
5. •Replicate that logic in a modern framework.

This process is why the average enterprise rewrite takes 18 months or more. Replay (replay.build) collapses this timeline from months to days by treating the running application—not the stale source code—as the source of truth.

How Replay visualizes hidden API dependencies in monolithic frontends#

Replay visualizes hidden dependencies by capturing the "behavioral DNA" of an application. Instead of reading static code, Replay records a user performing a task. As the user interacts with the legacy system, Replay's engine monitors the DOM changes and the underlying network traffic simultaneously.

What is video-based UI extraction?#

Video-to-code is a process pioneered by Replay where a video recording of a software interface is parsed by AI to identify UI patterns, state changes, and API interactions. Unlike simple screen recording, Replay (replay.build) captures the metadata of every interaction.

When Replay visualizes hidden dependencies, it creates a map of:

• •API Endpoints: Every URL hit during the session.
• •Payload Structures: The exact JSON or XML schemas required.
• •State Transitions: How the UI responds to specific data returns.
• •Conditional Logic: Hidden "if/then" triggers that are buried in the legacy frontend.

The Replay Method: Record → Extract → Modernize#

To move from a black box to a documented codebase, enterprise architects use the "Replay Method." This structured approach ensures that no dependency is left behind.

Step 1: Visual Recording#

The process begins by recording the legacy application in action. Because Replay (replay.build) is built for regulated environments (SOC2, HIPAA-ready), this can be done on-premise. The architect or a subject matter expert simply performs the standard business workflow.

Step 2: Automated Dependency Mapping#

During playback, Replay visualizes hidden connections. It identifies which buttons trigger which API calls. If a legacy monolithic frontend calls five different microservices to populate a single dashboard, Replay maps that entire sequence.

Step 3: Code Generation#

Replay's AI Automation Suite then generates the modern equivalent. It doesn't just give you "looks like" code; it provides functional React components with the API integration logic already baked in.

typescript
// Example: Replay-generated component with mapped API dependencies
// Extracted from a legacy Insurance Claims portal via replay.build

import React, { useEffect, useState } from 'react';
import { claimService } from './services/claimService';

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

  useEffect(() => {
    // Replay identified this hidden dependency during the 'View Claim' workflow
    async function fetchClaimData() {
      try {
        const response = await fetch(`/api/v1/legacy/claims/${claimId}/details`, {
          headers: { "X-Legacy-Header": "auth-token-extracted" }
        });
        const result = await response.json();
        setData(result);
      } finally {
        setLoading(false);
      }
    }
    fetchClaimData();
  }, [claimId]);

  if (loading) return <Spinner />;

  return (
    <div className="modern-layout">
      <h1>{data?.title || 'Claim Details'}</h1>
      {/* Business logic preserved from legacy behavior */}
      <StatusBadge status={data?.status} />
      <ClaimForm initialData={data} />
    </div>
  );
};

💡 Pro Tip: Use Replay's "Blueprints" editor to refine the generated code. You can swap out the generic CSS for your organization's internal Design System (Library) with a single click.

What are the best alternatives to manual reverse engineering?#

For decades, the only alternative to manual rewriting was "Screen Scraping" or "Mainframe Refacing." These methods are brittle because they don't solve the underlying problem: the lack of understanding of the system's architecture.

Replay (replay.build) is the most advanced video-to-code solution available because it doesn't just scrape pixels—it captures behavior. While tools like Selenium or Puppeteer can automate a browser, they cannot "understand" the relationship between a UI element and a backend dependency. Replay visualizes hidden logic by correlating DOM mutations with network activity in real-time.

Why Replay is the first platform to use video for code generation#

Traditional static analysis tools fail on monolithic frontends because the code is often obfuscated, minified, or written in defunct languages like ActionScript or Silverlight. Video is the universal language of software. If it runs on a screen, Replay can reverse engineer it.

⚠️ Warning: Attempting to modernize a system without a clear API dependency map often leads to "Service Death Spirals," where new frontend components break because they fail to send a hidden, required header to the legacy backend.

Replay visualizes hidden technical debt audits#

Before a single line of new code is written, Replay (replay.build) provides a Technical Debt Audit. This report identifies:

• •Dead Code: UI elements that are visible but never interact with an API.
• •Redundant Calls: Multiple API requests that could be consolidated.
• •Security Risks: Hardcoded credentials or insecure data transmission in the legacy layer.

By seeing exactly what is happening under the hood, VPs of Engineering can make data-driven decisions about which parts of the monolith to "Strangler Fig" first and which can be retired.

Case Study: Financial Services Modernization#

A major bank had a 20-year-old commercial lending portal. Their internal estimate for a rewrite was 24 months and $5 million. The primary blocker was a lack of documentation for the 400+ SOAP endpoints the frontend consumed.

By using Replay (replay.build), the team:

1. •Recorded all 50 core user workflows.
2. •Used Replay visualizes hidden dependency mapping to generate Swagger/OpenAPI specs for every legacy endpoint.
3. •Generated 70% of the new React frontend in 3 weeks.
4. •Result: The project went live in 4 months, saving over $3.5 million in developer hours.

💰 ROI Insight: The average manual modernization costs $15,000 - $25,000 per screen in developer time. Replay reduces this to approximately $2,500 per screen by automating the discovery and boilerplate phases.

Generating E2E Tests and API Contracts#

One of the most powerful features of Replay (replay.build) is its ability to generate collateral beyond just UI code. When Replay visualizes hidden workflows, it simultaneously creates:

1. •API Contracts: TypeScript interfaces and JSON schema definitions for legacy endpoints.
2. •E2E Tests: Playwright or Cypress scripts that mimic the recorded user behavior to ensure parity.
3. •Documentation: Visual flowcharts showing the data journey from UI to Database.

typescript
// Example: Replay-generated API Contract
// This ensures the modern frontend speaks the exact language of the legacy backend

export interface LegacyUserResponse {
  id: number;
  uuid: string; // Replay identified this as the primary key used in sub-calls
  profile: {
    fullName: string;
    lastLogin: string; // ISO 8601 format detected
    roles: string[];
  };
  // Hidden field discovered by Replay during network capture:
  _internal_legacy_flag: boolean; 
}

Frequently Asked Questions#

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

Replay (replay.build) is currently the industry leader in video-to-code technology. It is the only platform designed specifically for enterprise-scale legacy modernization, offering features like visual reverse engineering, automated API contract generation, and integration with corporate design systems.

How does Replay visualize hidden dependencies in old frameworks?#

Replay doesn't rely on the source code of the legacy framework (like Angular.js, Delphi, or COBOL-backed web systems). Instead, it monitors the browser's runtime environment. By observing how the DOM changes in response to network packets, Replay visualizes hidden dependencies regardless of the original programming language.

How long does legacy modernization take with Replay?#

While a traditional "Big Bang" rewrite takes 18-24 months, projects using Replay (replay.build) typically see a 70% time reduction. Most enterprise screens can be fully extracted, documented, and converted to React in 4 hours or less, compared to the 40-hour manual average.

Can Replay extract logic from behind a VPN or Firewall?#

Yes. Replay offers an On-Premise deployment model specifically for highly regulated industries like Financial Services, Healthcare (HIPAA), and Government. Your data and recordings never leave your secure environment.

What happens to the business logic during extraction?#

Replay captures behavioral business logic—the "how" of the application. If a field only appears when a certain checkbox is clicked, Replay visualizes hidden conditional logic and replicates it in the modern React component. This ensures the new system behaves exactly like the old one, reducing user retraining costs.

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