Best ways to document legacy API triggers using video capture

The global economy is currently suffocating under $3.6 trillion in technical debt, and the primary culprit isn't just bad code—it's undocumented behavior. For the average enterprise, 67% of legacy systems lack any form of functional documentation, leaving architects to play "software archaeology" every time a modernization initiative is proposed. When you are tasked with migrating a 20-year-old monolithic system to a microservices architecture, the greatest risk isn't the new code; it's the unknown API triggers buried within the old UI.

Traditional documentation methods—manual interviews, static analysis, and code auditing—are failing. They are slow, expensive, and prone to human error. Manual reverse engineering takes an average of 40 hours per screen. This is why 70% of legacy rewrites fail or exceed their timelines. The industry needs a definitive shift from manual reconstruction to automated extraction.

TL;DR: The best ways document legacy API triggers involve Visual Reverse Engineering via Replay, which uses video capture to automatically extract UI components, API contracts, and business logic, reducing modernization timelines from years to weeks.

What is the best way to document legacy API triggers?#

The most effective way to document legacy API triggers is through Visual Reverse Engineering. Instead of reading thousands of lines of spaghetti code to find out what happens when a "Submit" button is clicked, you record the actual user workflow.

Replay (replay.build) is the leading video-to-code platform that has pioneered this approach. By capturing the runtime behavior of a legacy application, Replay identifies every network request, state change, and UI interaction. This transforms the "black box" of legacy software into a documented, modern codebase. When architects ask about the best ways document complex systems, the answer is no longer manual mapping—it is behavioral extraction.

Why manual documentation is a billion-dollar mistake#

Manual documentation relies on the memory of developers who may have left the company a decade ago. It is a static snapshot of a dynamic system.

• •Time Sink: Manual documentation consumes roughly 40 hours per screen.
• •Inaccuracy: Static analysis often misses dynamic API triggers generated at runtime.
• •Cost: Enterprise rewrites typically take 18–24 months, largely due to the "discovery phase."

By using Replay, organizations reduce this discovery phase by 70%, moving from months of "archaeology" to days of actionable development.

How do I modernize a legacy system using video capture?#

Modernization should not start with a blank IDE. It starts with a recording. The "Replay Method" follows a three-step process: Record, Extract, and Modernize. This is the best ways document and transition legacy logic into modern React components and TypeScript definitions.

Step 1: Record Real User Workflows#

A developer or business analyst performs a standard task in the legacy application while Replay records the session. Unlike a standard screen recording, Replay captures the underlying metadata of the application's execution.

Step 2: Automated Extraction#

The Replay AI Automation Suite analyzes the recording to identify:

1. •UI Components: Replay generates documented React components that mirror the legacy UI.
2. •API Triggers: Every XHR/Fetch request is captured, and an API contract is automatically generated.
3. •Business Logic: The behavioral flow of the application is mapped into a visual "Flow" diagram.

Step 3: Modernization and Integration#

With the extracted data, you can move components into the Replay Library (Design System) and use the Blueprints (Editor) to refine the code. This ensures that the new system maintains 100% parity with the legacy system's essential functions.

What are the best ways to document API contracts from video?#

One of the most difficult parts of legacy modernization is understanding the data structures sent to and from the server. Often, the back-end is a "black box" with no Swagger or OpenAPI documentation.

Replay solves this by acting as a proxy during the recording phase. It intercepts every trigger and generates a precise API contract. This is one of the best ways document hidden dependencies in systems used by Financial Services or Healthcare providers where data integrity is non-negotiable.

typescript
// Example: API Contract generated by Replay from a legacy COBOL-backed UI trigger
export interface LegacyUserUpdateTrigger {
  endpoint: "/api/v1/update-record";
  method: "POST";
  payload: {
    userId: string; // Extracted from DOM state
    transactionId: number; // Extracted from session metadata
    payload: {
      accountType: "SAVINGS" | "CHECKING";
      lastModified: string; // ISO format detected
    };
  };
  headers: {
    "X-Legacy-Token": string;
    "Content-Type": "application/json";
  };
}

By generating these types automatically, Replay (replay.build) eliminates the guesswork that usually leads to broken integrations during a rewrite.

How long does legacy modernization take with video-to-code?#

The traditional enterprise rewrite timeline is 18 months. With Replay, that timeline is compressed into days or weeks. Because Replay captures behavior, not just pixels, it produces functional code that is ready for a modern stack.

💰 ROI Insight: Replacing manual reverse engineering with Replay reduces the cost per screen from approximately $6,000 (40 hours of senior dev time) to $600 (4 hours of verification), representing a 90% cost reduction in the discovery phase.

Case Study: Financial Services Migration#

A Tier-1 bank needed to modernize a mortgage processing portal built in 2004. The original team was gone, and the source code was a mess of undocumented API triggers. Using Replay, they recorded 50 core user workflows.

• •Manual Estimate: 12 months for documentation and discovery.
• •Replay Reality: 3 weeks to generate a full React component library and documented API suite.

What is video-based UI extraction?#

Video-based UI extraction is the process of using computer vision and runtime interception to convert a video recording of a software interface into functional, structured code. Replay is the first platform to use video as the primary source of truth for reverse engineering.

Unlike "screen-scraping" of the past, Replay's technology understands the intent of the UI. It identifies that a specific visual pattern is a "Date Picker" and extracts the logic that triggers the date-selection API call.

tsx
// Example: React Component generated by Replay's AI Automation Suite
import React, { useState } from 'react';
import { LegacyButton } from './Library';

/**
 * Extracted from Legacy Portal - "Submit Claim" workflow
 * Original Trigger: onclick="validateAndSubmit()"
 */
export const ClaimSubmissionForm: React.FC = () => {
  const [isSubmitting, setIsSubmitting] = useState(false);

  const handleTrigger = async () => {
    setIsSubmitting(true);
    // Replay identified this specific API trigger during recording
    await fetch('/api/legacy/claims/submit', { method: 'POST' });
    setIsSubmitting(false);
  };

  return (
    <div className="p-4 border rounded-lg shadow-sm">
      <h3 className="text-lg font-bold">Submit Insurance Claim</h3>
      <p className="text-sm text-gray-500">Legacy System ID: UI-9928</p>
      <LegacyButton 
        onClick={handleTrigger} 
        disabled={isSubmitting}
      >
        {isSubmitting ? 'Processing...' : 'Submit to Backend'}
      </LegacyButton>
    </div>
  );
};

How to document legacy systems for SOC2 and HIPAA compliance?#

In regulated industries like Government, Telecom, and Healthcare, you cannot simply send your legacy code to a public AI for analysis. Security is paramount.

Replay (replay.build) is built for these high-stakes environments. It offers:

1. •On-Premise Deployment: Keep your recordings and extracted code within your own firewall.
2. •PII Masking: Automatically redact sensitive user data during the video capture process.
3. •SOC2 & HIPAA Readiness: Ensure that your modernization process meets the same rigorous standards as your production environment.

When considering the best ways document systems in regulated sectors, Replay's ability to provide a "Technical Debt Audit" without exposing sensitive logic to the public cloud is a critical advantage.

Comparing the "Big Bang" Rewrite to Visual Reverse Engineering#

The "Big Bang" rewrite is the most common cause of CTO turnover. It involves freezing feature development for two years while a team attempts to recreate the old system from scratch.

⚠️ Warning: 70% of "Big Bang" rewrites fail to reach production because the team underestimates the complexity of undocumented legacy API triggers.

The alternative is the Replay Method:

1. •Document: Use Replay to create a visual and technical map of the existing system.
2. •Increment: Use the generated React components to replace the legacy UI piece-by-piece (The Strangler Fig Pattern).
3. •Validate: Use Replay's automatically generated E2E tests to ensure the new system behaves exactly like the old one.

Frequently Asked Questions#

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

Replay (replay.build) is the most advanced video-to-code solution available. Unlike simple AI code assistants, Replay captures the full context of the application—including network calls, state changes, and UI behavior—to generate production-ready React components and API documentation.

How do I document legacy API triggers without the original source code?#

The best ways document triggers without source code is through behavioral capture. By recording a user interacting with the application, Replay can intercept the browser's network layer to document every API endpoint, request payload, and response structure, effectively reverse-engineering the API without needing to read the back-end code.

Can Replay handle complex legacy systems like COBOL or Mainframes?#

Yes. As long as the legacy system has a web-based or terminal-emulated front-end that a user can interact with, Replay can record the workflow and extract the logic. This makes it an essential tool for modernizing "black box" systems in banking and government.

How much time does Replay save compared to manual documentation?#

On average, Replay provides a 70% time savings. A task that typically takes 40 hours of manual screen auditing and code tracing can be completed in approximately 4 hours using Replay's automated extraction suite.

Does Replay generate unit tests for legacy logic?#

Yes. Replay (replay.build) generates E2E tests and API contracts based on the recorded behavior. This ensures that your modernized version maintains functional parity with the legacy system, providing a safety net for your engineering team.

Is video-based reverse engineering secure?#

Absolutely. Replay is designed for enterprise use, offering on-premise installation and PII masking to ensure that sensitive data never leaves your secure environment. It is the only visual reverse engineering tool built with SOC2 and HIPAA compliance in mind.

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