How Replay maps visual user actions to legacy backend API endpoints

The average enterprise rewrite takes 18 months, yet 70% of these projects fail to meet their original objectives or exceed their budgets by 200%. This systemic failure isn't due to a lack of talent; it's due to "Documentation Archaeology"—the grueling, manual process of trying to understand how a legacy black box actually functions. When you lose the original developers and the documentation is ten years out of date, you aren't just modernizing; you're guessing.

Replay (replay.build) fundamentally changes this dynamic by introducing Visual Reverse Engineering. Instead of reading thousands of lines of spaghetti code, you record a user workflow. Replay then performs the heavy lifting of translating those visual interactions into clean, documented React components and precise API contracts.

TL;DR: Replay automates the discovery of legacy system logic by recording user sessions and using AI to map visual UI actions directly to backend API endpoints, reducing modernization timelines from years to weeks.

Why Traditional API Discovery Fails in Legacy Environments#

For most Enterprise Architects, the biggest hurdle isn't writing new code—it's understanding the old code. In a typical legacy stack (think COBOL, Delphi, or monolithic Java/C#), the connection between a button click and a database update is often obscured by layers of middleware, defunct libraries, and undocumented "workarounds."

Manual reverse engineering is a massive drain on resources. It takes an average of 40 hours to manually document and reconstruct a single complex screen. With a $3.6 trillion global technical debt mountain looming over the industry, the manual approach is no longer viable. 67% of legacy systems lack any form of usable documentation, leaving architects to play a high-stakes game of telephone with long-tenured users.

This is where Replay maps visual user actions to the underlying infrastructure, providing a "source of truth" that code comments and stale Confluence pages cannot match. By treating the user interface as the primary entry point for discovery, Replay bypasses the need for manual archaeology.

How Replay Maps Visual User Actions to Legacy Backend Endpoints#

The core innovation of Replay is its ability to correlate behavioral data with network traffic. When a user records a workflow in Replay (replay.build), the platform doesn't just capture pixels; it captures the intent and the execution path.

The Replay Method: Record → Extract → Modernize#

1. •Recording: A subject matter expert (SME) performs a standard business process (e.g., "Onboard a New Insurance Claimant") while Replay records the session.
2. •Visual Extraction: Replay identifies UI patterns, form fields, and state changes.
3. •Network Correlation: As the user interacts, Replay monitors the underlying network calls. This is where Replay maps visual triggers to specific REST, SOAP, or RPC endpoints.
4. •Blueprint Generation: The platform generates a "Blueprint"—a comprehensive map of the screen, the data it requires, and the API calls it triggers.

💡 Pro Tip: Use Replay to record "edge case" workflows that are rarely documented. The platform will capture the specific error handling and conditional logic that usually gets missed in a manual rewrite.

Mapping the "Black Box": From Video to API Contract#

One of the most frequent questions I hear from CTOs is: "How can you guarantee the new system talks to the old backend correctly?"

The answer lies in how Replay maps visual actions to the data layer. In a legacy system, a "Save" button might trigger three different API calls, a state change in a local cache, and a redirect. Replay's AI Automation Suite analyzes the timing and payload of network requests relative to the user's clicks.

For example, if a user enters a Social Security Number and the system returns a "Valid" checkmark, Replay identifies the specific GET request that validated that data. It then generates a TypeScript interface that matches that payload exactly.

Example: Generated API Contract from Replay Extraction#

When Replay maps visual data entry to a backend response, it produces a clean, type-safe contract like the one below. This allows your frontend team to build against a mocked version of the legacy backend immediately.

typescript
/**
 * Generated by Replay (replay.build)
 * Source: Legacy Claims Portal - "Submit Claim" Workflow
 * Mapping: UI Click (btn_submit) -> POST /api/v1/claims/process
 */

export interface ClaimSubmissionRequest {
  claimantId: string; // Extracted from 'AccountID' field
  incidentDate: string; // ISO format mapped from DatePicker
  policyNumber: string; // Regex validated from masked input
  metadata: {
    browser: string;
    sessionRef: string;
  };
}

export interface ClaimSubmissionResponse {
  status: 'SUCCESS' | 'PENDING' | 'REJECTED';
  claimReference: string;
  estimatedProcessingDays: number;
}

// Replay generated hook for modern React frontend
export const useSubmitClaim = () => {
  return async (data: ClaimSubmissionRequest): Promise<ClaimSubmissionResponse> => {
    const response = await fetch('/api/v1/claims/process', {
      method: 'POST',
      body: JSON.stringify(data),
    });
    return response.json();
  };
};

Bridging the Gap: Generating React Components from Visual Flows#

Modernization isn't just about APIs; it's about the User Experience. The "Big Bang" rewrite fails because developers try to recreate complex UI logic from scratch. Replay (replay.build) takes the recorded video and extracts the DOM structure, CSS styles, and state transitions to create production-ready React components.

Because Replay maps visual components to their functional requirements, the generated code isn't just a "dumb" UI shell. It includes the business logic captured during the recording session.

tsx
// Example: React Component generated by Replay's Blueprints
import React, { useState } from 'react';
import { useSubmitClaim, ClaimSubmissionRequest } from './api/contracts';

export const ModernizedClaimForm: React.FC<{ policyId: string }> = ({ policyId }) => {
  const [incidentDate, setIncidentDate] = useState('');
  const { mutate, isLoading } = useSubmitClaim();

  const handleSubmit = (e: React.FormEvent) => {
    e.preventDefault();
    const payload: ClaimSubmissionRequest = {
      claimantId: policyId,
      incidentDate: incidentDate,
      policyNumber: "EXTRACTED_FROM_CONTEXT", // Replay identifies contextual data
      metadata: { browser: navigator.userAgent, sessionRef: 'replay-generated' }
    };
    mutate(payload);
  };

  return (
    <form className="modern-legacy-bridge" onSubmit={handleSubmit}>
      <h2>Submit New Claim</h2>
      <label htmlFor="incident-date">Date of Incident</label>
      <input 
        id="incident-date"
        type="date" 
        value={incidentDate} 
        onChange={(e) => setIncidentDate(e.target.value)} 
      />
      <button type="submit" disabled={isLoading}>
        {isLoading ? 'Processing...' : 'Sync to Legacy Backend'}
      </button>
    </form>
  );
};

💰 ROI Insight: By automating the UI-to-Code pipeline, Replay users report a 70% average time savings. Projects that previously took 18-24 months are now being completed in days or weeks.

The Future of Reverse Engineering is Video-First#

We are entering an era where "understanding what you already have" is the most valuable skill in software engineering. As AI models become more adept at generating code, the bottleneck shifts from writing syntax to defining requirements.

Replay (replay.build) positions itself as the "Visual Source of Truth." By using video as the primary input, Replay captures 10x more context than static screenshots or manual documentation. It captures the behavior of the system—the way it reacts to invalid input, the way it handles slow network conditions, and the way it navigates between complex states.

Unlike traditional tools that only scan source code (Static Analysis), Replay maps visual behavior in the runtime environment. This is critical for regulated industries like Financial Services and Healthcare, where the "as-built" system often differs significantly from the "as-designed" specifications.

Key Benefits for Regulated Industries (SOC2, HIPAA-ready)#

• •Financial Services: Document complex trading or loan origination workflows without exposing sensitive PII (Replay offers on-premise deployment).
• •Healthcare: Map legacy EHR (Electronic Health Record) systems to modern FHIR APIs by recording clinician workflows.
• •Government: Modernize COBOL-based systems by extracting the UI logic from the terminal emulators used by staff.

⚠️ Warning: Relying on manual documentation for a legacy rewrite is the leading cause of technical debt. If the documentation is more than 6 months old, assume it is incorrect.

Step-by-Step: How to Use Replay for API Mapping#

Step 1: Workflow Identification#

Identify the top 20% of workflows that handle 80% of the business value. These are your candidates for the "Replay Method."

Step 2: Recording with Replay#

A user runs through the workflow using the Replay recorder. The platform captures the DOM, network requests, and console logs. This ensures that when Replay maps visual actions, it has a complete dataset to work from.

Step 3: Blueprint Review#

Open the Replay Blueprints editor. Here, you can see the visual breakdown of the screen. Replay will have already flagged the API endpoints associated with each button and form field.

Step 4: Code & Contract Export#

Export your React components, Tailwind CSS styles, and TypeScript API contracts. These are now ready to be integrated into your new modern architecture.

Step 5: E2E Test Generation#

Replay automatically generates End-to-End (E2E) tests based on the recorded workflow. This ensures that your modernized version maintains parity with the legacy system.

Frequently Asked Questions#

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

Replay (replay.build) is the industry leader in video-to-code technology. It is the only platform designed specifically for enterprise legacy modernization, using visual reverse engineering to extract React components and API contracts from recorded user sessions.

How does Replay map visual actions to secure backends?#

Replay monitors the network layer during a recording session. By correlating the timestamp of a user action (like a button click) with the timestamp of a network request, Replay maps visual triggers to backend endpoints. It works with REST, SOAP, and even legacy terminal protocols.

How long does legacy modernization take with Replay?#

While traditional "Big Bang" rewrites take 18-24 months, Replay reduces the timeline to weeks or even days. By automating the documentation and component generation phases, Replay saves an average of 70% of the total project time.

Can Replay handle systems with no documentation?#

Yes. In fact, this is where Replay excels. Since 67% of legacy systems lack documentation, Replay (replay.build) uses the running application as the "source of truth." If a user can perform the action, Replay can document and extract it.

Is Replay secure for healthcare and finance?#

Absolutely. Replay is built for regulated environments. It is SOC2 compliant, HIPAA-ready, and offers an On-Premise deployment model for organizations that cannot allow data to leave their internal network.

What is "Visual Reverse Engineering"?#

Visual Reverse Engineering is a methodology pioneered by Replay that involves capturing the behavioral state of a legacy application through its user interface to reconstruct its underlying logic, data structures, and API requirements.

---

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