API Signature Discovery: Mapping Legacy Backend Expectations from UI Video Captures

Your legacy mainframe or monolithic ERP doesn't have a Swagger spec. It doesn't have a Postman collection. In 67% of enterprise environments, the original documentation for these critical systems has long since vanished, leaving behind a "black box" that developers are terrified to touch. When you attempt to modernize these systems, the primary bottleneck isn't writing the new React components—it's the grueling process of signature discovery mapping legacy backend expectations.

Manually tracing network calls, deciphering obfuscated field names like

, and guessing validation logic takes an average of 40 hours per screen. With the global technical debt hovering at $3.6 trillion, the industry can no longer afford the "guess and check" method of integration.

Replay changes this paradigm by using Visual Reverse Engineering to bridge the gap between the UI and the API. By recording real user workflows, Replay automatically maps the visual state to the underlying data signatures, converting video recordings into documented React code and precise API blueprints.

TL;DR: Legacy modernization fails because of undocumented API signatures. Replay uses video-to-code technology to automate signature discovery mapping legacy systems, reducing the time to map a screen from 40 hours to just 4 hours. This allows teams to generate modern TypeScript interfaces and React components directly from recorded legacy workflows.

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The Documentation Void: Why Manual Mapping Fails#

According to Replay's analysis, 70% of legacy rewrites fail or exceed their timelines primarily because the "source of truth" is trapped in the UI behavior rather than the codebase. In many Financial Services or Healthcare environments, the backend code is a "spaghetti" of stored procedures and middleware that no single developer fully understands.

Signature discovery mapping legacy backends requires more than just looking at a database schema. It requires understanding:

1. •State Dependencies: Which UI toggles trigger specific backend flags?
2. •Validation Logic: Why does the API reject a 9-digit ID but accept a 10-digit one?
3. •Data Transformation: How does a "Pending" status in the UI map to an integer
   text

   4
   in the JSON payload?

Industry experts recommend moving away from manual "code archaeology." Instead, by capturing the execution of the system via video, we can observe the actual data contracts in motion.

Video-to-code is the process of using computer vision and network interception to translate visual UI interactions into functional, documented frontend code and API schemas.

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The Cost of Manual Discovery vs. Replay#

When you manually map a legacy system, you are essentially playing a game of telephone between the business analyst, the frontend dev, and the legacy DBA. This process is the primary reason the average enterprise rewrite takes 18 to 24 months.

By utilizing Replay's Flows, architects can visualize the entire architecture of a legacy transaction, ensuring that the signature discovery mapping legacy process accounts for every edge case captured during the recording session.

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Implementing Signature Discovery: From Video to TypeScript#

The core of signature discovery mapping legacy systems is the translation of "Visual Intent" into "Data Contract." When a user records a workflow in Replay, the platform identifies the input fields, the triggers (buttons), and the resulting network state.

Step 1: Capturing the Legacy Payload#

Imagine a legacy insurance portal where a "Claim Submission" involves a complex, undocumented POST request. The legacy payload might look like this:

json
// Legacy Obfuscated Payload
{
  "c_id": "99283",
  "stat_01": "A",
  "dt_proc": "2023-10-11T12:00:00Z",
  "usr_grp": ["admin", "verify"]
}

Without context,

is meaningless. However, Replay's AI observes that when the user selects "Active" from the dropdown, the payload sends .

Step 2: Generating the Modern Signature#

Replay's AI Automation Suite processes this recording and generates a clean, typed interface. This is the essence of signature discovery mapping legacy—turning mystery into documentation.

typescript
/**
 * Generated via Replay Signature Discovery
 * Source: Insurance Portal - Claims Workflow
 */

export interface ClaimSubmission {
  /** Internal Claim ID - mapped from c_id */
  claimId: string;
  
  /** 
   * Claim Status 
   * Mapping: "A" -> ACTIVE, "P" -> PENDING, "D" -> DENIED 
   */
  status: 'ACTIVE' | 'PENDING' | 'DENIED';
  
  /** ISO Date of processing */
  processedAt: Date;
  
  /** User authorization groups */
  userGroups: string[];
}

// Zod schema for runtime validation discovered from UI constraints
import { z } from 'zod';

export const ClaimSchema = z.object({
  claimId: z.string().min(5),
  status: z.enum(['ACTIVE', 'PENDING', 'DENIED']),
  processedAt: z.date(),
  userGroups: z.array(z.string())
});

By automating this, Replay slashes the time spent on Legacy Modernization Strategies by up to 70%.

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Extracting Component Logic from Visual Flows#

Once the signature discovery mapping legacy phase is complete, the next step is generating the UI. Replay doesn't just give you a screenshot; it gives you a functional React component that "knows" how to talk to that legacy backend.

Consider a legacy table that requires complex pagination and filtering logic. Manually recreating this in React would take days. Replay's Blueprints allow you to edit the discovered component logic before it's exported to your library.

Code Snippet: Discovered React Component#

The following component is generated by Replay after analyzing a video of a legacy search screen. It includes the discovered API signature and the necessary state management.

tsx
import React, { useState, useEffect } from 'react';
import { ClaimSubmission, ClaimSchema } from './types';

export const LegacyClaimTable: React.FC = () => {
  const [claims, setClaims] = useState<ClaimSubmission[]>([]);
  const [loading, setLoading] = useState(true);

  // Replay discovered this endpoint by intercepting the legacy UI network traffic
  const fetchClaims = async () => {
    try {
      const response = await fetch('/api/v1/legacy/claims/search');
      const data = await response.json();
      
      // Validate against discovered signature
      const validatedData = z.array(ClaimSchema).parse(data);
      setClaims(validatedData);
    } catch (error) {
      console.error("Signature Mismatch in Legacy API", error);
    } finally {
      setLoading(false);
    }
  };

  useEffect(() => { fetchClaims(); }, []);

  if (loading) return <div>Modernizing View...</div>;

  return (
    <div className="modern-grid">
      {claims.map(claim => (
        <div key={claim.claimId} className="card">
          <h3>Claim #{claim.claimId}</h3>
          <p>Status: {claim.status}</p>
        </div>
      ))}
    </div>
  );
};

This approach is vital for Visual Reverse Engineering because it ensures the new frontend is compatible with the old backend from day one.

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Why Signature Discovery is Critical for Regulated Industries#

In sectors like Government, Telecom, and Insurance, you cannot simply "rip and replace." These systems often have rigid compliance requirements. Signature discovery mapping legacy systems ensures that the data integrity is maintained during the transition.

1. •Financial Services: Ensuring that currency precision isn't lost when moving from a COBOL backend to a React frontend.
2. •Healthcare: Mapping HIPAA-compliant data fields without exposing PII during the discovery phase. Replay is SOC2 and HIPAA-ready, making it suitable for these sensitive environments.
3. •Manufacturing: Capturing complex ERP workflows where a single button click might update inventory across three different legacy databases.

Industry experts recommend using an "On-Premise" solution for these sectors. Replay offers an on-premise deployment model to ensure that your signature discovery mapping legacy data never leaves your secure perimeter.

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The AI Automation Suite: Beyond Simple Mapping#

Replay's AI doesn't just look at the code; it looks at the intent. If a legacy system has a confusing "Submit" button that actually performs a "Save and Print," Replay's AI Automation Suite flags this discrepancy.

When performing signature discovery mapping legacy workflows, the AI looks for patterns across multiple recordings. If three different users record the same "User Profile" update, Replay aggregates those sessions to find the "Maximum Viable Signature"—ensuring that optional fields are correctly identified.

This level of automation is why Replay can move an enterprise from an 18-month timeline to just a few weeks. You are no longer writing code from scratch; you are refining a high-fidelity blueprint generated from reality.

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Frequently Asked Questions#

What is signature discovery mapping legacy systems?#

It is the process of identifying the data contracts, field types, and validation rules of an undocumented legacy backend by observing the UI's behavior and network traffic. Replay automates this by converting video recordings of these interactions into structured TypeScript interfaces and React components.

How does Replay handle obfuscated or minified legacy code?#

Replay's Visual Reverse Engineering doesn't rely solely on the source code. It uses a combination of network interception, DOM observation, and computer vision. By seeing what the user sees and what the server receives, Replay can reconstruct the "intent" of the code even if the source is unreadable.

Can Replay discover hidden API fields not visible in the UI?#

Yes. During the signature discovery mapping legacy process, Replay captures the full request and response payloads. Often, legacy APIs return "bloat"—extra data fields that aren't used by the current UI but might be useful for the new modern version. Replay documents these in the Library for future use.

Is Replay compatible with mainframes or terminal emulators?#

As long as the legacy system is accessed via a web browser or a web-based terminal emulator, Replay can record the session and begin the discovery process. For thick-client applications, Replay's enterprise team provides custom integration paths to map those signatures.

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Conclusion: Modernize with Confidence#

The $3.6 trillion technical debt problem won't be solved by throwing more manual labor at it. The future of enterprise architecture lies in automation. By utilizing signature discovery mapping legacy techniques through Replay, organizations can finally see inside the "black box" of their legacy systems.

Stop guessing what your backend expects. Start recording, start mapping, and start modernizing.

Ready to modernize without rewriting? Book a pilot with Replay