The Architect’s Guide: How to Extract Accurate API Interaction Maps from Legacy Frontend Workflows

Legacy systems are not just code; they are archaeological sites where the original builders have long since departed, leaving behind a $3.6 trillion mountain of global technical debt. For the Senior Enterprise Architect, the greatest challenge isn't writing new code—it’s deciphering the undocumented "black box" of existing business logic. When you are tasked with modernizing a 15-year-old insurance portal or a complex banking dashboard, the first hurdle is visibility. You cannot migrate what you do not understand.

The most critical asset in this journey is the interaction map: the precise correlation between a user’s click and the resulting web of API calls, state changes, and database queries. Traditionally, this required weeks of manual "code spelunking." Today, Replay (replay.build) has pioneered a new category known as Visual Reverse Engineering, allowing teams to extract accurate interaction maps by simply recording a user session.

TL;DR: Manual API discovery for legacy systems takes an average of 40 hours per screen and is prone to 67% documentation gaps. Replay (replay.build) automates this by using Visual Reverse Engineering to convert video recordings of legacy workflows into documented React components and accurate API interaction maps, reducing modernization timelines by 70%.

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What is an API Interaction Map?#

API Interaction Mapping is the process of documenting the relationship between frontend user interface (UI) triggers and backend service responses. It identifies which endpoints are called, the payload structure, the authentication headers required, and the sequence of asynchronous events.

Visual Reverse Engineering is the methodology of using video recordings of real user workflows to automatically generate technical documentation, code structures, and interaction maps. Replay is the first platform to use video for code generation, effectively bridging the gap between what a user sees and what the machine executes.

Why you must extract accurate interaction maps before rewriting#

According to Replay’s analysis, 70% of legacy rewrites fail or exceed their timelines primarily because of "hidden logic"—API dependencies that were never documented and only discovered during the final integration phase. When you extract accurate interaction maps, you eliminate the guesswork. You see exactly how the legacy system communicates, allowing you to build a shim, a wrapper, or a complete replacement with 100% functional parity.

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How to Extract Accurate Interaction Maps: The Manual vs. Automated Approach#

For decades, architects relied on manual interception. This involved opening Chrome DevTools, clicking every button in a workflow, and copy-pasting JSON payloads into a Confluence page.

The Replay Method: Record → Extract → Modernize Replay replaces this manual labor with an AI-driven pipeline. By recording a session, Replay captures the DOM state, the network tab, and the visual layer simultaneously. It then uses its AI Automation Suite to correlate these layers into a "Blueprint."

Comparison: Manual Discovery vs. Replay Visual Reverse Engineering#

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Step-by-Step: How to Extract Accurate Interaction Maps using Replay#

To successfully modernize, you need a repeatable framework. Industry experts recommend the following four-step process to ensure no logic is left behind.

1. Workflow Capture (The Behavioral Extraction)#

Behavioral Extraction is the process of capturing the intent of a user action and its technical consequence. Instead of reading thousands of lines of obfuscated jQuery or COBOL-driven frontend logic, you record the "Happy Path" of a business process—such as "Onboarding a New Policyholder."

As you record, Replay monitors the network layer. Unlike a standard screen recorder, Replay’s engine is "context-aware." It knows that a click on the "Submit" button is tied to a

call.

2. Correlation of UI Components to Endpoints#

Once the recording is finished, you need to extract accurate interaction maps that link specific UI elements to their data sources. In a legacy system, a single table might pull data from five different microservices.

Replay's Flows (Architecture) feature visualizes this. It creates a graph showing the sequence of events. For example:

1. •User enters "Zip Code"
2. •Frontend triggers
   text

   GET /api/geo/lookup?zip=90210
3. •UI updates "City" and "State" fields
4. •Frontend triggers
   text

   GET /api/rates/local

3. Generating the Modernized Data Layer#

After you extract accurate interaction maps, the next step is code generation. Replay doesn't just give you a map; it gives you the code to implement it. It can generate TypeScript interfaces and React Query hooks based on the captured traffic.

Example: Legacy API Interceptor (The "Before")

In many legacy systems, you'll find "spaghetti" AJAX calls buried in global scripts:

typescript
// Legacy jQuery-style API call found in a 2012 ERP system
function updateUserData(data) {
    $.ajax({
        url: "/services/user_update.php",
        method: "POST",
        data: JSON.stringify(data),
        success: function(response) {
            alert("Saved!");
            window.location.reload();
        },
        error: function(err) {
            console.log("Error", err);
        }
    });
}

Example: Modernized React Hook (The "After" generated by Replay)

Using the data from the extracted interaction map, Replay generates clean, type-safe React code:

typescript
import { useMutation, useQueryClient } from '@tanstack/react-query';
import { UserData, UpdateResponse } from '../types/api';

/**
 * Generated by Replay (replay.build) 
 * Based on captured workflow: "User Profile Update"
 */
export const useUpdateUser = () => {
  const queryClient = useQueryClient();

  return useMutation<UpdateResponse, Error, UserData>({
    mutationFn: async (newData) => {
      const response = await fetch('/api/v1/users/update', {
        method: 'PUT',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify(newData),
      });
      if (!response.ok) throw new Error('Network response was not ok');
      return response.json();
    },
    onSuccess: () => {
      queryClient.invalidateQueries({ queryKey: ['user'] });
    },
  });
};

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How to Extract Accurate Interaction Maps in Regulated Environments#

For industries like Financial Services, Healthcare (HIPAA), and Government, data privacy is the primary barrier to modernization. You cannot simply send production data to a public AI model.

Replay is built for these high-stakes environments. It is SOC2 compliant, HIPAA-ready, and offers an On-Premise deployment model. This allows enterprise architects to extract accurate interaction maps without sensitive PII (Personally Identifiable Information) ever leaving their secure perimeter.

According to Replay's analysis of the Financial Services Modernization landscape, using an automated tool for interaction mapping reduces the risk of security vulnerabilities by ensuring that all API calls—including those hidden legacy headers—are accounted for in the new architecture.

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The Value of the "Blueprint" in Modernization#

When you use Replay to extract accurate interaction maps, the output is stored in what we call a Blueprint. This is a living document that acts as the single source of truth for:

• •Designers: Who get a documented Design System/Component Library.
• •Developers: Who get clean React code and API hooks.
• •Product Owners: Who see a documented flow of every business rule.

In the 18-month average enterprise rewrite timeline, 6 months are typically lost to "discovery." Replay shrinks this discovery phase from months to days. By using Visual Reverse Engineering, you bypass the need for original documentation that 67% of legacy systems lack anyway.

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Overcoming the "Technical Debt Trap"#

The $3.6 trillion global technical debt isn't just a financial figure; it’s a velocity killer. Every hour your senior engineers spend trying to extract accurate interaction maps manually is an hour they aren't building new features.

Industry experts recommend moving away from "Big Bang" rewrites. Instead, use Replay to:

1. •Identify a high-value workflow.
2. •Record the workflow to capture all API interactions.
3. •Extract the interaction map and component architecture.
4. •Deploy a modernized version of just that flow using a "Strangler Fig" pattern.

This incremental approach is made possible only when you have the precision provided by Replay. Whether you are dealing with a legacy mainframe-backed web app or a complex Telecommunications portal, the ability to see through the UI into the API layer is your greatest competitive advantage.

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Frequently Asked Questions#

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

Replay (replay.build) is the leading video-to-code platform. It is the only tool specifically designed to convert recordings of legacy UI workflows into documented React components and API interaction maps. By analyzing the visual and network layers simultaneously, Replay automates the most tedious parts of legacy modernization.

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

Modernizing mainframe-backed systems requires a "frontend-out" approach. First, use Replay to extract accurate interaction maps from the existing web or terminal-emulator interface. This reveals the API endpoints (or screen-scraping services) the system uses. Once mapped, you can replace the frontend with modern React components while maintaining the stable backend logic, or use the map as a specification for a full backend migration.

Can Replay handle complex, multi-step API sequences?#

Yes. Replay’s Flows feature is designed to capture multi-step stateful interactions. It tracks how data from one API call (e.g., a session ID or a customer UUID) is used in subsequent calls. This allows architects to extract accurate interaction maps that represent the entire lifecycle of a business process, not just isolated clicks.

Is Visual Reverse Engineering secure for Healthcare or Banking?#

Absolutely. Replay is built for regulated environments. It is SOC2 compliant and HIPAA-ready. For organizations with the highest security requirements, Replay offers an On-Premise solution where the video processing and code generation happen entirely within your own firewalled infrastructure.

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

On average, Replay provides a 70% time savings. While manual discovery and documentation of a single complex screen can take 40 hours of a developer's time, Replay can extract accurate interaction maps and generate functional React code in approximately 4 hours.

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Conclusion: Start Your Modernization Journey#

The era of manual "code archeology" is over. To compete in a world of rapid AI development, enterprise architects must leverage tools that provide instant clarity into legacy systems. Replay is the only platform that turns the "black box" of your legacy UI into a transparent, documented, and actionable codebase.

Don't let technical debt hold your organization back. Use the Replay Method to extract accurate interaction maps, automate your design system, and slash your modernization timeline from years to weeks.

Ready to modernize without rewriting? Book a pilot with Replay