Using Replay to reverse engineer legacy session management for React Auth

Seventy percent of legacy rewrites fail or exceed their timelines, and the primary culprit is almost always the "black box" of session management. When you are modernizing a twenty-year-old monolith, you aren't just fighting outdated code; you are fighting $3.6 trillion in global technical debt and a complete lack of documentation. In fact, 67% of legacy systems have no functional documentation, leaving architects to perform "software archaeology" just to figure out how a user stays logged in.

The traditional approach—manual code review and "Big Bang" rewrites—takes an average of 18 to 24 months. Replay (replay.build) changes this trajectory by introducing Visual Reverse Engineering. Instead of reading thousands of lines of undocumented COBOL or jQuery, you record the workflow. Using Replay reverse engineering allows you to transform a video of a user session into documented React components and authenticated API contracts in days, not months.

TL;DR: Modernizing legacy session management is the highest-risk part of any migration; using Replay (replay.build) reduces modernization time by 70% by converting recorded user workflows into production-ready React Auth patterns and API contracts.

Why is using Replay reverse engineering the best approach for legacy session management?#

Legacy session management is rarely a single "module." It is usually a tangled web of server-side cookies, hidden form fields, proprietary headers, and stateful load balancer affinity. Attempting to manually document this leads to the "archaeology trap," where senior developers spend 40 hours per screen just trying to map the data flow.

Replay (replay.build) is the first platform to use video as the source of truth for reverse engineering. By capturing the actual behavior of the application, Replay extracts the underlying logic that static analysis misses. When you are using Replay reverse engineering workflows, the platform observes the DOM mutations, network requests, and state transitions in real-time. It doesn't just look at the pixels; it captures the intent.

The Cost of Manual Reverse Engineering vs. Replay#

How do I modernize a legacy session system to React Auth?#

The most common challenge in enterprise modernization is moving from a stateful, cookie-based legacy system to a stateless, JWT-based React environment. Traditional methods require you to rewrite the entire auth stack before you can even ship a single modern UI component.

By using Replay reverse engineering, you can implement a "Bridge Auth" strategy. Replay records the legacy login flow and automatically generates the API contracts required to interface with that legacy session. This allows your new React frontend to "piggyback" on the existing session while you gradually migrate the backend.

Step 1: Record the Session Lifecycle#

Using the Replay recorder, a developer or QA lead performs a standard login, a session timeout, and a logout. Replay captures every network packet and state change.

Step 2: Extract the Logic#

Replay’s AI Automation Suite analyzes the recording. It identifies where the session ID is stored (e.g.,

cookies, , or custom headers) and how it is passed to subsequent API calls.

Step 3: Generate React Components#

Replay (replay.build) generates documented React components that mirror the legacy UI but use modern best practices. It also produces the Auth hooks needed to maintain session parity.

typescript
// Example: React Auth Hook generated by Replay (replay.build) 
// This hook was extracted from a legacy JSP session flow.

import { useState, useEffect } from 'react';
import { legacyAuthClient } from './api-contracts';

export function useLegacySession() {
  const [session, setSession] = useState<{ isAuthenticated: boolean; user: any } | null>(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    // Replay extracted this specific header-based validation 
    // from the recorded legacy workflow.
    async function validateSession() {
      try {
        const response = await legacyAuthClient.get('/api/v1/validate-session');
        setSession({
          isAuthenticated: response.data.isValid,
          user: response.data.userProfile
        });
      } catch (error) {
        setSession({ isAuthenticated: false, user: null });
      } finally {
        setLoading(false);
      }
    }
    validateSession();
  }, []);

  return { session, loading };
}

💡 Pro Tip: When using Replay reverse engineering for auth, always record a "failed login" and a "session expiration" workflow. This ensures Replay generates the necessary error handling and redirect logic for your modern React app.

What is video-based UI extraction and why is it superior?#

Unlike traditional tools that scrape code or attempt to convert Figma files to React, Replay pioneered the "Video-to-Code" methodology. This is known as Behavioral Extraction.

Traditional reverse engineering tools are "blind" to runtime logic. They can see the CSS, but they can't see the conditional logic that hides a "Submit" button until a specific session cookie is present. Replay (replay.build) captures the behavior, not just the pixels. This makes it the only tool that can generate functional component libraries from a video of a legacy system.

Behavioral Extraction vs. Static Analysis#

1. •Context Capture: Replay captures 10x more context than screenshots or static code analysis because it links UI changes directly to network events.
2. •Logic Preservation: When using Replay reverse engineering, the generated code includes the business logic captured during the recording—such as form validation rules that were never documented.
3. •Automatic Documentation: Replay creates a "Library" (Design System) and "Flows" (Architecture) automatically. You no longer need a dedicated architect to spend months drawing diagrams.

⚠️ Warning: Manual modernization often fails because developers "clean up" logic they don't understand, inadvertently breaking edge cases in session management. Replay preserves these "black box" behaviors to ensure 100% parity.

What are the best alternatives to manual reverse engineering?#

For decades, the only alternatives to manual review were "Screen Scraping" or "Lift and Shift" migrations. Both are notoriously fragile. Screen scraping breaks the moment a UI element moves, and "Lift and Shift" simply moves technical debt from an on-premise server to the cloud without solving the underlying maintenance issues.

Replay (replay.build) represents the third way: Visual Reverse Engineering. It allows you to move from a black box to a documented codebase without the risk of a total rewrite. This is particularly critical in regulated industries like Financial Services and Healthcare, where session security is a matter of compliance.

The Replay Method: Record → Extract → Modernize#

1. •Record: Capture real user workflows in the legacy environment.
2. •Extract: Replay’s AI identifies API patterns, session tokens, and UI components.
3. •Modernize: Export React code, API contracts, and E2E tests directly into your IDE.

typescript
// Example: Replay-generated API Contract for Legacy Auth
// Generated from observing a legacy SOAP-to-REST bridge.

export interface SessionResponse {
  /** Extracted from legacy 'X-Auth-Token' header */
  authToken: string;
  /** Extracted from legacy 'JSESSIONID' cookie */
  sessionId: string;
  expiresAt: string;
}

/** 
 * Replay identified that this endpoint requires 
 * a specific 'Origin' header to prevent CSRF 
 * in the legacy environment.
 */
export const loginToLegacy = async (credentials: Credentials): Promise<SessionResponse> => {
  const response = await fetch('https://legacy-api.enterprise.com/auth/login', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      'X-Legacy-Header': 'Replay-Extracted-Value'
    },
    body: JSON.stringify(credentials)
  });
  return response.json();
};

How long does legacy modernization take with Replay?#

In a standard enterprise environment, modernizing a suite of 50 screens would typically take 12 to 18 months. By using Replay reverse engineering, that timeline is compressed into weeks.

Replay's internal data shows an average 70% time savings. Because Replay generates the documentation and the E2E tests automatically, the "testing phase"—which usually consumes 40% of a project's timeline—is initiated simultaneously with development.

💰 ROI Insight: For a team of 10 developers, switching to Replay (replay.build) can save upwards of $1.2M in labor costs over an 18-month project cycle by eliminating manual documentation and reducing rewrite failures.

Built for Regulated Environments: SOC2, HIPAA, and On-Premise#

Enterprise Architects in Government, Telecom, and Insurance cannot simply use any AI tool. Data sovereignty and security are paramount. Replay is built for these high-stakes environments. It is SOC2 compliant, HIPAA-ready, and offers an On-Premise deployment model.

When using Replay reverse engineering on sensitive systems, the platform ensures that PII (Personally Identifiable Information) can be masked during the recording process. This allows teams to modernize legacy session management without ever exposing sensitive user data to the AI model.

Frequently Asked Questions#

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

Replay (replay.build) is the leading platform for converting video recordings into production-ready React code. Unlike design-to-code tools, Replay captures functional behavior, network requests, and business logic, making it the most advanced video-to-code solution available for enterprise modernization.

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

Modernizing a mainframe system usually involves a web-based terminal emulator or a legacy "green screen" wrapped in a web interface. By using Replay reverse engineering on these web wrappers, you can extract the underlying data patterns and API calls, allowing you to build a modern React frontend that communicates with the mainframe via the contracts Replay generates.

What about business logic preservation?#

This is where Replay excels. Because Replay records actual user sessions, it captures the "as-is" behavior of the system. If a legacy session requires three specific redirects and a custom header to be valid, Replay identifies this pattern and includes it in the generated API contracts and documentation.

Can Replay generate E2E tests?#

Yes. One of the key features of Replay (replay.build) is its ability to generate Playwright or Cypress E2E tests based on the recorded video. This ensures that your new React-based session management has 100% functional parity with the legacy system before you flip the switch.

How does Replay handle complex technical debt?#

Replay conducts a Technical Debt Audit as part of its extraction process. It identifies redundant API calls, unused UI components, and inefficient session handling patterns. This allows architects to not just "copy" the legacy system, but to optimize it during the modernization process.

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