The Black Box Problem: Legacy COBOL-to-React Pipelines Preserving 30 Years of Business Logic Without Source Code

The most expensive code in your organization is the code nobody understands. For the average Tier-1 bank or insurance provider, the core business logic—the rules that govern interest calculations, claims processing, and risk assessment—is trapped inside a COBOL mainframe environment that hasn't been documented since the late 1990s. When leadership demands a "modernization," the standard response is a high-risk, multi-year rewrite that has a 70% chance of failure.

The challenge isn't just the language; it's the lack of visibility. According to Replay's analysis, 67% of legacy systems lack any form of accurate technical documentation. This creates a paradox: you cannot move to the cloud because you don't know what the system actually does, but you cannot stay on the mainframe because the talent pool is evaporating and the $3.6 trillion global technical debt is accruing interest every day.

We are seeing a fundamental shift in how architects approach this. Instead of traditional "rip and replace" strategies, the industry is moving toward legacy coboltoreact pipelines preserving business logic through visual reverse engineering. By treating the UI as the "source of truth," organizations can extract functional requirements without ever touching the fragile COBOL source code.

TL;DR: Modernizing COBOL systems typically takes 18-24 months and often fails due to lost business logic. By using Replay, enterprises can record user workflows to automatically generate documented React components and Design Systems. This "Visual Reverse Engineering" approach reduces modernization timelines by 70%, transforming a 40-hour-per-screen manual rewrite into a 4-hour automated process, all while preserving 30 years of undocumented business rules.

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The Failure of Traditional Modernization Strategies#

Traditional modernization follows a predictable, painful path: "The Big Bang Rewrite." Developers attempt to read thousands of lines of procedural COBOL, map them to modern microservices, and then build a React frontend from scratch.

Industry experts recommend moving away from this model because it ignores the "Hidden Logic" problem. Over 30 years, edge cases have been hardcoded into the system. These rules aren't in the documentation; they are in the fingers of the users who have navigated these green screens for decades. When you rewrite the source, you lose the nuances of the workflow.

Video-to-code is the process of capturing real-time user interactions with a legacy interface and using computer vision and AI to translate those visual patterns into structured code, state machines, and component libraries.

By focusing on legacy coboltoreact pipelines preserving the user experience, architects can ensure that the new React application performs exactly like the legacy system, but with the benefits of a modern tech stack.

The Documentation Gap: Why Source Access is a Red Herring#

Many architects believe that having the COBOL source code is the key to success. In reality, the source code is often a liability.

1. •Dead Code: Large portions of the codebase may no longer be executed.
2. •Missing Dependencies: The code relies on specific mainframe environment variables or third-party libraries that no longer exist.
3. •Logic Drift: The way the software is actually used by the business has diverged from how the code was originally written.

Replay bypasses these issues by focusing on the output. If a user enters a claim number and the screen displays a specific validation error, that is a business rule. By capturing that interaction, we can replicate the validation logic in a modern React environment without needing to decipher the underlying COBOL

loops.

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Building Legacy COBOL-to-React Pipelines Preserving Functional Integrity#

To build an effective pipeline, you need to treat the legacy UI as a data source. This is where Visual Reverse Engineering comes into play. The pipeline consists of four distinct phases: Capture, Extraction, Mapping, and Generation.

Phase 1: The Capture (Recording the Workflow)#

Instead of interviewing users for weeks, you record them performing their daily tasks. These recordings capture every state change, every input validation, and every navigational shortcut. This is the foundation of legacy coboltoreact pipelines preserving the actual utility of the system.

Phase 2: Extraction (The Replay Library)#

Once recorded, the Replay Library analyzes the video frames to identify recurring UI patterns. It recognizes that a specific sequence of characters on a green screen represents a "Data Grid" or a "Customer Search Form."

Phase 3: Mapping to React#

The extracted patterns are then mapped to a modern Design System. This isn't just a visual copy; it's a structural mapping. The "Function Key" navigation of a mainframe is mapped to React Router hooks or state-driven navigation.

Phase 4: Code Generation#

Finally, the pipeline outputs clean, documented TypeScript and React code. Unlike generic AI code generators, this code is structured specifically for your enterprise's architecture.

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Technical Deep Dive: From Mainframe State to React Props#

When we talk about legacy coboltoreact pipelines preserving logic, we are talking about state management. A COBOL screen is essentially a snapshot of a flat record buffer. To translate this to React, we must convert that flat buffer into a structured component tree.

Consider a legacy "Policy Summary" screen. In the old system, this might be a series of

statements at specific coordinate positions. In the modern pipeline, we extract these as a functional component library.

Example: Legacy Data Extraction to React Component#

Below is a conceptual representation of how a legacy screen's data structure is transformed into a modern, type-safe React component using the outputs generated by Replay.

typescript
// Generated via Replay Blueprints
import React from 'react';
import { Card, Grid, Typography, StatusBadge } from '@/components/ui';

interface PolicySummaryProps {
  policyNumber: string;
  effectiveDate: string;
  premiumAmount: number;
  status: 'ACTIVE' | 'PENDING' | 'LAPSED';
  onRenew: (id: string) => void;
}

/**
 * Reconstructed from Legacy Workflow: "Policy_Inquiry_v4"
 * This component preserves the exact field mapping found in the 
 * mainframe 'POL-SUM-REC' buffer without requiring the original copybook.
 */
export const PolicySummary: React.FC<PolicySummaryProps> = ({
  policyNumber,
  effectiveDate,
  premiumAmount,
  status,
  onRenew,
}) => {
  return (
    <Card className="p-6 border-l-4 border-primary">
      <Grid container spacing={4}>
        <Grid item xs={12} md={4}>
          <Typography variant="label">Policy Number</Typography>
          <Typography variant="h2">{policyNumber}</Typography>
        </Grid>
        <Grid item xs={12} md={4}>
          <Typography variant="label">Effective Date</Typography>
          <Typography variant="body1">{effectiveDate}</Typography>
        </Grid>
        <Grid item xs={12} md={4}>
          <StatusBadge status={status} />
        </Grid>
      </Grid>
      <div className="mt-4 flex justify-end">
        <button 
          onClick={() => onRenew(policyNumber)}
          className="bg-blue-600 text-white px-4 py-2 rounded"
        >
          Execute Renewal (F3)
        </button>
      </div>
    </Card>
  );
};

The magic here isn't just the UI—it's the

function. In the legacy system, this was triggered by the key. Our legacy coboltoreact pipelines preserving approach maps that keyboard interrupt to a modern event handler, ensuring the workflow remains intuitive for the existing workforce while moving the execution to a modern API.

Handling Undocumented State Transitions#

One of the biggest hurdles in technical debt reduction is the "hidden" state. For instance, a field might only become editable if three other fields are filled in a specific order.

According to Replay's analysis, these conditional states are rarely documented in the COBOL source in a way that is easily readable by modern tools. Replay’s "Flows" feature maps these transitions visually.

typescript
// State machine generated from recorded user behavior
// Captures the "Locked Field" logic observed in the legacy UI
const usePolicyFormState = (initialData: any) => {
  const [state, setState] = React.useState(initialData);
  const [isEditable, setIsEditable] = React.useState(false);

  React.useEffect(() => {
    // Logic extracted from observed user workflows:
    // Field 'Premium' only becomes editable after 'Risk_Score' > 50
    if (state.riskScore > 50 && state.agentVerified) {
      setIsEditable(true);
    }
  }, [state.riskScore, state.agentVerified]);

  return { state, setState, isEditable };
};

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Why "No Source Access" is a Competitive Advantage#

It sounds counterintuitive, but ignoring the source code can actually accelerate modernization. When teams have access to the COBOL, they get bogged down in "code archeology." They spend months trying to understand how the engine works rather than what the engine produces.

By focusing on legacy coboltoreact pipelines preserving the functional output, you achieve:

1. •Speed: You eliminate the "Analysis Paralysis" phase of the project.
2. •Accuracy: You build based on what the system actually does today, not what it was designed to do 20 years ago.
3. •Decoupling: You can build the entire React frontend and Design System before the backend APIs are even finished.

This is particularly critical in Design System modernization, where the goal is to create a consistent look and feel across a fragmented portfolio of legacy applications.

Security and Compliance in Regulated Environments#

For Financial Services and Healthcare, "moving to the cloud" isn't just a technical challenge; it's a regulatory one. Modernizing a COBOL system requires strict adherence to SOC2 and HIPAA standards.

Replay is built for these environments. Because the platform can be deployed On-Premise, sensitive data never leaves your secure perimeter. The legacy coboltoreact pipelines preserving sensitive business logic can run entirely within your air-gapped environment, converting video recordings of internal tools into clean code without exposing PII (Personally Identifiable Information).

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The Economics of Visual Reverse Engineering#

The average enterprise rewrite timeline is 18 months. During that time, the business is frozen. No new features can be added to the legacy system, and the new system is a moving target.

Using Replay, that timeline is compressed into weeks.

• •Manual approach: 40 hours per screen (Research + Design + Code + Test).
• •Replay approach: 4 hours per screen (Record + Generate + Refine).

According to Replay's analysis, this 70% average time savings allows organizations to reallocate their most expensive resources—senior engineers—away from "translation" and toward "innovation." Instead of spending 18 months reaching parity with the old system, they spend 3 months reaching parity and 15 months building new value for the customer.

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Strategic Implementation: A Step-by-Step Guide#

If you are an Enterprise Architect looking to implement legacy coboltoreact pipelines preserving legacy logic, follow this roadmap:

1. Identify "High-Value, Low-Documentation" Workflows#

Start with the screens that are critical to the business but have the most technical debt. These are usually the primary data entry or inquiry screens used by internal staff.

2. Record with Replay#

Deploy the Replay recorder to a subset of expert users. Capture 5-10 iterations of each core workflow to ensure all edge cases and validation errors are triggered.

3. Generate the Component Library#

Use the Replay Library to aggregate these recordings into a unified set of React components. This will serve as your new "Source of Truth" for the UI.

4. Map to Modern APIs#

While the UI is being generated, your backend team can focus on creating the REST or GraphQL wrappers around the mainframe data. Because the React components are already built to match the legacy data structures, the integration is seamless.

5. Validate and Iterate#

Use the original recordings as your "User Acceptance Testing" (UAT) baseline. If the React app behaves exactly like the recording, the business logic has been successfully preserved.

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Frequently Asked Questions#

Can I use Replay if I don't have the original COBOL source code?#

Yes. In fact, Replay is specifically designed for scenarios where source code is missing, obfuscated, or too complex to parse. By using visual reverse engineering, we extract the business logic from the user interface and workflow behavior rather than the underlying procedural code. This ensures that your legacy coboltoreact pipelines preserving logic are based on actual system behavior.

How does Replay handle complex mainframe navigation like PF keys?#

Replay's AI identifies keyboard interrupts and navigational patterns within the video stream. It then maps these to modern React equivalents, such as button clicks, keyboard shortcuts (using hooks like

), or automated state transitions. This allows you to maintain the "muscle memory" of your power users while providing a modern web experience.

What is the output of the Replay pipeline?#

The pipeline outputs production-ready TypeScript and React code, along with a documented Design System. This includes functional components, state management logic, and "Blueprints" that describe the architecture of the flows. The code is modular, following clean architecture principles, and is ready to be integrated into your CI/CD pipeline.

Is this approach secure for highly regulated industries like banking?#

Absolutely. Replay offers On-Premise deployment and is built to be SOC2 and HIPAA-ready. Since the process involves recording UIs, we provide tools to mask sensitive PII during the capture phase, ensuring that only the structural and logical patterns are analyzed by the AI.

How much time can we really save on a large-scale migration?#

On average, Replay reduces the time spent on UI modernization by 70%. For a typical enterprise screen that would take 40 hours to manually document, design, and code, Replay completes the process in approximately 4 hours. This shifts the modernization timeline from years to months.

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Ready to modernize without rewriting? Book a pilot with Replay and see how visual reverse engineering can transform your legacy debt into a modern asset.