COBOL to React: Mapping 1M Lines of Mainframe Banking Logic

Technical debt is no longer a balance sheet footnote; it is a structural threat to financial stability. Global technical debt has ballooned to a staggering $3.6 trillion, much of it locked within the green-screen terminals of the 1970s and 80s. When an enterprise attempts to bridge the gap between COBOL and React, the primary obstacle isn't just the syntax—it is the sheer volume of undocumented business logic. Mapping 1M lines of mainframe code to a modern frontend manually is a recipe for the 70% of legacy rewrites that fail or exceed their timelines.

Standard modernization approaches involve "Big Bang" migrations that take an average of 18 months. By the time the new system is ready, the business requirements have shifted, and the documentation—which 67% of legacy systems lack entirely—is still missing. To solve this, we must move away from manual source-code analysis and toward Visual Reverse Engineering.

TL;DR: Modernizing 1M lines of COBOL to React is traditionally an 18-24 month ordeal with a high failure rate. By using Replay, enterprises can bypass manual documentation gaps through Visual Reverse Engineering. Replay records real user workflows to generate documented React components and design systems, reducing the time per screen from 40 hours to just 4 hours—an average 70% time savings.

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The 18-Month Trap: Why Manual Mapping Fails#

When architects discuss cobol react mapping lines, they often focus on the wrong end of the pipe. They attempt to read COBOL source code to understand what the React UI should do. However, in a banking environment with 1M+ lines of code, that COBOL is often a "spaghetti" of patches, middleware hacks, and dead code.

According to Replay's analysis, manual mapping of legacy logic to modern components takes approximately 40 hours per screen. For a standard core banking platform with 200+ screens, you are looking at years of development before a single customer sees a login page.

The Documentation Vacuum#

Most mainframe systems have been maintained by generations of developers, many of whom have retired. This creates a "documentation vacuum" where the only source of truth is the running application. Manual efforts to map cobol react mapping lines usually result in:

1. •Logic Drift: The new React component doesn't quite match the legacy calculation.
2. •Scope Creep: Trying to "clean up" the logic while migrating it.
3. •Data Mismatches: React's JSON-first state management clashing with COBOL’s fixed-width file structures.

Learn more about legacy modernization strategies

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Visual Reverse Engineering: A New Framework#

Video-to-code is the process of capturing live application interactions and programmatically converting those visual elements and workflows into structured code and documentation.

Instead of staring at a COBOL

, Replay records the actual workflows of bank tellers and underwriters. This "Visual Reverse Engineering" approach treats the UI as the ultimate specification. By recording the screen, Replay's AI Automation Suite identifies patterns, data inputs, and state changes, then maps those directly to a React Design System.

Replay vs. Manual Modernization#

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Technical Deep Dive: Mapping Logic to Components#

When we talk about cobol react mapping lines, we are essentially talking about state transformation. A COBOL screen (CICS) is often a flat map of fields. React, conversely, is a tree of components.

To bridge this, we use Replay to generate a "Blueprint"—a functional map of the legacy screen. This Blueprint identifies that a specific set of lines in the mainframe terminal represents an

component.

Step 1: Capturing the Workflow#

The process begins by recording a user performing a high-value task, such as "Processing a Wire Transfer." Replay captures every state transition. Industry experts recommend focusing on "Flows" rather than individual screens to ensure the React architecture maintains the integrity of the business process.

Step 2: Generating the Component Library#

Once the recording is processed, Replay populates the Library. This is not just a collection of CSS; it is a full Design System of React components that mirror the functional requirements of the legacy system but with modern UX patterns.

typescript
// Example of a React component generated via Replay's mapping
import React from 'react';
import { useLegacyData } from './hooks/useLegacyData';

interface AccountBalanceProps {
  accountId: string;
  currency: 'USD' | 'EUR';
}

/**
 * Mapped from COBOL Screen: ACCT-SUMM-01
 * Logic: Handles real-time balance calculation and overage flags
 */
export const AccountBalance: React.FC<AccountBalanceProps> = ({ accountId, currency }) => {
  const { data, loading, error } = useLegacyData(accountId);

  if (loading) return <SkeletonLoader />;
  if (error) return <ErrorMessage message="Mainframe Connection Timeout" />;

  return (
    <div className="p-4 border rounded-lg shadow-sm bg-white">
      <h3 className="text-sm font-medium text-gray-500">Available Balance</h3>
      <p className="text-2xl font-bold">
        {currency === 'USD' ? '$' : '€'}
        {data.availableBalance.toLocaleString()}
      </p>
      {data.isOverdrawn && (
        <span className="text-red-600 text-xs font-semibold">
          Warning: Account Overdrawn
        </span>
      )}
    </div>
  );
};

Step 3: Mapping Complex Logic#

The hardest part of cobol react mapping lines is the conditional logic hidden in the mainframe. For example, a "Gold Member" might have a different transaction limit than a "Standard Member." Replay’s AI Automation Suite detects these variations during the recording phase by analyzing different user sessions and highlighting the conditional branches in the resulting React code.

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Architecting for Regulated Environments#

For Financial Services and Healthcare, "moving fast and breaking things" is not an option. Modernizing a system with 1M lines of logic requires a platform that respects SOC2, HIPAA, and On-Premise requirements.

Replay is built for these environments. The platform allows architects to modernize without sending sensitive production data to a public cloud. By utilizing Blueprints, teams can edit and refine the mapped logic in a secure, collaborative editor before a single line of code is pushed to the repository.

Visual Reverse Engineering is the process of using recorded UI interactions to reconstruct the underlying application architecture, data flows, and business rules into a modern tech stack.

From 18 Months to Weeks#

The traditional "Big Bang" rewrite fails because it attempts to solve a $3.6 trillion problem with manual labor. When you leverage Replay, you are not just "coding"; you are automating the discovery of your own business rules.

According to Replay's analysis, enterprises that use visual mapping see a 70% reduction in development time. This is because the "discovery" phase—usually the longest part of a legacy project—is handled during the recording of flows.

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Implementation Strategy: The "Flow-Based" Approach#

When mapping cobol react mapping lines, don't attempt to migrate the entire 1M lines at once. Instead, categorize your mainframe into "Flows."

1. •Read-Only Flows: Account balances, transaction history. (Easiest to map).
2. •Transactional Flows: Wire transfers, bill pay, profile updates. (Requires careful state mapping).
3. •Admin/Batch Flows: End-of-day processing, reporting. (Often stays in COBOL/Mainframe longest).

By using the Flows feature in Replay, you can visually map the architecture of these processes. This allows for a "Strangler Fig" pattern, where the React frontend slowly replaces legacy screens one flow at a time.

typescript
// Example of a mapped Flow handler in React
// This handles the transition from a 'Search' screen to a 'Details' screen
// as captured in a Replay recording.

import { useNavigate } from 'react-router-dom';

export const useAccountNavigation = () => {
  const navigate = useNavigate();

  const handleAccountSelect = (accountNumber: string) => {
    // Logic mapped from Legacy Screen 'SEARCH-RES-04'
    // The mainframe would typically perform a 'GO TO' or 'LINK' here.
    console.log(`Mapping transition for Account: ${accountNumber}`);
    navigate(`/accounts/${accountNumber}/overview`);
  };

  return { handleAccountSelect };
};

Read more about UI to code automation

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The Role of AI in Mapping 1M Lines#

Manual mapping of cobol react mapping lines is prone to human error. A developer might miss a specific

condition buried in a 5,000-line COBOL program. Replay’s AI Automation Suite acts as a second set of eyes, identifying edge cases that were captured during the recording phase but might have been overlooked by the developer.

This AI doesn't just "guess" the code; it uses the visual evidence of the recording to ensure the React component behaves exactly like the original terminal screen. This "ground truth" approach is why Replay can offer such significant time savings.

Why Visual Evidence Matters#

In regulated industries like Insurance and Government, you must be able to prove that the new system performs exactly like the old one. Visual Reverse Engineering provides an audit trail. You can point to a recording of the legacy system and compare it side-by-side with the new React component.

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Frequently Asked Questions#

How does Replay handle 1M lines of COBOL without access to the source code?#

Replay focuses on Visual Reverse Engineering. By recording the UI and the workflows (the "outputs" of the COBOL logic), Replay can reconstruct the functional requirements and state transitions into React components. You don't need perfect COBOL documentation because Replay documents the behavior of the system in real-time.

Is the code generated by Replay maintainable?#

Yes. Unlike "black box" migration tools that output unreadable machine-generated code, Replay generates a clean, documented React Design System and Component Library. The code follows modern best practices (TypeScript, functional components, hooks) and is designed to be owned and maintained by your internal engineering team.

Can Replay work with green-screen terminal emulators?#

Absolutely. Replay is designed to record any web-accessible or desktop-based UI, including terminal emulators used to access mainframe systems. As long as a user can perform the workflow, Replay can capture it and begin the cobol react mapping lines process.

How does this impact the $3.6 trillion technical debt?#

By reducing the cost and time of modernization by 70%, Replay makes it economically viable to tackle technical debt that was previously considered "too expensive to fix." It turns 18-month risks into 18-week wins.

Is Replay SOC2 and HIPAA compliant?#

Yes. Replay is built for enterprise and regulated environments. We offer SOC2 compliance, are HIPAA-ready, and provide On-Premise deployment options for organizations with strict data residency requirements.

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Conclusion: The Path to Modernization#

Mapping 1M lines of logic from COBOL to React is the "Final Boss" of enterprise architecture. The traditional methods of manual documentation and manual coding are failing the industry, leading to billions in wasted spend and stagnant innovation.

By shifting to a Visual Reverse Engineering mindset with Replay, organizations can finally break the cycle of failed rewrites. You can convert your legacy workflows into a modern React Design System in weeks, not years, saving 70% of your development time and ensuring your business logic is preserved for the next generation of banking.

Ready to modernize without rewriting? Book a pilot with Replay