Mainframe UI Modernization: Why Your 24-Month Rewrite is Already Failing

The average enterprise rewrite takes 18 to 24 months, yet 70% of these projects either fail or significantly exceed their original timelines and budgets. In the financial services sector, this isn't just a statistical nuance—it’s a systemic risk. We are currently sitting on a global technical debt mountain worth $3.6 trillion, much of it locked inside COBOL-based mainframe systems that provide the backbone for global ledgering, clearing, and retail banking.

The "Big Bang" rewrite is a relic of the past. The risk of losing decades of embedded business logic—logic that exists only in the code because the original architects retired years ago—is too high. Today, the most successful Enterprise Architects are moving away from "archaeology-based" manual rewrites and toward automated visual reverse engineering.

TL;DR: Mainframe UI modernization fails when treated as a manual rewrite; using visual reverse engineering to extract logic into React components reduces modernization timelines by 70%.

The High Cost of Manual Archaeology#

When a Tier-1 bank decides to modernize a legacy lending portal, the first 6 months are typically spent in "discovery." This is a polite term for developers staring at 3270 terminal emulators (green screens) and trying to guess what happens when a user hits

The data is sobering: 67% of legacy systems lack up-to-date documentation. This creates a "Black Box" problem. If you don't know exactly what the system does, you cannot replace it. Manual extraction of a single complex mainframe screen into a modern React equivalent takes an average of 40 hours per screen. For a banking application with 200+ screens, that’s 8,000 man-hours just for the UI layer, before a single API is integrated.

Bridging the Gap: From CICS to React#

Mainframe UI modernization isn't just about making things look "pretty." It’s about state management. A legacy CICS (Customer Information Control System) screen manages state across multiple "maps." In a modern architecture, we need to translate these procedural screen transitions into a declarative, component-based React architecture.

The Problem with Traditional "Screen Scraping"#

Traditional screen scraping tools simply wrap the terminal protocol (TN3270) in a web-based shell. This is a "lipstick on a pig" approach. It doesn't solve the technical debt; it just moves it to a different browser tab. You still have no API contracts, no E2E tests, and no reusable design system.

The Replay Approach: Visual Reverse Engineering#

Instead of reading COBOL source code to understand the UI, Replay records real user workflows. It captures the interaction, the data flow, and the state changes. It then uses AI to generate documented React components and TypeScript interfaces that mirror the legacy behavior but utilize modern best practices.

💰 ROI Insight: Manual screen extraction takes ~40 hours. Replay reduces this to 4 hours per screen by automating the generation of the component scaffold and business logic mapping.

Implementation: Mapping Legacy State to Modern Props#

When extracting a legacy banking form—such as a commercial loan application—the challenge is preserving the validation logic that was previously hardcoded into the mainframe map.

Below is an example of how a captured mainframe interaction is transformed into a clean, type-safe React component using the Replay extraction engine.

typescript
// Generated by Replay Visual Reverse Engineering
// Source: LN_APP_042 (Commercial Loan Entry Screen)

import React, { useState, useEffect } from 'react';
import { Button, TextField, Alert } from '@enterprise-ds/core';

interface LoanApplicationProps {
  initialData?: Record<string, any>;
  onTransition: (nextScreen: string, payload: any) => void;
}

/**
 * @description Migrated from Legacy Screen LN_APP_042. 
 * Preserves PF-key logic and field-level validation rules.
 */
export const CommercialLoanForm: React.FC<LoanApplicationProps> = ({ 
  initialData, 
  onTransition 
}) => {
  const [formData, setFormData] = useState(initialData || {});
  const [errors, setErrors] = useState<string[]>([]);

  // Mapping legacy PF3 (Exit) and PF12 (Submit)
  const handleKeyDown = (e: React.KeyboardEvent) => {
    if (e.key === 'F3') onTransition('MENU_MAIN', formData);
    if (e.key === 'F12') handleSubmit();
  };

  const handleSubmit = async () => {
    // Logic extracted from mainframe field validation rules
    if (!formData.accountNumber || formData.accountNumber.length < 10) {
      setErrors(['Invalid Account Number: Must be 10 digits (Legacy Rule: ERR-042)']);
      return;
    }
    
    onTransition('LN_CONFIRM', formData);
  };

  return (
    <div className="p-6 bg-white rounded-lg shadow" onKeyDown={handleKeyDown}>
      <h2 className="text-xl font-bold mb-4">Commercial Loan Application</h2>
      {errors.map(err => <Alert type="error" message={err} key={err} />)}
      
      <TextField
        label="Account Number (FLD-001)"
        value={formData.accountNumber}
        onChange={(val) => setFormData({...formData, accountNumber: val})}
        maxLength={10}
      />
      
      <div className="mt-6 flex gap-4">
        <Button onClick={() => onTransition('MENU_MAIN', {})} variant="secondary">
          [F3] Cancel
        </Button>
        <Button onClick={handleSubmit} variant="primary">
          [F12] Process Application
        </Button>
      </div>
    </div>
  );
};

💡 Pro Tip: When modernizing banking UIs, always preserve the legacy field IDs (e.g.,

text

FLD-001

) in your code comments or data attributes. This makes it significantly easier for mainframe developers and modern frontend engineers to communicate during the integration phase.

The 5-Step Modernization Workflow#

To move from a "Black Box" mainframe to a documented React codebase, follow this structured methodology used by Replay for enterprise-grade deployments.

Step 1: Workflow Recording#

Instead of reading code, record a subject matter expert (SME) performing the actual task in the legacy system. Replay captures the DOM (if web-based) or the terminal stream, along with the underlying network calls.

Step 2: Component Extraction#

Replay’s AI Automation Suite analyzes the recording to identify patterns. It recognizes that a specific grid on the green screen is actually a "Transaction History" table. It then generates the React component using your organization's Design System (via the Replay Library feature).

Step 3: API Contract Generation#

Mainframes rarely have clean REST APIs. They often use MQ, SOAP, or direct CICS calls. Replay generates the API contract (OpenAPI/Swagger) based on the data captured during the recording, providing a blueprint for the backend team to build the "Strangler" facade.

Step 4: Technical Debt Audit#

The system automatically flags redundant workflows. If three different screens are used to perform the same "Address Update" function, Replay identifies this duplication, allowing you to consolidate three legacy screens into one modern React component.

Step 5: E2E Test Parity#

One of the biggest risks in banking is regression. Replay generates Playwright or Cypress E2E tests based on the recorded legacy workflow. You can run these against the new React UI to ensure the business outcome is identical to the legacy system.

⚠️ Warning: Do not attempt to "improve" the business logic during the first phase of extraction. Focus on functional parity first. Once you have a documented React codebase, you can iterate on the UX.

Solving the Security and Compliance Gap#

For Financial Services and Healthcare, "cloud-only" is often a non-starter for core systems. Mainframe UI modernization involves sensitive PII (Personally Identifiable Information) and financial records.

Replay is built for these regulated environments:

• •SOC2 Type II & HIPAA Ready: Compliance is baked into the extraction process.
• •On-Premise Deployment: For Tier-1 banks, Replay can be deployed entirely within your VPC or on-premise data center.
• •Data Masking: Sensitive data captured during recording is automatically masked before it reaches the AI extraction engine.

Architecting for the Future: The "Flows" Model#

In a complex banking environment, a single user action might span multiple legacy systems (e.g., a mainframe core, a mid-range AS/400, and a modern microservice).

The Replay Flows feature allows architects to visualize these cross-system journeys. By recording the end-to-end process, Replay builds a visual architecture map. This moves the organization from "tribal knowledge" to a "documented codebase" where every screen transition is mapped and every API call is accounted for.

typescript
// Example: Generated API Contract for a Legacy Mainframe Endpoint
// Path: /api/v1/ledger/balance-inquiry

/**
 * @summary Extracted from Mainframe Transaction 'BAL1'
 * @description Retrieves current balance. Requires CICS Region: PROD01
 */
export interface BalanceInquiryRequest {
  account_id: string; // Legacy Field: ACCT-IN
  branch_code: string; // Legacy Field: BR-CD
}

export interface BalanceInquiryResponse {
  available_balance: number; // Legacy Field: AV-BAL
  pending_transactions: number; // Legacy Field: PND-TX
  currency: string; // Default: 'USD'
}

Frequently Asked Questions#

How does Replay handle complex business logic hidden in COBOL?#

Replay doesn't just look at the UI; it looks at the behavior of the system. By capturing the inputs and the resulting outputs across multiple scenarios, Replay can generate documentation and test cases that describe the business logic. While it doesn't "rewrite" the COBOL, it provides the "Black Box" specifications needed to rebuild that logic in a modern language if desired.

What is the average time savings compared to manual modernization?#

Our partners report an average of 70% time savings. Specifically, the transition from discovery to a functional React prototype moves from months to days. A typical enterprise screen that takes 40 hours to manually document, design, and code can be processed by Replay in under 4 hours.

Can Replay integrate with our existing Design System?#

Yes. The Replay Library feature allows you to upload your existing React component library. When Replay extracts a legacy UI, it maps the legacy elements (buttons, inputs, tables) directly to your specific design system components, ensuring the modernized UI is on-brand from day one.

Does this replace the need for developers?#

No. Replay is a "Force Multiplier" for Enterprise Architects and Senior Developers. It automates the "grunt work" of reverse engineering and boilerplate generation, allowing your high-cost engineering talent to focus on complex integration, security, and performance optimization.

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