Modernizing Core Banking: Bridging the Gap Between Mainframe Logic and React UIs

The "Big Bang" rewrite is the most expensive suicide mission in the enterprise. In the financial services sector, where core banking systems often rely on decades-old COBOL or PL/I logic running on IBM z/OS, the urge to "just start over" has led to the graveyard of 70% of legacy modernization projects. When you are dealing with $3.6 trillion in global technical debt, you cannot afford to guess.

The problem isn't the destination—everyone wants a cloud-native, React-based microservices architecture—the problem is the archaeology required to get there. Most legacy systems are "black boxes" where the original developers retired years ago, and 67% of these systems lack any meaningful documentation. You aren't just migrating code; you are performing an organ transplant on a patient whose anatomy you don't fully understand.

TL;DR: Modernizing core banking requires moving away from manual "code archaeology" toward visual reverse engineering, using real user workflows to generate documented React components and API contracts in days rather than years.

The High Cost of Manual Archaeology#

Traditional modernization follows a predictable, painful path: business analysts spend months interviewing users, developers spend more months digging through mainframe logs, and architects attempt to map green-screen terminal interactions to modern RESTful patterns.

On average, manually reverse-engineering a single complex legacy screen takes 40 hours. In a core banking environment with 500+ screens (think loan origination, KYC workflows, and ledger management), that’s 20,000 man-hours just to understand what you currently have. This is why the average enterprise rewrite timeline stretches to 18-24 months—and why most fail before the first deployment.

Comparison: Modernization Methodologies#

Breaking the Black Box with Replay#

The future of modernization isn't rewriting from scratch; it's understanding what you already have by using video as the source of truth. Replay changes the paradigm by recording real user workflows as they interact with the legacy system.

Instead of reading 30-year-old COBOL, Replay observes the inputs, outputs, and state changes. It then uses an AI Automation Suite to transform these recordings into modern, documented React components and standardized API contracts. This reduces the time per screen from 40 hours to just 4 hours—a 90% reduction in manual effort.

Step 1: Assessment and Recording#

We start by recording actual bankers performing their daily tasks. This captures the "hidden" business logic—the weird edge cases and workarounds that never made it into the official documentation. Replay's engine tracks every DOM change (or terminal emulator state change) and maps it to data flows.

Step 2: Visual Extraction#

Replay’s "Blueprints" editor takes these recordings and identifies UI patterns. It doesn't just take a screenshot; it identifies functional units. It extracts the underlying data structures and business rules inherent in the form validations and navigation flows.

Step 3: Component Generation#

From these Blueprints, Replay generates clean, type-safe React components. These aren't just "spaghetti code" exports; they are structured to fit into your modern Design System (Library).

typescript
// Example: Generated React component from a legacy Core Banking "Account Summary" screen
// Extracted via Replay Visual Reverse Engineering

import React, { useState, useEffect } from 'react';
import { Button, Card, Table, StatusBadge } from '@/components/ui-library';
import { useAccountData } from '@/hooks/api-contracts';

interface AccountProps {
  accountId: string;
  onTransactionSelect: (id: string) => void;
}

export const LegacyAccountSummary: React.FC<AccountProps> = ({ accountId, onTransactionSelect }) => {
  const { data, loading, error } = useAccountData(accountId);

  // Business logic preserved: Only show "High Risk" badge if balance < $1000 
  // and account has been active for less than 90 days.
  const isHighRisk = data?.balance < 1000 && data?.accountAgeDays < 90;

  if (loading) return <SkeletonLoader />;

  return (
    <Card title={`Account: ${data?.accountNumber}`}>
      <div className="flex justify-between items-center mb-4">
        <span className="text-2xl font-bold">${data?.balance.toLocaleString()}</span>
        {isHighRisk && <StatusBadge variant="danger">High Risk</StatusBadge>}
      </div>
      
      <Table 
        data={data?.recentTransactions}
        columns={[
          { header: 'Date', key: 'date' },
          { header: 'Description', key: 'desc' },
          { header: 'Amount', key: 'amount', cell: (val) => `$${val}` }
        ]}
        onRowClick={(row) => onTransactionSelect(row.id)}
      />
      
      <div className="mt-4 flex gap-2">
        <Button onClick={() => window.print()}>Export Ledger</Button>
        <Button variant="primary">Initiate Transfer</Button>
      </div>
    </Card>
  );
};

💰 ROI Insight: By automating the extraction of the UI and the logic mapping, Replay users report an average of 70% time savings on the total project lifecycle. For a mid-sized bank, this represents millions of dollars in reclaimed developer productivity.

Bridging the Logic Gap: API Contracts#

The hardest part of modernizing core banking isn't the UI—it's the middle tier. How do you talk to a mainframe from a modern web app? Usually, this involves a messy layer of "middleware" that no one wants to touch.

Replay automates the creation of API contracts by observing the data exchange during user sessions. It generates OpenAPI (Swagger) specifications that represent the actual data requirements of the front-end, rather than the theoretical structures found in outdated DB2 schemas.

yaml
# Generated API Contract for Legacy Ledger Access
openapi: 3.0.0
info:
  title: Core Banking Ledger API
  version: 1.0.0
paths:
  /accounts/{accountId}/transactions:
    get:
      summary: Retrieves transaction history based on legacy screen workflow
      parameters:
        - name: accountId
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/TransactionHistory'
components:
  schemas:
    TransactionHistory:
      type: object
      properties:
        balance:
          type: number
        accountNumber:
          type: string
        recentTransactions:
          type: array
          items:
            type: object
            properties:
              id: { type: string }
              date: { type: string, format: date }
              amount: { type: number }

Security and Compliance in Regulated Environments#

In banking, "cloud-native" often comes with a "not on my watch" from the CISO. Modernizing core systems requires more than just code; it requires a platform that respects the regulatory constraints of Financial Services, Healthcare, and Government.

⚠️ Warning: Never use "black box" AI tools that require sending your sensitive financial data or source code to a third-party cloud.

Replay is built for these environments:

• •SOC2 & HIPAA Ready: Compliance is baked into the platform architecture.
• •On-Premise Available: For Tier-1 banks, Replay can be deployed entirely within your VPC or air-gapped data center.
• •Technical Debt Audit: Every component generated comes with a full audit trail, showing exactly which legacy workflow it was derived from.

The Replay Workflow: From Recording to React#

Step 1: Capture the Truth#

Deploy the Replay recorder to a subset of power users. As they navigate the legacy core banking system, Replay captures the visual state, the network calls, and the user intent. This creates a "Digital Twin" of your legacy application.

Step 2: Mapping the Architecture#

Use the Flows feature to visualize the application's architecture. Most legacy systems have circular dependencies and "spaghetti" navigation. Replay flattens this, showing you the logical flow of the business process, not just the technical debt of the code.

Step 3: Design System Integration#

Import your modern Figma components into the Library. Replay’s AI Suite maps legacy UI elements (like a green-screen data grid) to your modern React Table components. This ensures that the output isn't just "new code," but code that follows your specific enterprise standards.

Step 4: E2E Test Generation#

One of the biggest risks in modernization is regression. Replay generates Playwright or Cypress E2E tests based on the original recordings. This ensures that the new React UI behaves exactly like the legacy system it replaces.

💡 Pro Tip: Use the generated E2E tests as a "validation suite" during User Acceptance Testing (UAT). If the React app passes the tests generated from the legacy recording, you have mathematical proof of functional parity.

Challenging the "Manual First" Status Quo#

The industry has been conditioned to believe that modernization must be slow to be safe. We challenge that. Manual documentation is inherently flawed because it relies on human memory and interpretation.

By using Replay, you are moving from subjective interpretation to objective extraction. You aren't asking a developer what they think the COBOL logic does; you are observing what the system actually does and codifying it.

Why Visual Reverse Engineering Wins:#

• •Preserves Tribal Knowledge: Captures the "how" of the workflow, even if the "why" was lost 20 years ago.
• •Eliminates Documentation Debt: The platform generates the documentation as a byproduct of the extraction.
• •Reduces Developer Burnout: No one wants to spend two years manually porting legacy forms. Replay lets developers focus on building new features rather than excavating old ones.

Frequently Asked Questions#

How long does core banking extraction take?#

While a full core banking replacement is a massive undertaking, individual modules (like "Account Opening" or "Credit Scoring") can be extracted and documented in 2-4 weeks. This is a significant improvement over the 6-9 months typically required for manual discovery.

What about business logic preservation?#

Replay captures the observable business logic—validations, state changes, and data transformations. While the most complex backend calculations remain on the mainframe (accessed via the generated API contracts), the UI logic and orchestration are perfectly preserved and migrated to the React layer.

Does Replay work with terminal emulators (Green Screens)?#

Yes. Replay is designed to work with web-based legacy systems, Citrix-delivered apps, and terminal emulators. If a user can see it and interact with it, Replay can reverse-engineer it.

How does this handle technical debt?#

Replay includes a Technical Debt Audit feature. During extraction, it identifies redundant workflows, unused fields, and circular navigation paths. This allows you to "clean as you go," ensuring that you don't just migrate your legacy problems to a modern framework.

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