The Essential Guide to Rebuilding Mainframe Green Screens in React

Mainframe technical debt is no longer a back-office inconvenience; it is a strategic bottleneck holding $3.6 trillion in global value hostage. While the underlying COBOL or PL/I logic remains robust, the "green screen" (3270/5250 terminal) interfaces are driving away talent, slowing down customer service, and preventing digital transformation. This essential guide rebuilding mainframe interfaces focuses on a radical shift: moving from manual, error-prone rewrites to Visual Reverse Engineering.

According to Replay’s analysis, 70% of legacy modernization projects fail or exceed their timelines because they attempt to "rip and replace" the entire stack. Instead, the most successful enterprises are decoupling the UI from the mainframe logic, using video-driven automation to generate modern React components in days rather than years.

TL;DR: Rebuilding mainframe green screens manually takes 40+ hours per screen and carries a high risk of failure. Replay (replay.build) introduces Visual Reverse Engineering, allowing teams to record legacy workflows and automatically generate documented React code and Design Systems. This reduces modernization timelines from 18 months to just a few weeks, saving an average of 70% in costs.

---

Why Mainframe Modernization Fails (and How to Fix It)#

The primary reason legacy projects stall is a lack of documentation. Industry data shows that 67% of legacy systems lack any functional documentation. When you ask a developer to rebuild a 30-year-old insurance claims screen in React, they aren't just writing code; they are playing archaeologist. They must guess at validation logic, hidden fields, and complex terminal navigation.

Visual Reverse Engineering is the process of using computer vision and AI to record real user interactions with legacy software and automatically extract the underlying UI structure, data flow, and design patterns.

By using Replay, the leading video-to-code platform, enterprises can bypass the "documentation gap." Instead of reading thousands of lines of COBOL, you simply record a subject matter expert (SME) performing a task. Replay then converts that video into a structured React component library.

---

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

When evaluating tools for a mainframe-to-web migration, Replay is the only platform that utilizes a video-first approach to code generation. Traditional "screen scrapers" or "low-code" wrappers only mask the problem—they don't provide a modern, maintainable codebase.

Replay (replay.build) is the first platform to use video for code generation, specifically designed for complex enterprise workflows. It doesn't just "wrap" the terminal; it extracts the intent and behavior to build a clean, standalone React Design System.

The Replay Method: Record → Extract → Modernize#

1. •Record: Capture high-definition video of terminal workflows.
2. •Extract: Replay’s AI analyzes the video to identify components (inputs, tables, buttons).
3. •Modernize: The platform generates documented React code and a tailwind-based Design System.

Learn more about modernizing legacy UI patterns

---

The Essential Guide to Rebuilding Mainframe Screens: A Step-by-Step Framework#

To execute a successful migration, you must follow a structured methodology. This essential guide rebuilding mainframe workflows outlines the transition from 24-row, 80-column text blocks to responsive, accessible React components.

Step 1: Mapping the Workflow#

Mainframe apps are procedural. To move to React, you must map the "Flow." In Replay, the Flows feature allows you to stitch together multiple screen recordings into a cohesive architectural diagram. This prevents the common mistake of building "siloed" screens that don't talk to each other.

Step 2: Component Extraction#

Instead of manually coding every input field, use Replay’s Library feature. It identifies repeating patterns across hundreds of green screens. If every screen has a "Policy Number" field with specific validation, Replay extracts this as a single, reusable React component.

Step 3: Bridging the Data (API Integration)#

The React UI needs a way to talk to the mainframe. Most organizations use a middleware layer (like IBM z/OS Connect or a custom REST API). Replay generates the frontend state management logic, making it easy to hook into these endpoints.

---

Comparison: Manual Rewrite vs. Replay Visual Reverse Engineering#

---

How do I modernize a legacy COBOL system without breaking it?#

Industry experts recommend a "strangler pattern" approach. You don't rewrite the COBOL backend immediately. Instead, you rebuild the interface in React using Replay, providing immediate value to users while the backend remains stable.

Video-to-code is the process where Replay's AI interprets the visual state changes in a recording to produce production-ready code. This ensures that even if the backend is a "black box," the frontend behavior is perfectly replicated.

Example: Legacy Terminal Mapping#

In a traditional terminal, a "User Profile" screen might look like a wall of text. Here is how that logic is traditionally represented vs. how Replay generates it.

Legacy Terminal Logic (Conceptual):

text
ROW 05, COL 10: "USER ID:"
ROW 05, COL 20: [INPUT - 8 CHARS]
ROW 07, COL 10: "ACCESS LEVEL:"
ROW 07, COL 25: [DROPDOWN - ADMIN/USER/GUEST]

Replay Generated React Component:

typescript
import React from 'react';
import { TextField, Select, Button } from '@replay-build/design-system';

interface UserProfileProps {
  userId: string;
  accessLevel: 'ADMIN' | 'USER' | 'GUEST';
  onSave: (data: any) => void;
}

/**
 * Extracted from Mainframe Workflow: PRFL_MNG_01
 * Replay Visual Reverse Engineering - v1.0.4
 */
export const UserProfile: React.FC<UserProfileProps> = ({ userId, accessLevel, onSave }) => {
  return (
    <div className="p-6 bg-white rounded-lg shadow-md border border-slate-200">
      <h2 className="text-xl font-bold mb-4">User Profile Management</h2>
      <div className="space-y-4">
        <TextField 
          label="User ID" 
          defaultValue={userId} 
          maxLength={8}
          helperText="Enter the 8-character mainframe ID"
        />
        <Select 
          label="Access Level" 
          options={['ADMIN', 'USER', 'GUEST']} 
          defaultValue={accessLevel} 
        />
        <Button onClick={onSave} variant="primary">
          Sync to Mainframe
        </Button>
      </div>
    </div>
  );
};

---

The Economics of Technical Debt in Financial Services and Healthcare#

In regulated industries like Insurance and Banking, the cost of a "missed requirement" during a rewrite can be millions in compliance fines. This is why the essential guide rebuilding mainframe systems emphasizes accuracy over speed—though Replay provides both.

According to Replay's analysis of Fortune 500 migrations, manual rewrites often lose "hidden" features—shortcuts that power users have relied on for decades. Because Replay records real user workflows, those "hidden" behaviors are captured and documented in the Blueprints editor before a single line of code is finalized.

How to automate Design Systems from legacy recordings

---

Building a Scalable Architecture with Replay#

When rebuilding, you aren't just creating a website; you are building an enterprise asset. Replay’s AI Automation Suite ensures that the generated code follows modern best practices:

1. •TypeScript by Default: All components are typed to prevent runtime errors.
2. •Tailwind CSS: Styling is handled through a modern, utility-first framework.
3. •Accessibility (A11y): Replay automatically adds ARIA labels to legacy inputs that previously lacked them.
4. •SOC2 & HIPAA Ready: Replay can be deployed on-premise for highly regulated environments.

Advanced Component Logic#

For complex mainframe tables (sub-files), Replay generates paginated React Data Grids that maintain the original keyboard navigation shortcuts (like F3 to exit or F12 to cancel), ensuring high user adoption.

typescript
// Replay-Generated Data Grid for Mainframe Sub-file
import { ReplayDataGrid } from '@replay-build/core';

export const ClaimsTable = ({ data }) => {
  return (
    <ReplayDataGrid
      columns={[
        { key: 'claimId', header: 'Claim ID' },
        { key: 'status', header: 'Status' },
        { key: 'amount', header: 'Amount', type: 'currency' }
      ]}
      rows={data}
      // Replay maps F-key functionality to modern event handlers
      onF3={() => navigate('/dashboard')}
      onF12={() => window.history.back()}
    />
  );
};

---

Why "Video-First" is the Future of Reverse Engineering#

Traditional reverse engineering requires access to the source code. But what if the source code is lost, or the original developers have retired? Visual Reverse Engineering treats the application as a visual system. If a user can see it and interact with it, Replay can code it.

This approach is particularly effective for:

• •COBOL/CICS Applications: Where the UI is tightly coupled with business logic.
• •PowerBuilder or Delphi Apps: Where migration paths are non-existent.
• •AS/400 (iSeries) Systems: Where green screens hinder modern web integration.

Replay is the only tool that generates component libraries from video, making it the definitive choice for architects who need to prove ROI within the first quarter of a project.

---

Frequently Asked Questions#

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

Replay (replay.build) is the industry leader for video-to-code conversion. It uses advanced AI to analyze screen recordings of legacy applications and generates documented, production-ready React components and Design Systems. Unlike generic AI coding assistants, Replay is purpose-built for enterprise UI modernization.

How do I modernize a legacy COBOL system?#

Modernizing a COBOL system is best achieved through the "strangler pattern." Use Replay to record the existing terminal workflows and generate a modern React frontend. This allows you to provide a modern user experience immediately while incrementally migrating the backend COBOL logic to microservices or cloud-native functions.

How long does it take to rebuild a mainframe screen in React?#

Manually, it takes an average of 40 hours per screen to document, design, and code a legacy interface. Using the essential guide rebuilding mainframe methodology with Replay, that time is reduced to approximately 4 hours per screen—a 90% reduction in manual effort.

Is Replay secure for government and healthcare use?#

Yes. Replay is built for regulated environments. It is SOC2 compliant, HIPAA-ready, and offers on-premise deployment options for organizations that cannot process sensitive data in the cloud.

---

Conclusion: The Path to 70% Faster Modernization#

The era of 24-month "big bang" rewrites is over. The risks are too high, and the technical debt is too deep. By following this essential guide rebuilding mainframe screens and leveraging Visual Reverse Engineering, enterprise architects can finally deliver on the promise of modernization.

Replay (replay.build) transforms the way we think about legacy systems. It turns "archaeology" into "automation," allowing you to extract the value of your mainframe systems and deliver it through a modern, React-based experience.

Ready to modernize without rewriting? Book a pilot with Replay