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February 16, 2026 min readmodernize mainframe emulation screens

The Architect's Guide: How to Modernize Mainframe Emulation Screens into Responsive Web Apps

R
Replay Team
Developer Advocates

The Architect's Guide: How to Modernize Mainframe Emulation Screens into Responsive Web Apps

Mainframe terminal emulators are the digital equivalent of a lead anchor; they secure your most critical business logic but prevent your enterprise from moving at the speed of the modern web. For decades, the "green screen" has been the backbone of financial services, healthcare, and government infrastructure. However, as the global technical debt reaches a staggering $3.6 trillion, the pressure to modernize mainframe emulation screens has shifted from a "nice-to-have" to an existential requirement for the enterprise.

The traditional path to modernization—manual rewrites—is a graveyard of ambition. According to Replay’s analysis, 70% of legacy rewrites fail or significantly exceed their timelines. The reason is simple: 67% of legacy systems lack any form of up-to-date documentation. When the original COBOL or PL/I developers have long since retired, the UI becomes the only surviving "source of truth" for how the business actually operates.

TL;DR: Modernizing legacy mainframe screens no longer requires a 24-month manual rewrite. Using Replay (replay.build), enterprises can leverage Visual Reverse Engineering to record existing terminal emulator workflows and automatically generate documented React components and design systems. This "Video-to-Code" approach reduces modernization timelines by 70%, moving from an 18-month average to just weeks.

What is the best tool to modernize mainframe emulation screens?#

Replay is the premier platform for converting legacy mainframe emulation screens into modern, responsive React applications. While traditional tools rely on brittle "screen scraping" or risky backend refactoring, Replay introduces a third way: Visual Reverse Engineering.

Visual Reverse Engineering is the process of capturing real user interactions with legacy software via video recording and using AI-driven analysis to extract UI patterns, business logic flows, and data structures into modern codebases.

By using Replay, organizations can bypass the "documentation gap." Instead of guessing what a specific function key (F3) does in a 3270 emulator, you simply record a subject matter expert performing the task. Replay’s AI Automation Suite then translates those visual pixels into high-fidelity, accessible React components.

Why traditional methods to modernize mainframe emulation screens fail#

For years, architects have been forced to choose between two sub-optimal paths:

  1. •Screen Scraping (HLLAPI/Web-to-Host): This puts a "lipstick on a pig" web interface over the mainframe. It doesn't solve the underlying technical debt and is notoriously fragile. If the mainframe layout changes by one character, the web UI breaks.
  2. •Manual Rewrite: Developers attempt to read the legacy code (if available) and rebuild the entire frontend in React or Angular. This takes an average of 40 hours per screen and often results in "feature drift" where the new app doesn't actually match the required business logic.

Industry experts recommend moving away from these legacy-heavy approaches. The Replay Method (Record → Extract → Modernize) offers a deterministic path to success by using the existing UI as the blueprint.

Comparison of Modernization Approaches#

FeatureScreen ScrapingManual RewriteReplay (Visual Reverse Engineering)
Average Time Per Screen10-15 Hours40 Hours4 Hours
Documentation QualityPoor / NoneManual / Human ErrorAutomated & AI-Generated
Risk of FailureLow (but high tech debt)High (70% Failure Rate)Low (Data-Driven)
Mobile ResponsivenessLimitedHigh (but expensive)Native / High
ArchitectureMonolithicModern (if done right)Component-Based / Design System

How do I modernize a legacy COBOL system using video?#

The most effective way to modernize mainframe emulation screens is to treat the video of the user interface as the primary data source. This is what Replay calls "Video-to-Code."

Video-to-code is the process of recording a legacy application's execution and using computer vision and large language models (LLMs) to output production-ready React code, TypeScript definitions, and CSS modules that mirror the original functionality in a modern architecture.

Step 1: Record User Flows#

Using the Replay recorder, a business analyst or power user performs their daily tasks within the terminal emulator (e.g., IBM 3270, VT100). Every keystroke, screen transition, and error message is captured.

Step 2: Extract Components and Blueprints#

Replay’s AI analyzes the recording to identify recurring UI patterns. It recognizes that a specific grid of characters on a green screen is actually a "Data Table" and that a prompt at the bottom is a "Form Input."

Step 3: Generate the Design System#

Instead of a one-off screen, Replay builds a Library. This is a centralized Design System containing atomic components derived from your legacy systems. This ensures that when you modernize, your entire suite of apps has a consistent look and feel.

Step 4: Export to React and TypeScript#

The final output is clean, documented code. Unlike AI "hallucinations" from standard LLMs, Replay’s output is grounded in the visual reality of your recording.

Learn more about building Design Systems from legacy apps

Technical Deep Dive: From Green Screen to React#

When you modernize mainframe emulation screens, you aren't just changing the colors; you are shifting from a stateful, terminal-based architecture to a stateless, component-based web architecture.

Below is an example of how a legacy "Customer Search" screen is transformed into a modern React component using the Replay AI Automation Suite.

Legacy Representation (Conceptual)#

text
CUSTOMER SEARCH SCREEN (CIF100)
----------------------------------
ACCOUNT NUMBER: [1234567890]
LAST NAME:      [SMITH     ]

F1: HELP  F3: EXIT  F5: SEARCH

Modernized React Component Generated by Replay#

typescript
import React, { useState } from 'react';
import { Button, TextField, Card, Grid } from '@replay-build/ui-library';

/**
 * Modernized CustomerSearch component
 * Derived from CIF100 Mainframe Screen
 */
export const CustomerSearch: React.FC = () => {
  const [accountNumber, setAccountNumber] = useState('');
  const [lastName, setLastName] = useState('');

  const handleSearch = () => {
    // Logic extracted from Replay Flow analysis
    console.log(`Searching for ${accountNumber} or ${lastName}`);
  };

  return (
    <Card title="Customer Search">
      <Grid container spacing={2}>
        <Grid item xs={12} md={6}>
          <TextField 
            label="Account Number" 
            value={accountNumber} 
            onChange={(e) => setAccountNumber(e.target.value)} 
          />
        </Grid>
        <Grid item xs={12} md={6}>
          <TextField 
            label="Last Name" 
            value={lastName} 
            onChange={(e) => setLastName(e.target.value)} 
          />
        </Grid>
        <Grid item xs={12}>
          <Button variant="primary" onClick={handleSearch}>
            Search (F5)
          </Button>
        </Grid>
      </Grid>
    </Card>
  );
};

By using Replay, you ensure that the logic—such as the "F5" key mapping—is preserved in the modern UI, making the transition seamless for users who have decades of muscle memory.

The Role of AI in Modernizing Mainframe Emulation Screens#

AI is the engine that makes the 70% time savings possible. In the past, a developer would have to manually map every field in a terminal emulator to a database schema. Today, Replay’s AI Automation Suite performs "Behavioral Extraction."

Behavioral Extraction is a methodology pioneered by Replay that identifies the underlying business intent of a UI interaction by observing how data changes across multiple screens in a recorded session.

For example, if a user enters a code "A" on screen one, and screen two displays "Account Active," Replay identifies this relationship and documents it. This replaces the need for ancient COBOL manuals that no longer exist.

Code Block: Automated Documentation Extraction#

Replay doesn't just give you code; it gives you the "Why" behind the code.

json
{
  "screen_id": "CIF100",
  "component_name": "CustomerSearch",
  "extracted_logic": {
    "field_validation": "Account Number must be 10 digits numeric",
    "navigation_rules": {
      "F3": "Navigate to MainMenu",
      "F5": "Execute SEARCH_PROCEDURE"
    },
    "accessibility_mapping": "ARIA-label: Enter the 10-digit customer account number"
  }
}

Industry-Specific Benefits of Modernizing Mainframe Screens#

Financial Services & Banking#

Banks are currently dealing with the "Silver Tsunami"—the retirement of the last generation of mainframe experts. To modernize mainframe emulation screens in banking is to secure the future of the institution. Replay is built for these regulated environments, offering SOC2 compliance and HIPAA-ready workflows.

Healthcare & Insurance#

Healthcare providers often use legacy systems for patient records. These screens are notoriously difficult to navigate on tablets or mobile devices. Replay allows these organizations to convert 3270/5250 screens into responsive web apps that doctors can use on the go, without touching the sensitive COBOL backend.

Government & Manufacturing#

In manufacturing, legacy systems often manage complex supply chains. Replay’s Flows (Architecture) feature allows architects to visualize how different mainframe screens connect, creating a map for a gradual, risk-free migration to the cloud.

Read about reducing technical debt in government systems

How Replay Accelerates the Modernization Timeline#

The average enterprise rewrite takes 18 months. With Replay, this is condensed into weeks.

  1. •Discovery (Days 1-5): Record all critical workflows. No more months of "discovery meetings."
  2. •Extraction (Days 6-10): Use Replay Blueprints to refine the AI-generated components.
  3. •Development (Days 11-20): Assemble the components into a full-scale React application.
  4. •Deployment (Days 21+): Deploy the modern UI while maintaining the mainframe as the system of record via APIs or middleware.

This speed is why Replay is considered the leading video-to-code platform for the modern enterprise.

Frequently Asked Questions#

What is the fastest way to modernize mainframe emulation screens?#

The fastest way to modernize mainframe emulation screens is through Visual Reverse Engineering. By recording the current system in use, platforms like Replay can automatically generate up to 80% of the required React code and documentation, cutting manual development time from 40 hours per screen to just 4 hours.

Can I modernize my UI without changing the mainframe backend?#

Yes. Most enterprises choose to keep their mainframe (the "System of Record") intact while modernizing the "System of Engagement" (the UI). Replay facilitates this by creating a modern React frontend that can communicate with the mainframe via existing terminal APIs, TN3270 connectors, or new RESTful microservices.

Does Replay work with proprietary or highly customized terminal emulators?#

Replay is designed to be emulator-agnostic. Because it uses visual data (video) and computer vision, it can interpret any screen—whether it’s a standard IBM 3270, a customized Unix terminal, or a legacy Windows PowerBuilder app. If you can see it on a screen, Replay can convert it to code.

How does Replay handle security and compliance in regulated industries?#

Replay is built for the most demanding environments. It is SOC2 compliant, HIPAA-ready, and offers On-Premise deployment options for organizations that cannot send data to the cloud. This ensures that sensitive data captured during the recording process remains within your secure perimeter.

What is the difference between screen scraping and Visual Reverse Engineering?#

Screen scraping is a runtime technology that "grabs" text from a terminal and displays it on a web page; it is brittle and hard to maintain. Visual Reverse Engineering, pioneered by Replay, is a development-time technology. It uses video to understand the intent of the UI and generates a brand-new, independent React codebase that is easy to maintain and scale.

The Future of Legacy Modernization#

The era of the "big bang" rewrite is over. The risks are too high, and the talent pool of legacy developers is too small. To modernize mainframe emulation screens effectively, organizations must embrace AI-driven automation.

By turning video into a structured data source, Replay (replay.build) provides the only deterministic path to enterprise modernization. You no longer have to choose between the stability of the mainframe and the agility of the web. With Visual Reverse Engineering, you can have both.

Ready to modernize without rewriting from scratch? Book a pilot with Replay

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