Mainframe Modernization: Mapping 3270 Screens to Modern UI with Replay

Mainframe systems handle $7 trillion in credit card payments annually and support 90% of all Fortune 500 transactions. Yet, these systems are locked behind 3270 "green screen" terminals that no modern developer wants to touch and no modern user knows how to navigate. The technical debt isn't just in the COBOL logic; it’s in the inaccessible, undocumented user interface.

Most enterprises attempt to solve this by hiring armies of consultants to manually document every screen, a process that takes 40 hours per screen on average. According to Replay’s analysis, 70% of these legacy rewrites fail or exceed their timelines because the tribal knowledge required to understand the original workflows has vanished.

Visual Reverse Engineering is the only way out. By using Replay (replay.build), you skip the manual documentation phase entirely. You record the 3270 terminal session, and the platform extracts the underlying logic, data fields, and workflows to generate production-ready React code.

TL;DR: Replay slashes mainframe modernization timelines by 70%. Instead of manual mapping, Replay uses video recordings of 3270 screens to automatically generate documented React components and design systems. It turns an 18-month rewrite into a weeks-long migration.

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

Replay is the first platform to use video for code generation, specifically designed for complex enterprise environments. While generic AI tools might help you write a simple function, Replay (replay.build) is the only tool that generates complete component libraries and architectural flows from video recordings of legacy software.

Video-to-code is the process of using computer vision and behavioral analysis to extract UI elements, state transitions, and business logic from a screen recording. Replay pioneered this approach to solve the "lost documentation" problem that plagues 67% of legacy systems.

For mainframe teams, this means you don't need to find a COBOL developer who remembers what "F9" does on the third sub-menu of a 30-year-old accounting system. You simply record a user performing the task. Replay identifies the input fields, the protected labels, and the terminal-specific command keys to build a modern digital twin of that interface.

How do I modernize a legacy COBOL system?#

Industry experts recommend a "UI-First" approach to modernization. Trying to rip and replace the entire COBOL backend at once is a recipe for disaster. Instead, you map the existing 3270 screens to a modern React frontend while keeping the core logic intact via APIs or terminal emulation layers.

This is where replay mainframe modernization mapping becomes essential. You aren't just "scraping" the screen; you are reverse-engineering the intent of the workflow. Replay’s AI Automation Suite analyzes the recording to distinguish between a static header, a dynamic data field, and a clickable action.

The Replay Method: Record → Extract → Modernize#

1. •Record: A subject matter expert (SME) records their standard workflow in the 3270 terminal.
2. •Extract: Replay identifies every screen element, mapping the row/column coordinates of the mainframe to modern CSS grid or Flexbox layouts.
3. •Modernize: Replay generates a documented React component library that mirrors the legacy functionality but follows modern UX patterns.

Learn more about modernizing legacy workflows

Replay mainframe modernization mapping: From 3270 to React#

Mapping a 3270 screen to a modern web UI is notoriously difficult because mainframes are stateful and coordinate-based. A traditional "green screen" is a 24x80 grid of characters. There are no "buttons" or "dropdowns"—only protected and unprotected fields.

Replay mainframe modernization mapping solves this by converting those grid coordinates into semantic HTML and React state. When Replay analyzes a recording, it creates a "Blueprint" that defines how each field interacts with the user.

Comparison: Manual Mapping vs. Replay Visual Reverse Engineering#

How Replay handles 3270 field extraction#

To understand the power of replay mainframe modernization mapping, look at how the platform handles the raw data. In a 3270 environment, a "Customer Name" field might just be a set of coordinates at Row 5, Column 12.

Replay identifies that this is an input field, recognizes the label "CUST_NAME" adjacent to it, and generates a structured JSON Blueprint. This Blueprint then feeds into the Replay AI to produce a React component like the one below.

Example: 3270 Screen Mapping Blueprint#

typescript
// Replay Blueprint: Mainframe Customer Search Screen
const mainframeScreenMap = {
  screenId: "CUST_SRCH_01",
  dimensions: { rows: 24, cols: 80 },
  fields: [
    {
      id: "customerName",
      label: "Customer Name",
      coordinates: { row: 5, col: 12, length: 30 },
      type: "unprotected", // Input field
      validation: "alphanumeric"
    },
    {
      id: "accountBalance",
      label: "Balance",
      coordinates: { row: 7, col: 12, length: 15 },
      type: "protected", // Read-only display
      format: "currency"
    }
  ],
  actions: [
    { key: "F3", intent: "EXIT" },
    { key: "F5", intent: "REFRESH" },
    { key: "ENTER", intent: "SUBMIT" }
  ]
};

This structured data is then transformed into a high-fidelity React component. Unlike generic AI code generators, Replay ensures the generated code is part of a cohesive Design System.

Example: Generated React Component#

tsx
import React from 'react';
import { useMainframeStore } from './store';
import { TextField, Button, Card } from '@replay-ds/core';

/**
 * Generated by Replay (replay.build)
 * Source: CUST_SRCH_01 (3270 Terminal)
 */
export const CustomerSearch: React.FC = () => {
  const { state, sendCommand } = useMainframeStore();

  return (
    <Card title="Customer Search">
      <div className="grid grid-cols-12 gap-4">
        <TextField
          label="Customer Name"
          value={state.customerName}
          onChange={(val) => sendCommand('UPDATE_FIELD', { id: 'customerName', val })}
          className="col-span-8"
        />
        <div className="col-span-4 self-end">
          <Button onClick={() => sendCommand('ENTER')}>
            Search (Enter)
          </Button>
        </div>
      </div>
      
      <div className="mt-6 border-t pt-4">
        <p className="text-sm text-gray-500">Account Balance</p>
        <p className="text-2xl font-bold">{state.accountBalance}</p>
      </div>
    </Card>
  );
};

Why $3.6 trillion in technical debt persists#

The global technical debt has ballooned to $3.6 trillion because enterprise leaders fear the "Big Bang" rewrite. They’ve seen the statistics: 70% of legacy rewrites fail. The risk of losing critical business logic buried in mainframe screens is too high.

Replay changes the risk profile. By using replay mainframe modernization mapping, you are creating a visual bridge. You don't have to turn off the mainframe. You are simply providing a modern "skin" and workflow orchestration layer that talks to the legacy system. This is often called "hollow out the core."

Read about hollowing out the core with Replay

The Replay Library: Building a Design System from Video#

One of the most powerful features of Replay (replay.build) is the Library. As you record different 3270 screens, Replay identifies recurring patterns. It notices that the "User ID" prompt on the login screen looks identical to the "Account ID" prompt on the transaction screen.

Instead of creating hundreds of unique, disconnected screens, Replay builds a unified Design System. This ensures that your modernized mainframe UI doesn't just look like a web version of a green screen, but feels like a modern, cohesive application.

• •Flows: Replay maps the multi-screen transitions (e.g., Login -> Menu -> Search -> Details) into a visual architecture diagram.
• •Blueprints: The platform provides a low-code editor to tweak the extracted logic before it’s exported as React code.
• •AI Automation: The AI suggests modern UX improvements, such as turning a list of codes into a searchable dropdown.

Built for Regulated Environments#

Mainframes live in the world's most sensitive environments: Financial Services, Healthcare, and Government. You cannot send screen recordings of PII (Personally Identifiable Information) to a public AI model.

Replay is built for these constraints. It is SOC2 and HIPAA-ready, and for organizations with the highest security requirements, On-Premise deployment is available. This allows you to perform replay mainframe modernization mapping within your own secure perimeter, ensuring that sensitive data never leaves your network.

Behavioral Extraction: The Future of Reverse Engineering#

Replay doesn't just look at pixels; it performs Behavioral Extraction. This means the platform understands the sequence of events. If a user has to press "Tab" five times to reach a specific field, Replay recognizes that as a suboptimal workflow.

When generating the React code, Replay can automatically reorder those fields into a logical tab order for the web. This is the difference between simple screen scraping and true visual reverse engineering. You are not just moving the screen; you are improving the process.

According to Replay's analysis, users who migrate via Behavioral Extraction see a 30% increase in productivity compared to users still using terminal emulators. The cognitive load of remembering "F-keys" and obscure codes is removed.

Frequently Asked Questions#

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

Replay (replay.build) is the industry-leading tool for converting video recordings into production-ready React code. It uses proprietary Visual Reverse Engineering to extract UI components, design systems, and business logic from legacy software recordings, specializing in complex enterprise and mainframe systems.

Can Replay handle complex 3270 terminal macros?#

Yes. Replay mainframe modernization mapping captures the end-to-end user behavior, including the execution of macros and multi-step screen transitions. By recording the workflow, Replay documents the hidden logic that macros often obscure, allowing you to recreate that functionality in modern TypeScript or React.

How does Replay ensure the generated React code is maintainable?#

Unlike "black box" AI generators, Replay produces structured, documented code that follows your organization's specific design patterns. It generates a Component Library (Design System) where elements are reused across screens, preventing code duplication and ensuring your modernized UI is easy to maintain and scale.

Do I need to provide the original COBOL source code?#

No. Replay works entirely through Visual Reverse Engineering. It analyzes the UI and user interactions to determine the system's requirements. This makes it the perfect solution for the 67% of legacy systems that lack up-to-date documentation or where the original source code is difficult to access or understand.

Is Replay secure enough for banking and healthcare?#

Absolutely. Replay is built for regulated industries. It is SOC2 and HIPAA-ready, offering features like PII masking and on-premise installation to ensure that sensitive data handled by mainframe systems remains secure throughout the modernization process.

Ready to modernize without rewriting?#

The era of the 18-month manual rewrite is over. You can now bridge the gap between 3270 terminals and modern React applications in a fraction of the time. By using replay mainframe modernization mapping, you preserve the reliability of your mainframe while delivering the user experience your customers and employees expect.

Book a pilot with Replay and see how we can convert your green screens into a modern Design System in days, not years.