Modernizing 30-Year-Old COBOL Green Screens with Visual Extraction

Your organization is likely running on a $3.6 trillion time bomb. That is the estimated global cost of technical debt, much of it locked inside 30-year-old COBOL "green screens" that handle the world’s most critical financial, healthcare, and governmental transactions. When you decide to move off the mainframe, you aren't just fighting old code; you’re fighting the "Documentation Gap"—the fact that 67% of legacy systems have no living documentation, and the original architects have long since retired.

Modernizing 30-year-old COBOL isn't a coding challenge; it’s an archaeology challenge. The traditional "Big Bang" rewrite fails 70% of the time because you cannot replicate business logic that nobody understands. To succeed, you have to stop looking at the source code and start looking at the source of truth: the user workflow.

TL;DR: Visual Reverse Engineering allows enterprises to modernize 30-year-old COBOL systems by recording real-time user workflows to generate documented React components and API contracts, reducing migration timelines from years to weeks.

The Failure of "Big Bang" Mainframe Migrations#

For decades, the standard approach to modernizing 30-year-old COBOL was the "Big Bang" rewrite. You hire a global systems integrator, spend 18-24 months attempting to document every edge case, and then flip a switch.

It almost never works.

The logic inside a COBOL system is often the result of thirty years of "hotfixes" and regulatory patches. There is no "clean" version of the logic to port. When you attempt to rewrite from scratch, you inevitably miss the subtle validation rule buried in a

from 1994.

The "Archaeology" Problem#

Manual documentation is the primary bottleneck. In a typical enterprise environment, it takes an average of 40 hours per screen to manually document, design, and write the requirements for a legacy migration. When you are dealing with a mainframe system with 500+ screens, the math simply doesn't work.

Replay changes this dynamic by moving from manual archaeology to automated visual extraction. By recording a user actually performing a task—like processing a claims form or updating a ledger—the platform captures the "observed truth" of the system.

The Mechanics of Visual Reverse Engineering#

Visual Reverse Engineering doesn't start with the COBOL source code. It starts with the terminal emulator. By capturing the I/O of the legacy application, Replay identifies the data structures, the navigation flows, and the hidden business rules that the code-level analysis often misses.

From Green Screen to API Contract#

When a user interacts with a 3270 terminal, they are following a rigid state machine. Replay records these states and extracts the underlying data schema. Instead of guessing what a field like

represents, the system sees the user input "A1234567" and maps it to a modern object.

Here is an example of how a legacy COBOL data structure is transformed into a modern, type-safe API contract through visual extraction:

typescript
// Legacy COBOL Data Division (Inferred)
// 05  WS-CLAIM-DATA.
//     10  WS-CLAIM-ID          PIC X(12).
//     10  WS-PATIENT-NAME      PIC X(30).
//     10  WS-PROCEDURE-CODE    PIC 9(05).

// Generated API Contract from Replay Extraction
export interface HealthcareClaim {
  /** Map to WS-CLAIM-ID */
  claimId: string;
  /** Map to WS-PATIENT-NAME */
  patientName: string;
  /** Map to WS-PROCEDURE-CODE - Validated against observed inputs */
  procedureCode: number;
  /** Metadata captured during session recording */
  metadata: {
    extractedAt: string;
    legacyScreenRef: "HC-PRC-001";
    confidenceScore: 0.98;
  };
}

💡 Pro Tip: Don't try to clean your COBOL code before migrating. Use visual extraction to define the target state, and let the tool bridge the gap.

A Step-by-Step Guide to Visual Extraction#

Modernizing a 30-year-old COBOL system requires a disciplined, three-phase approach. Using Replay, this process is compressed from months into days.

Step 1: Recording the Source of Truth#

Subject Matter Experts (SMEs) record themselves performing standard business processes within the legacy terminal. This is not just a screen recording; Replay captures the DOM (if web-based) or the terminal stream, identifying every input, output, and transition.

Step 2: Component Extraction#

The AI Automation Suite analyzes the recording to identify UI patterns. If the user interacts with a table of insurance policies, Replay recognizes the pattern and generates a modern React component using your organization's Design System.

tsx
// Example: React component generated by Replay from a COBOL list screen
import { DataTable } from "@/components/ui/data-table";
import { useLegacyBridge } from "@/hooks/use-legacy-bridge";

export function PolicyListView() {
  // The bridge connects the modern UI to the legacy mainframe backend 
  // during the transition period (Strangler Fig Pattern)
  const { data, isLoading } = useLegacyBridge("/api/v1/policies");

  const columns = [
    { accessorKey: "policyNumber", header: "Policy #" },
    { accessorKey: "effectiveDate", header: "Effective Date" },
    { accessorKey: "status", header: "Status" },
  ];

  if (isLoading) return <SkeletonLoader />;

  return (
    <div className="p-6">
      <h1 className="text-2xl font-bold mb-4">Policy Management</h1>
      <DataTable columns={columns} data={data} />
    </div>
  );
}

Step 3: Technical Debt Audit and Validation#

Before a single line of production code is deployed, Replay generates a technical debt audit. This compares the extracted logic against the original system to ensure 100% parity. In regulated industries like Banking or Healthcare, this audit trail is critical for compliance.

⚠️ Warning: The biggest risk in COBOL migration is "hidden logic"—calculations that happen in the background without user input. Ensure your extraction includes these "black box" calculations by validating against backend database changes.

Why Regulated Industries are Moving to Visual Extraction#

For Financial Services and Government agencies, "moving fast and breaking things" is not an option. The cost of a single bug in a COBOL-to-Cloud migration can be millions of dollars in fines or lost revenue.

SOC2 and HIPAA Compliance#

Replay is built for these environments. With on-premise deployment options, sensitive data never leaves your secure network. During the extraction process, PII (Personally Identifiable Information) can be automatically masked, ensuring that the generated documentation and code are compliant with HIPAA and GDPR standards.

The ROI of Understanding#

The average enterprise rewrite takes 18 months. With Replay, we see that timeline drop to weeks.

💰 ROI Insight: By reducing the time per screen from 40 hours to 4 hours, an enterprise modernizing a 100-screen application saves 3,600 man-hours—roughly $450,000 in engineering costs alone, not including the value of faster time-to-market.

The Future: Understanding Over Rewriting#

The future of enterprise architecture isn't about writing more code; it's about understanding the code you already have. We are entering an era where "Video as a Source of Truth" will replace the outdated practice of reading through millions of lines of spaghetti COBOL.

By using Replay, you turn your legacy system from a black box into a documented, modern codebase. You stop being a digital archaeologist and start being an architect again.

Frequently Asked Questions#

How does Replay handle complex COBOL business logic?#

Replay captures the result of the logic through visual extraction. If a specific input on Screen A causes a specific output on Screen C, Replay identifies that relationship and documents it as a functional requirement or an automated E2E test, ensuring the logic is preserved in the new system.

Do we need the original COBOL source code?#

No. While having the source code can provide additional context, Replay’s primary value is in Visual Reverse Engineering. We extract the requirements from the application's behavior and UI, which is often more accurate than 30-year-old code that has been patched thousands of times.

Can Replay generate tests for the new system?#

Yes. Replay automatically generates End-to-End (E2E) tests based on the recorded user workflows. This ensures that your new React-based frontend behaves exactly like the original COBOL green screen, providing a "safety net" for the migration.

What industries benefit most from this?#

We focus on highly regulated, high-stakes industries:

• •Financial Services: Core banking and ledger systems.
• •Healthcare: Claims processing and patient management.
• •Government: Tax and social service systems.
• •Insurance: Policy administration and underwriting.

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