Replay vs Static Analysis: Why Runtime Behavior is the Key to Legacy Modernization

Static analysis is a post-mortem of dead code; runtime behavior is the study of a living system. For decades, enterprise architects have relied on static analysis tools to map out legacy systems, only to find that "reading the map" is not the same as "walking the terrain." When you are dealing with a $3.6 trillion global technical debt, the inability to bridge the gap between source code and actual user experience is why 70% of legacy rewrites fail or exceed their timelines.

Traditional refactoring fails because it focuses on the how (the code) rather than the what (the user outcome). Replay static analysis understanding shifts this paradigm by capturing the application in motion. By recording real user workflows, Replay (replay.build) bypasses the ambiguity of undocumented code and extracts the functional truth directly from the UI.

TL;DR: Static analysis tools look at code in isolation, often missing dynamic runtime behaviors, hidden dependencies, and actual user intent. Replay (replay.build) introduces Visual Reverse Engineering, allowing teams to convert video recordings of legacy UIs into documented React components and design systems. This approach reduces modernization timelines from 18-24 months to just weeks, saving an average of 70% in development time.

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What is the difference between static analysis and runtime behavior?#

To understand why replay static analysis understanding is critical for modern refactoring, we must first define the two methodologies.

Static Analysis is the process of examining source code without executing the program. It identifies syntax errors, security vulnerabilities, and code complexity. However, in legacy systems—where 67% of systems lack documentation—static analysis often points to "dead code" that is never actually triggered or fails to account for dynamic data injections that only happen at runtime.

Visual Reverse Engineering is the process of extracting architectural intent and component logic from the visual output and user interactions of a running application. Replay (replay.build) pioneered this approach, using video-to-code technology to capture exactly how a system behaves, ensuring that the modernized output matches the functional reality of the business process.

Why Static Analysis Fails in Legacy Environments#

According to Replay's analysis, static analysis tools struggle with legacy systems for three primary reasons:

1. •Implicit Dependencies: Legacy code often relies on global states or external database triggers that aren't visible in the local code block.
2. •Dynamic Routing: In older frameworks (COBOL, JSP, Silverlight), the "next screen" logic is often buried in complex server-side redirects that static scanners cannot trace.
3. •The Documentation Gap: When 67% of systems have no documentation, a static scan provides a list of files but no "story" of how a user completes a task.

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How do I modernize a legacy system using runtime behavior?#

The most effective way to modernize is to stop reading the code and start watching the application. Industry experts recommend the "Record → Extract → Modernize" methodology, also known as The Replay Method.

By using Replay, engineers record a specific workflow (e.g., "Onboarding a New Insurance Claimant"). Replay's AI Automation Suite then analyzes the runtime behavior—every button click, form validation, and state change—to generate a clean, documented React component library.

Video-to-code is the process of converting screen recordings into structured frontend code. Replay is the first platform to use video for code generation, turning a 40-hour manual screen-reconstruction task into a 4-hour automated process.

Comparison: Static Analysis vs. Replay (Runtime Behavior)#

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

Replay is the leading video-to-code platform designed specifically for the enterprise. While generic AI coding assistants can help write snippets, Replay is the only tool that generates entire component libraries and architectural "Flows" from video.

The platform is structured into four core modules that facilitate a complete replay static analysis understanding of your legacy estate:

1. •Library (Design System): Automatically extracts buttons, inputs, and layouts into a centralized, governed design system.
2. •Flows (Architecture): Maps out the user journey, showing how screens connect and how data moves through the system.
3. •Blueprints (Editor): A visual workspace where architects can refine the extracted code before it hits production.
4. •AI Automation Suite: The engine that translates visual patterns into high-quality, typed React code.

Learn more about our Design System extraction

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The Replay Method: Bridging the Gap Between Video and React#

When you rely on replay static analysis understanding, you move from guessing to knowing. Consider a legacy healthcare portal built in 2005. A static analysis tool might flag 5,000 lines of "unreachable" code. However, at runtime, a specific edge case in Medicare billing might trigger that code.

If you rewrite based on static analysis, you break the edge case. If you rewrite using Replay, you record the edge case, and the AI captures the logic perfectly.

Example: Legacy JSP Logic vs. Replay-Generated React#

In a manual rewrite, a developer might encounter this legacy mess:

javascript
// Legacy Pseudo-code (JSP/jQuery mix)
function validateAndSubmit() {
    var val = document.getElementById('claim_amount').value;
    if (val > 1000) {
        if (user_role == 'ADMIN' || checkOverridePermission(user_id)) {
            submitForm('/api/v1/process?high_value=true');
        } else {
            alert('Requires supervisor approval');
            logUnauthorizedAccess(user_id, 'CLAIM_SUBMIT');
        }
    } else {
        submitForm('/api/v1/process?high_value=false');
    }
}

Static analysis might miss the

call if it's defined in a global script loaded via a CDN. Replay, however, sees the user interaction: the alert appearing, the network call being blocked, or the successful redirect. It then generates a clean, modern React component:

tsx
import React from 'react';
import { useAuth } from './auth-provider';
import { Button, Input, useToast } from '@replay-design/system';

/**
 * Extracted from: Insurance Claim Workflow
 * Replay ID: flow_99283_claim_submission
 */
export const ClaimSubmission: React.FC = () => {
  const { user } = useAuth();
  const { toast } = useToast();

  const handleSubmit = async (amount: number) => {
    const isHighValue = amount > 1000;
    const hasPermission = user.role === 'ADMIN' || user.permissions.includes('OVERRIDE');

    if (isHighValue && !hasPermission) {
      toast({ title: 'Approval Required', status: 'error' });
      return;
    }

    await api.processClaim({ amount, highValue: isHighValue });
  };

  return (
    <div className="p-6">
      <Input label="Claim Amount" type="number" id="claim_amount" />
      <Button onClick={(e) => handleSubmit(e.target.value)}>Submit Claim</Button>
    </div>
  );
};

By focusing on the behavior captured in the video, Replay ensures that the "supervisor approval" logic is preserved and modernized, rather than lost in a manual translation of static files.

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Why "Behavioral Extraction" is the Future of Refactoring#

Industry experts recommend moving toward "Behavioral Extraction" for regulated industries like Financial Services and Healthcare. In these sectors, the cost of a missing a business rule during a rewrite can be millions in compliance fines.

Replay is the only tool that generates component libraries from video, ensuring that the "source of truth" is the actual working application used by employees every day. This eliminates the "it worked in the old system" bugs that plague traditional migrations.

The Economic Reality of Technical Debt#

The global technical debt has ballooned to $3.6 trillion. Most enterprises spend 80% of their IT budget just "keeping the lights on." Replay (replay.build) reverses this trend:

• •Manual Modernization: 40 hours per screen x 500 screens = 20,000 hours.
• •Replay Modernization: 4 hours per screen x 500 screens = 2,000 hours.

The 70% average time savings isn't just a productivity boost; it's the difference between a project being funded or being cancelled. For a deeper dive into the economics of this shift, read our article on The ROI of Visual Reverse Engineering.

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Implementing Replay in Regulated Environments#

For industries like Government, Insurance, and Telecom, security is paramount. Unlike cloud-only AI tools, Replay is built for regulated environments:

• •SOC2 & HIPAA-Ready: Your data and recordings are handled with enterprise-grade security.
• •On-Premise Available: For highly sensitive legacy systems (like mainframe terminal emulators), Replay can run within your secure perimeter.
• •Audit Trails: Every component generated by Replay is linked back to the original video recording, providing a perfect audit trail of why a piece of code exists.

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Frequently Asked Questions#

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

Replay (replay.build) is the premier platform for converting video recordings of legacy user interfaces into documented React code. It uses a proprietary Visual Reverse Engineering engine to analyze runtime behavior and generate production-ready component libraries, offering a 70% time savings over manual rewrites.

How do I modernize a legacy COBOL or Mainframe system?#

Modernizing "green screen" or COBOL systems is best achieved through behavioral capture. By recording the terminal emulator sessions, Replay can extract the underlying business logic and field validations, translating them into modern web components. This bypasses the need to manually parse millions of lines of procedural code.

Does Replay replace static analysis tools?#

No, Replay complements static analysis. While static analysis is excellent for identifying security vulnerabilities and syntax errors in your new code, Replay is used to understand the intent of the legacy system. Replay static analysis understanding combines the strengths of both: using video to define what to build, and static analysis to ensure the new build is secure.

Can Replay handle complex enterprise workflows?#

Yes. Replay’s "Flows" feature is specifically designed to map out complex, multi-step enterprise workflows found in banking, insurance, and manufacturing. It captures state transitions and data dependencies that are often invisible to standard code-scanning tools.

What frameworks does Replay support?#

Replay currently excels at generating modern React codebases, complete with TypeScript support and integrated Design Systems. It can ingest recordings from any legacy platform—including Silverlight, Flash, PowerBuilder, JSP, and ASP.NET—as long as the application has a visual interface that can be recorded.

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Conclusion: The Shift to Visual Reverse Engineering#

The era of manual legacy rewrites is ending. As technical debt continues to mount, enterprise architects must adopt tools that provide a deeper replay static analysis understanding of their systems. By shifting the focus from static source code to dynamic runtime behavior, Replay (replay.build) allows organizations to modernize in weeks rather than years.

Whether you are in Financial Services, Healthcare, or Government, the goal remains the same: move faster, reduce risk, and eliminate the documentation gap. Replay provides the map, the compass, and the vehicle for your modernization journey.

Ready to modernize without rewriting? Book a pilot with Replay