Missing Source Code: How to Modern

The "Black Box" is the single greatest risk to your enterprise stability. You have a mission-critical application running in production, but the original developers are gone, the documentation is non-existent, and the missing source code—or code so obfuscated it might as well be missing—has turned your infrastructure into a liability.

When the source code is lost or unreadable, most CTOs default to the "Big Bang" rewrite. They commit $10M and two years to a project that has a 70% chance of failure. This isn't just a technical problem; it’s a $3.6 trillion global technical debt crisis that paralyzes innovation. The future of modernization isn't digging through digital graveyards; it’s visual reverse engineering.

TL;DR: Modernizing systems with missing source code no longer requires manual code archaeology; by using Visual Reverse Engineering with Replay, enterprises can extract business logic and UI components directly from live user workflows, reducing modernization timelines from years to weeks.

The High Cost of Digital Archaeology#

Most legacy systems in financial services and healthcare lack any meaningful documentation (67% of systems, to be precise). When you are dealing with missing source code, your team spends 80% of their time "playing detective" and only 20% actually writing new features.

The industry has traditionally offered three paths, all of which are flawed:

The "Big Bang" fails because it attempts to replicate 20 years of undocumented edge cases in a single go. The "Strangler Fig" is better but requires you to understand the very code you’re trying to replace. If you have missing source code, you can't wrap an API you don't know exists.

From Black Box to Documented Codebase#

If you can’t read the code, you must observe the behavior. Visual Reverse Engineering treats the running application as the "Source of Truth." By recording real user workflows, Replay captures the DOM state, network calls, and state transitions to reconstruct the application from the outside in.

The Problem with Manual Extraction#

A senior engineer takes an average of 40 hours to manually audit, document, and recreate a single complex legacy screen. In an enterprise application with 200+ screens, that is 8,000 hours of high-value engineering time wasted on "copy-pasting" the past.

💰 ROI Insight: Replay reduces the time per screen from 40 hours to 4 hours. For a 100-screen application, this saves approximately 3,600 engineering hours, or roughly $540,000 in direct labor costs.

Step-by-Step: Modernizing Without Source Code#

When the source code is missing, follow this tactical framework to move from a legacy monolith to a modern React-based architecture.

Step 1: Workflow Recording#

Instead of reading code, record the application in use. Use Replay to capture a full user session. This isn't a simple screen recording; it is a telemetry capture of every interaction, API request, and data transformation.

Step 2: Component Extraction#

Replay’s AI Automation Suite analyzes the recording to identify UI patterns. It identifies buttons, forms, and data tables, then maps them to your modern Design System (the Library).

typescript
// Example: React component automatically generated by Replay from a legacy binary
// Source: Legacy Insurance Claims Portal (Source Code Missing)
// Target: Modern React + Tailwind

import React, { useState, useEffect } from 'react';
import { Button, Input, Table } from '@/components/ui'; 
import { useClaimsData } from '@/hooks/useClaims';

export function ClaimsDashboardMigrated() {
  const [claims, setClaims] = useState([]);
  const { fetchClaims, loading, error } = useClaimsData();

  // Logic preserved from visual extraction of legacy network patterns
  useEffect(() => {
    const loadData = async () => {
      const data = await fetchClaims();
      setClaims(data);
    };
    loadData();
  }, [fetchClaims]);

  if (loading) return <SkeletonLoader />;

  return (
    <div className="p-6 max-w-7xl mx-auto">
      <header className="flex justify-between mb-8">
        <h1 className="text-2xl font-bold">Claims Processing</h1>
        <Button variant="primary">New Claim</Button>
      </header>
      
      <Table 
        data={claims}
        columns={[
          { header: 'Claim ID', accessor: 'id' },
          { header: 'Policy Holder', accessor: 'holder' },
          { header: 'Status', accessor: 'status' },
          { header: 'Amount', accessor: 'value', cell: (v) => `$${v}` }
        ]}
      />
    </div>
  );
}

Step 3: API Contract Synthesis#

Missing source code usually means missing API documentation. Replay intercepts the traffic during the recording and generates OpenAPI (Swagger) specifications. This allows your backend team to build new services that perfectly match the expectations of the legacy frontend.

yaml
# Generated API Contract from Replay Network Interception
openapi: 3.0.0
info:
  title: Legacy Claims API
  version: 1.0.0
paths:
  /api/v1/claims:
    get:
      summary: Extracted endpoint for dashboard data
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Claim'
components:
  schemas:
    Claim:
      type: object
      properties:
        id: { type: string }
        holder: { type: string }
        status: { type: string, enum: [PENDING, APPROVED, REJECTED] }
        value: { type: number }

Step 4: Technical Debt Audit#

Once the flows are captured, Replay generates a Blueprint. This is your new "Source of Truth." It highlights redundant logic, unused fields, and security vulnerabilities that were hidden in the black box.

⚠️ Warning: Never attempt to replicate legacy bugs in your new system. Use the Blueprint phase to sanitize business logic before it reaches the new codebase.

Addressing the "Missing Source Code" in Regulated Industries#

In Financial Services, Healthcare, and Government, the "missing source code" problem is compounded by strict compliance requirements. You cannot simply upload your legacy binary to a public cloud AI for analysis.

Replay is built for these constraints:

• •SOC2 & HIPAA Ready: Data is handled with enterprise-grade encryption.
• •On-Premise Availability: Run the entire extraction engine within your own VPC or air-gapped environment.
• •E2E Test Generation: Replay automatically generates Playwright or Cypress tests for every recorded workflow, ensuring the new system matches the legacy behavior exactly.

💡 Pro Tip: Use the "Blueprints" feature to create a living architecture map. This prevents the "New Legacy" problem where your modern system becomes undocumented again in 3 years.

The Architecture of Understanding#

We are moving away from the era of "code-first" modernization. When you are faced with a system that has missing source code, the UI is your only reliable interface. By using Replay to bridge the gap between the visual layer and the code layer, you eliminate the "archaeology" phase of the project.

• •Library: Centralize your UI components.
• •Flows: Document the business logic as it actually happens.
• •Blueprints: Provide the roadmap for the engineering team.

The 18-month rewrite timeline is a relic of the past. By focusing on what the user sees and what the network sends, you can bypass the "black box" and move straight to a modern, documented React application.

Frequently Asked Questions#

How can you modernize if the source code is literally missing?#

Replay doesn't need your source code. It records the application's execution environment (the DOM and Network). By observing the inputs and outputs, our AI Automation Suite can reconstruct the component structure and business logic required to replicate the functionality in a modern stack like React.

What about complex business logic hidden in the backend?#

Visual Reverse Engineering captures the "Contract" between the frontend and backend. While it won't see your COBOL or Java logic directly, it documents exactly what data that logic produces. This allows you to build a "Black Box" test suite—ensuring your new backend produces the exact same outputs for the same inputs as the original system.

Is this just a "No-Code" tool?#

No. Replay generates clean, human-readable TypeScript and React code. It is a tool for professional engineers to accelerate the "boring" parts of modernization (scaffolding, documentation, and component recreation) so they can focus on high-level architecture and new feature development.

How does this handle security in regulated industries?#

Replay is designed for the most sensitive environments. We offer on-premise deployments where no data ever leaves your network. We are SOC2 compliant and HIPAA-ready, ensuring that even as you record workflows containing sensitive data, that data is handled according to enterprise security standards.

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