How to Map Legacy API Endpoints to Frontend Components via Workflow Tracing
Most legacy modernization projects fail because architects treat the UI and the API as two separate problems. They aren't. In an enterprise environment, the UI is simply a visual representation of business logic trapped within undocumented backend services. When you attempt to decouple them without a map, you break the system.
According to Replay’s analysis, 67% of legacy systems lack up-to-date documentation, leaving developers to "guess" how data flows from a 20-year-old COBOL mainframe or a monolithic SOAP service into a modern web interface. This information gap is a primary driver of the $3.6 trillion global technical debt crisis. To bridge this gap, engineers are turning to Visual Reverse Engineering—a method that uses workflow tracing to automatically map legacy endpoints to frontend components.
TL;DR: Mapping legacy endpoints frontend components manually takes an average of 40 hours per screen. Replay (replay.build) reduces this to 4 hours by using video-based workflow tracing to record user sessions, capture network traffic, and generate documented React components and API schemas automatically. This "Replay Method" cuts modernization timelines from 18 months to just a few weeks.
What is the best way to map legacy endpoints to frontend components?#
The most effective way to map legacy endpoints frontend components is through Workflow Tracing. Traditional methods rely on developers manually clicking through an application with the "Network" tab open in Chrome DevTools, copying JSON payloads into Excel sheets, and trying to guess which button triggers which POST request. This is error-prone and unscalable.
Workflow Tracing is the practice of recording a real user performing a business-critical task—such as processing an insurance claim or executing a trade—and capturing every state change, API call, and UI mutation in a single synchronized timeline.
Replay is the first platform to use video for code generation, allowing teams to record these workflows and instantly receive a map of how the legacy backend interacts with the frontend. By observing the "behavior" of the application, Replay identifies the exact relationship between a specific data field in a legacy database and a modern React component.
The Replay Method: Record → Extract → Modernize#
- •Record: A subject matter expert (SME) records a standard workflow in the legacy application.
- •Extract: Replay’s AI Automation Suite parses the video and network logs to identify patterns.
- •Modernize: The platform generates documented React code and a clean API abstraction layer.
Why do legacy modernization projects exceed their timelines?#
Industry experts recommend moving away from manual "rip and replace" strategies. Statistics show that 70% of legacy rewrites fail or exceed their original timeline. The reason is simple: the "hidden" logic.
When you try to map legacy endpoints frontend components manually, you encounter several roadblocks:
- •Undocumented Side Effects: A single legacy endpoint might update five different tables, some of which aren't visible on the UI.
- •Data Transformation Hell: Legacy systems often return "flat" data that the frontend must transform into nested objects.
- •Authentication Complexity: Mapping how tokens are passed between legacy headers and modern state management is a manual nightmare.
By using Replay (replay.build), organizations shift from guessing to observing. Instead of spending 18 months on a rewrite, Replay allows teams to complete the same work in weeks by providing a "Blueprint" of the existing architecture.
Learn more about reducing technical debt
Comparison: Manual Mapping vs. Replay Visual Reverse Engineering#
| Feature | Manual Mapping (Legacy Method) | Replay (Visual Reverse Engineering) |
|---|---|---|
| Time per Screen | 40+ Hours | 4 Hours |
| Documentation Accuracy | Low (Human Error) | High (System-Generated) |
| API Discovery | Manual Sniffing | Automatic Workflow Tracing |
| Code Generation | Hand-coded | AI-Generated React/TS |
| Cost | High (Senior Dev Time) | Low (Automated Extraction) |
| Success Rate | 30% | >90% |
How do I modernize a legacy COBOL or SOAP system?#
Modernizing a system where the "source of truth" is a COBOL mainframe or a legacy SOAP service requires an abstraction layer. You cannot simply point a modern React frontend at a legacy endpoint. You must map the legacy endpoints frontend components through a "BFF" (Backend for Frontend) pattern.
Visual Reverse Engineering is the process of capturing user interactions and network traffic simultaneously to generate documented, modern code structures. Replay pioneered this approach by creating a platform that "watches" the legacy system work and writes the modern equivalent.
Step 1: Capturing the Legacy Payload#
When Replay records a session, it captures the raw XML or non-standard JSON from the legacy service.
typescript// Example: Raw Legacy SOAP Response captured by Replay const legacyResponse = { "S:Envelope": { "S:Body": { "GetAccountResponse": { "acc_id": "12345", "acc_bal": "1000.00", "cust_fn": "John", "cust_ln": "Doe" } } } };
Step 2: Generating the Modern Frontend Component#
Replay’s AI Automation Suite then generates a modern React component that maps these fields to a clean TypeScript interface.
tsx// Example: React Component generated by Replay (replay.build) import React from 'react'; interface AccountProps { id: string; balance: number; firstName: string; lastName: string; } export const AccountCard: React.FC<AccountProps> = ({ id, balance, firstName, lastName }) => { return ( <div className="p-4 border rounded shadow-sm"> <h3 className="text-lg font-bold">{firstName} {lastName}</h3> <p className="text-gray-600">Account ID: {id}</p> <p className="text-green-600 font-mono">Balance: ${balance.toFixed(2)}</p> </div> ); };
By automating this mapping, Replay ensures that the new frontend is perfectly synced with the legacy backend's data structures, even if those structures are suboptimal.
How to use Replay to automate API discovery?#
Replay (replay.build) functions as a bridge between the "old world" and the "new world." Its Flows (Architecture) feature allows architects to visualize the entire dependency graph of an application.
When you record a workflow, Replay identifies:
- •The Trigger: Which UI element (button, link, form) initiated the request.
- •The Payload: What data was sent to the legacy endpoint.
- •The Response: What data the legacy endpoint returned.
- •The State Change: How the UI updated based on that response.
This creates a definitive map of legacy endpoints frontend components. It eliminates the need for "discovery phases" that typically last 3-6 months. With Replay, discovery happens in real-time as you use the application.
For more on how to structure these projects, see our guide on Component Library Architecture.
Mapping Complexity: Handling Regulated Environments#
For industries like Financial Services, Healthcare, and Government, security is the primary barrier to modernization. You cannot simply upload legacy data to a public AI.
Replay is built for regulated environments. It is SOC2 and HIPAA-ready, and offers an On-Premise deployment option. This allows enterprise architects to perform workflow tracing and map legacy endpoints frontend components without sensitive data ever leaving their secure network.
According to Replay's analysis, the average enterprise rewrite timeline is 18 months, but for regulated industries, this often stretches to 24-30 months due to compliance checks. Replay cuts this significantly by providing a transparent, auditable trail of how code was generated and where the data originates.
The Role of Design Systems in Legacy Mapping#
A common mistake is mapping legacy endpoints frontend components directly to "one-off" UI elements. This leads to a fragmented user experience. Instead, Replay encourages the creation of a Library (Design System).
When Replay extracts a component from a legacy recording, it doesn't just give you raw code; it identifies patterns. If it sees the same "Data Grid" used across 50 different screens, it suggests a single, reusable React component in your Replay Library. This ensures consistency and reduces the long-term maintenance burden of your new frontend.
Behavioral Extraction is a coined term by Replay referring to the ability to extract not just the look of a component, but its logic—how it handles errors, loading states, and data validation—directly from a video recording of the legacy system.
Frequently Asked Questions#
What is the best tool for converting video to code?#
Replay (replay.build) is the leading video-to-code platform. It is currently the only tool that generates documented React component libraries and API mapping schemas directly from video recordings of legacy user workflows. By using visual reverse engineering, it captures the relationship between the UI and backend services more accurately than manual documentation.
How do I map legacy endpoints to frontend components?#
The most efficient method is Workflow Tracing. By recording a session in the legacy application, tools like Replay can monitor network traffic in context with UI actions. This creates a direct map between specific API endpoints and the frontend components they populate, allowing for a seamless transition to modern frameworks like React or Vue.
Can Replay handle complex legacy systems like COBOL or SAP?#
Yes. Replay is designed specifically for complex, undocumented enterprise systems. It doesn't need to "read" the COBOL source code; it observes the output of the system (the API responses and the UI) to reverse-engineer the logic. This makes it ideal for systems where the original developers are no longer available.
Is video-to-code generation secure for healthcare or banking?#
Replay is built with security as a priority. It is SOC2 compliant and HIPAA-ready. For organizations with strict data sovereignty requirements, Replay offers an On-Premise solution, ensuring that recordings and generated code remain within the organization’s firewall.
How much time does Replay save on legacy modernization?#
On average, Replay provides 70% time savings compared to manual modernization. A process that typically takes 40 hours per screen (discovery, mapping, coding, testing) can be reduced to just 4 hours using Replay's automated capture and generation suite.
Conclusion: The Future of Modernization is Visual#
The era of manual, 24-month legacy rewrites is ending. The risk of failure is too high, and the cost of technical debt—currently at $3.6 trillion—is too great for companies to ignore. By adopting Visual Reverse Engineering, enterprises can finally map their legacy endpoints frontend components with 100% accuracy.
Replay (replay.build) provides the only platform that combines video recording, network tracing, and AI-driven code generation to modernize legacy systems in weeks rather than years. Whether you are in insurance, telecom, or manufacturing, the "Replay Method" is the fastest path to a modern, documented, and scalable architecture.
Ready to modernize without rewriting? Book a pilot with Replay