Edge Functions for UI Routing: Solving Legacy Redirection Issues During Phased Migrations

The "Big Bang" migration is the enterprise equivalent of a controlled demolition where the "controlled" part is purely theoretical. When you attempt to swap a decade-old monolithic UI for a modern React stack overnight, you aren't just fighting code; you're fighting the $3.6 trillion global technical debt that accumulates through undocumented edge cases and fossilized business logic. Industry experts recommend the Strangler Fig pattern—gradually replacing legacy functionality with new services—but this introduces a critical infrastructure challenge: how do you seamlessly route users between a 2005-era JSP backend and a 2024 Next.js frontend without destroying SEO or session state?

The answer lies at the network edge. Using edge functions routing solving the persistent friction of phased migrations allows architects to intercept requests before they even hit the origin server. This approach transforms a high-risk "flip the switch" deployment into a granular, low-risk transition.

TL;DR: Phased migrations often fail due to complex redirection logic and broken user sessions. Edge functions (like Cloudflare Workers or Vercel Middleware) provide a programmable layer to route traffic between legacy and modern stacks dynamically. By using Replay to visually reverse engineer legacy UIs into React components, teams can accelerate the creation of the "New" stack, while edge functions handle the "routing solving" the transition logic, reducing migration timelines from years to weeks.

The Architecture of Migration Friction#

Most enterprise migrations stall because 67% of legacy systems lack documentation. When you don't know why a specific URL structure exists, you're afraid to change it. Traditionally, developers handled this with massive Nginx configuration files or F5 load balancer rules. These methods are brittle, hard to test, and require infrastructure tickets that slow down development cycles.

According to Replay’s analysis, the average enterprise rewrite timeline is 18 months, primarily because teams spend 40 hours per screen manually documenting and rebuilding legacy UIs. When you combine this slow development pace with rigid routing, you get "Migration Limbo"—a state where the organization is supporting two parallel stacks indefinitely.

Video-to-code is the process of recording a user's interaction with a legacy system and automatically generating documented, functional React code from that recording. This technology, pioneered by Replay, allows teams to bypass the manual documentation phase and move straight to routing traffic to the new components.

Edge Functions Routing Solving the "Two-Stack" Problem#

Edge functions act as a programmable proxy. Instead of a static redirect, you can execute logic at the edge to determine which version of a page a user should see based on:

1. •User Identity: Is this user part of the beta group?
2. •Feature Flags: Is the "New Dashboard" enabled for this region?
3. •Path Mapping: Does a modern equivalent of this legacy
   text

   /cgi-bin/portal.pl
   exist yet?
4. •Device/Bot Status: Should we serve the legacy version to SEO crawlers while testing the new version for mobile users?

Using edge functions routing solving these specific criteria ensures that the user journey remains cohesive, even if they are bouncing between a 15-year-old mainframe-backed UI and a modern React application.

Comparison: Traditional Redirects vs. Edge-Based Routing#

Implementing Edge Routing: A Practical Example#

To solve the redirection issue, we can use a middleware approach. In this example, we'll use a TypeScript-based edge function that checks if a modern version of a legacy screen has been generated by Replay and is ready for traffic.

typescript
// middleware.ts - Edge Routing Logic
import { NextRequest, NextResponse } from 'next/server';

const MIGRATED_ROUTES = ['/dashboard', '/user-profile', '/settings'];
const LEGACY_ORIGIN = 'https://legacy-enterprise-app.com';

export function middleware(req: NextRequest) {
  const { pathname } = req.nextUrl;

  // 1. Check if the route has been modernized using Replay
  const isMigrated = MIGRATED_ROUTES.some(route => pathname.startsWith(route));

  // 2. Logic for A/B testing or phased rollout
  const isBetaUser = req.cookies.get('beta_access')?.value === 'true';

  if (isMigrated && isBetaUser) {
    // Serve the modern React component
    return NextResponse.next();
  }

  // 3. Fallback to Legacy System for un-migrated paths
  const url = new URL(pathname, LEGACY_ORIGIN);
  return NextResponse.rewrite(url);
}

export const config = {
  matcher: [
    /*
     * Match all request paths except for the ones starting with:
     * - api (API routes)
     * - _next/static (static files)
     * - _next/image (image optimization files)
     * - favicon.ico (favicon file)
     */
    '/((?!api|_next/static|_next/image|favicon.ico).*)',
  ],
};

This code demonstrates edge functions routing solving the "all-or-nothing" problem. We can selectively route users based on their cookie status or the specific path. If a path hasn't been modernized yet, the user is transparently proxied to the legacy origin. They never see a "404 Not Found" or a jarring redirect; the URL in their browser stays the same.

Accelerating the "New" Stack with Replay#

The bottleneck in this strategy is usually the speed at which you can create the modern targets for your edge functions to route to. Manual modernization is a grind. Visual Reverse Engineering changes the math. Instead of 40 hours per screen, Replay reduces the time to 4 hours.

When you record a workflow in your legacy app, Replay's AI Automation Suite analyzes the DOM, the CSS, and the data flow. It then generates:

1. •A Clean React Component: No "spaghetti" code, just modern, functional components.
2. •A Design System: Extraction of colors, spacing, and typography into a reusable Replay Library.
3. •Architecture Flows: Documentation of how the legacy screen interacted with back-end APIs.

By using Replay, you can populate your

array significantly faster. This is crucial because 70% of legacy rewrites fail or exceed their timeline. Speed isn't just a luxury; it's a risk mitigation strategy.

Learn more about modernizing legacy systems

Handling Session Persistence Across Generations#

One of the biggest hurdles in edge functions routing solving migration issues is session state. If a user logs into the legacy app, they expect to be logged into the new React app, and vice-versa.

Industry experts recommend using a shared "Session Bridge" at the edge. The edge function can intercept the legacy session cookie, validate it against the legacy database (or a shared Redis cache), and then inject a modern JWT for the React frontend to use.

typescript
// session-bridge.ts - Example of Edge Session Sync
async function syncSession(req: NextRequest) {
  const legacySession = req.cookies.get('ASP.NET_SessionId')?.value;
  
  if (legacySession && !req.cookies.has('modern_auth_token')) {
    // Call an internal microservice to validate legacy session
    const user = await validateLegacySession(legacySession);
    
    if (user) {
      const response = NextResponse.next();
      // Inject modern token for the React app generated by Replay
      response.cookies.set('modern_auth_token', user.jwt, { 
        httpOnly: true, 
        secure: true 
      });
      return response;
    }
  }
  return NextResponse.next();
}

This ensures that as you use edge functions routing solving the path redirection, the user's identity remains consistent. This is particularly vital in regulated industries like Financial Services and Healthcare, where session hijacking or dropped sessions can lead to compliance failures. Replay is built for these environments, offering SOC2 and HIPAA-ready configurations, including on-premise deployment for highly sensitive data.

The Cost of Inaction: Technical Debt vs. Modernization#

The global technical debt of $3.6 trillion isn't just a number; it's the cost of lost agility. Every day your developers spend patching a legacy UI is a day they aren't building new features.

Consider the following:

• •Manual Modernization: 100 screens * 40 hours/screen = 4,000 hours.
• •Replay Modernization: 100 screens * 4 hours/screen = 400 hours.

By saving 3,600 hours, a team of five developers saves nearly 18 months of collective labor. When you combine this efficiency with the precision of edge functions, the ROI is undeniable. You aren't just moving code; you're reclaiming your engineering velocity.

How to extract a Design System from legacy code

SEO and Performance Benefits of Edge Routing#

A common fear with phased migrations is the "SEO suicide" caused by a maze of 301 and 302 redirects. Search engines penalize slow-loading pages and complex redirect chains. Edge functions routing solving the navigation logic happens at the server level (the "edge"), meaning the browser (and the search crawler) receives a 200 OK status immediately.

Because the routing happens at the Point of Presence (PoP) closest to the user, the latency is negligible—often under 10ms. This is far superior to routing traffic back to a central load balancer and then out to a different cloud provider.

According to Replay's analysis, companies that implement edge-based routing see a 30% improvement in Core Web Vitals compared to those using traditional server-side redirects during migrations. This is because the edge can also handle "Image Optimization" and "HTML Rewriting" on the fly, cleaning up legacy output before it reaches the client.

Visual Reverse Engineering: The Missing Link#

Visual Reverse Engineering is the process of using AI to interpret the visual and structural output of a legacy application to recreate its source code in a modern framework.

When you use Replay, you aren't just getting a "screenshot to code" conversion. You are getting a deep structural analysis. Replay’s Blueprints (Editor) allow you to refine the generated code, ensuring it meets your enterprise standards. This high-fidelity output is what makes edge functions routing solving the migration possible; you finally have code that is worth routing to.

Steps to a Successful Phased Migration:#

1. •Inventory: Use Replay to record all critical user flows.
2. •Generate: Convert those flows into a Replay Library of React components.
3. •Deploy: Host the new React application on a modern platform (Vercel, AWS, etc.).
4. •Route: Implement edge functions to steer traffic between the old and new stacks.
5. •Iterate: Gradually move more routes into the
   text

   MIGRATED_ROUTES
   list until the legacy app can be decommissioned.

Frequently Asked Questions#

Does using edge functions for routing increase latency?#

No, in most cases it decreases perceived latency. Because edge functions run at the network edge (closest to the user), the routing decision happens almost instantly. This avoids the "round-trip" penalty of sending a request to a central server only to have it redirected elsewhere.

How does Replay handle complex legacy logic that isn't visible in the UI?#

While Visual Reverse Engineering focuses on the UI and interaction layer, Replay's AI Automation Suite also analyzes network requests and data structures. This allows it to document how the UI interacts with the backend. For complex "hidden" business logic, industry experts recommend keeping that logic in the backend (or migrating it to microservices) while Replay handles the frontend modernization.

Is edge functions routing solving the problem of state management?#

It provides the infrastructure to solve it. By intercepting requests, edge functions can synchronize cookies and headers between the legacy and modern environments. However, you still need a strategy for where your "source of truth" for data lives during the transition period.

Can Replay work with legacy systems that are behind a firewall?#

Yes. Replay offers On-Premise deployment options specifically designed for regulated industries like Government, Telecom, and Manufacturing. You can record and generate code within your own secure environment, ensuring that sensitive data never leaves your network.

What happens if an edge function fails?#

Modern edge platforms have built-in failover mechanisms. If an edge function fails to execute, you can configure a default "fail-open" route that sends the user to the legacy origin by default, ensuring that the application remains accessible even if the migration logic encounters an error.

Conclusion#

The path to modernization doesn't have to be a death march. By leveraging edge functions routing solving the complexities of phased redirection, and using Replay to automate the creation of modern React components, enterprise architects can finally break the cycle of technical debt.

Stop manual documentation. Stop "Big Bang" deployments. Start routing your way to a modern stack, one screen at a time.

Ready to modernize without rewriting? Book a pilot with Replay