Middleware & Routing

10. Middleware & Routing

10.1 What is Middleware?

Middleware runs before every request is processed — at the edge, before the cache is checked. It's the right place for:

// middleware.ts (runs on Edge Runtime)
import { NextResponse } from 'next/server';
import type { NextRequest } from 'next/server';

export function middleware(request: NextRequest) {
  const token = request.cookies.get('auth-token');
  
  // Authentication redirect
  if (!token && request.nextUrl.pathname.startsWith('/dashboard')) {
    return NextResponse.redirect(new URL('/login', request.url));
  }
  
  // A/B testing
  const variant = Math.random() < 0.5 ? 'a' : 'b';
  const response = NextResponse.next();
  response.cookies.set('ab-variant', variant);
  return response;
  
  // Geolocation personalisation
  const country = request.geo?.country;
  if (country === 'DE') {
    return NextResponse.redirect(new URL('/de' + request.nextUrl.pathname, request.url));
  }
}

export const config = {
  matcher: ['/((?!api|_next/static|_next/image|favicon.ico).*)'],
};

10.2 What Middleware should NOT do

• Database queries (no persistent connections in Edge Runtime)
• Complex business logic (35ms CPU limit)
• Large npm packages (size limit: 1MB)
• Caching responses

10.3 Vercel's Routing Middleware (formerly Edge Middleware)

Since mid-2025, Vercel unified "Edge Middleware" and "Edge Functions" under "Vercel Functions using the Edge Runtime." Middleware is now the term for request interception before routing.