Next.js Middleware Deep Dive: A/B Testing and Tenant Routing

What Middleware Is For Next.js middleware runs before a request reaches your route handler—at the edge, before any rendering. It's the right place for: Authentication checks Redirects based on user state Rate limiting A/B testing assignment Geolocation-based routing Request logging Basic Setup // middleware.ts (in root, next to app/) import { NextResponse } from 'next/server'; import type { NextRequest } from 'next/server'; export function middleware(request: NextRequest) { return NextResponse.next(); } // Which paths it runs on export const config = { matcher: [ '/((?!_next/static|_next/image|favicon.ico|public/).*)', ], }; Authentication Middleware import { NextResponse } from 'next/server'; import type { NextRequest } from 'next/server'; import { verifyToken } from './lib/auth'; const PUBLIC_PATHS = ['/login', '/signup', '/api/auth', '/', '/pricing']; export async function middleware(request: NextRequest) { const { pathname } = request.nextUrl; // Allow public paths if (PUBLIC_PATHS