How Next.js 15 Middleware Handles Auth, Rate Limiting, and Edge Logic

Next.js 15 middleware is a single edge-layer function that intercepts every matching request before it reaches any route handler, enabling developers to centralise authentication, rate limiting, security headers, and more in one place. Despite its versatility, many teams use it only for basic authentication and scatter other concerns across individual API routes. A detailed developer guide outlines production-ready patterns including Supabase JWT verification with automatic token refresh, Upstash Redis-backed sliding-window rate limiting, geolocation handling, bot blocking, and CSP header management. The guide notes that all patterns run on the Vercel Edge Runtime, though most are portable to other deployment targets, with geolocation via request headers being a Vercel-specific feature. Proper matcher configuration is highlighted as critical, as middleware executes on every matched path and should explicitly exclude static assets to avoid unnecessary overhead.