Think of two types of shops:

• Edge Runtime = A vending machine. Small, fast, everywhere — but only does simple tasks (take money, give product).
• Node.js Runtime = A full supermarket. Big, slower, located further away — but can do everything (fresh produce, deli counter, bakery).

The Edge Runtime runs your code at CDN edge nodes — hundreds of locations worldwide, very close to users. It uses a lightweight V8 isolate (not full Node.js).

export const runtime = "edge"; // 👈 Run at the edge

export async function GET(request: Request) {
  const { geo } = request as any;

  return Response.json({
    country: geo?.country ?? "Unknown",
    city: geo?.city ?? "Unknown",
    latency: "sub-10ms",
  });
}

• Middleware (always runs on Edge)
• Route handlers with export const runtime = "edge"
• Some page rendering (limited)

flowchart TB
    Q{"What does your\ncode need to do?"}
    Q -->|"Simple checks\\n(auth, redirects, geo)\"| Edge["⚡ Edge Runtime\nFast, global, lightweight"]
    Q -->|"Complex work\\n(DB queries, file ops)\"| Node["🖥️ Node.js Runtime\nFull power, no limits"]

    Edge --> EM["Middleware\nURL rewrites\nAuth checks\nGeo redirects"]
    Edge --> ER["Edge API Routes\nGeo lookups\nQuick transformations"]

    Node --> NH["Heavy API Routes\nDatabase operations\nFile processing"]
    Node --> NP["Pages with DB data\nComplex server logic"]

    style Q fill:#f59e0b,color:#000
    style Edge fill:#7c3aed,color:#fff
    style Node fill:#059669,color:#fff
    style EM fill:#9333ea,color:#fff
    style ER fill:#9333ea,color:#fff
    style NH fill:#4f46e5,color:#fff
    style NP fill:#4f46e5,color:#fff

You cannot use these in Edge Runtime:

Middleware bundles must be under 1MB. Heavy libraries will fail the build.

• Edge Runtime = fast, runs on CDN nodes worldwide, but limited capabilities (no file system, no DB)
• Node.js Runtime = full server capabilities, runs in one region, but more powerful
• Middleware always runs on Edge — keep it fast and lightweight
• Use Edge for auth checks, redirects, geo-detection; use Node.js for DB queries, file operations, heavy computations
• If you’re not sure, start with Node.js — it can do everything Edge can, just slower globally