Edge Functions: What They Are, When to Use Them, and When Not To

// This works in Node.js but NOT at the edge
import fs from "fs";
import { createConnection } from "net";
import { execSync } from "child_process";
 
const file = fs.readFileSync("/etc/hosts");        // ❌ No filesystem
const conn = createConnection({ port: 5432 });     // ❌ No raw TCP
const result = execSync("ls -la");                  // ❌ No child processes
process.env.DATABASE_URL;                           // ⚠️  Available but static, set at deploy time

// These all work at the edge
const response = await fetch("https://api.example.com/data");
const url = new URL(request.url);
const headers = new Headers({ "Content-Type": "application/json" });
const encoder = new TextEncoder();
const encoded = encoder.encode("hello");
const hash = await crypto.subtle.digest("SHA-256", encoded);
const id = crypto.randomUUID();
 
// Web Streams API
const stream = new ReadableStream({
  start(controller) {
    controller.enqueue("chunk 1");
    controller.enqueue("chunk 2");
    controller.close();
  },
});
 
// Cache API
const cache = caches.default;
await cache.put(request, response.clone());

// ❌ Don't do this at the edge
import dayjs from "dayjs";
import * as yup from "yup";
import lodash from "lodash";
 
// ✅ Do this instead — use built-in APIs
const date = new Date().toISOString();
const isValid = typeof input === "string" && input.length < 200;
const unique = [...new Set(items)];

// middleware.ts — runs at the edge on every request
import { NextRequest, NextResponse } from "next/server";
 
export const config = {
  matcher: ["/", "/shop/:path*"],
};
 
export function middleware(request: NextRequest) {
  const country = request.geo?.country ?? "US";
  const city = request.geo?.city ?? "Unknown";
  const region = request.geo?.region ?? "Unknown";
 
  // Redirect to country-specific store
  if (request.nextUrl.pathname === "/shop") {
    const storeMap: Record<string, string> = {
      DE: "/shop/eu",
      FR: "/shop/eu",
      GB: "/shop/uk",
      JP: "/shop/jp",
      TR: "/shop/tr",
    };
 
    const storePath = storeMap[country] ?? "/shop/us";
    if (request.nextUrl.pathname !== storePath) {
      return NextResponse.redirect(new URL(storePath, request.url));
    }
  }
 
  // Add geo headers for downstream use
  const response = NextResponse.next();
  response.headers.set("x-user-country", country);
  response.headers.set("x-user-city", city);
  response.headers.set("x-user-region", region);
  return response;
}

import { NextRequest, NextResponse } from "next/server";
 
export function middleware(request: NextRequest) {
  // Check if user already has a variant assignment
  const existingVariant = request.cookies.get("ab-variant")?.value;
 
  if (existingVariant) {
    // Rewrite to the correct variant page
    const url = request.nextUrl.clone();
    url.pathname = `/variants/${existingVariant}${url.pathname}`;
    return NextResponse.rewrite(url);
  }
 
  // Assign a new variant (50/50 split)
  const variant = Math.random() < 0.5 ? "control" : "treatment";
 
  const url = request.nextUrl.clone();
  url.pathname = `/variants/${variant}${url.pathname}`;
 
  const response = NextResponse.rewrite(url);
  response.cookies.set("ab-variant", variant, {
    maxAge: 60 * 60 * 24 * 30, // 30 days
    httpOnly: true,
    sameSite: "lax",
  });
 
  return response;
}

import { jwtVerify, importSPKI } from "jose";
import { NextRequest, NextResponse } from "next/server";
 
const PUBLIC_KEY = `-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA...
-----END PUBLIC KEY-----`;
 
export async function middleware(request: NextRequest) {
  const token = request.cookies.get("session-token")?.value;
 
  if (!token) {
    return NextResponse.redirect(new URL("/login", request.url));
  }
 
  try {
    const key = await importSPKI(PUBLIC_KEY, "RS256");
    const { payload } = await jwtVerify(token, key, {
      algorithms: ["RS256"],
      issuer: "https://auth.example.com",
    });
 
    // Pass user info downstream as headers
    const response = NextResponse.next();
    response.headers.set("x-user-id", payload.sub as string);
    response.headers.set("x-user-role", payload.role as string);
    return response;
  } catch {
    // Token expired or invalid
    const response = NextResponse.redirect(new URL("/login", request.url));
    response.cookies.delete("session-token");
    return response;
  }
}

import { NextRequest, NextResponse } from "next/server";
 
// Simple in-memory rate limiter (per edge location)
const rateLimitMap = new Map<string, { count: number; timestamp: number }>();
 
export function middleware(request: NextRequest) {
  const ip = request.headers.get("x-forwarded-for")?.split(",").pop()?.trim()
    ?? "unknown";
  const ua = request.headers.get("user-agent") ?? "";
 
  // Block known bad bots
  const badBots = ["AhrefsBot", "SemrushBot", "MJ12bot", "DotBot"];
  if (badBots.some((bot) => ua.includes(bot))) {
    return new NextResponse("Forbidden", { status: 403 });
  }
 
  // Simple rate limiting
  const now = Date.now();
  const windowMs = 60_000; // 1 minute
  const maxRequests = 100;
 
  const entry = rateLimitMap.get(ip);
  if (entry && now - entry.timestamp < windowMs) {
    entry.count++;
    if (entry.count > maxRequests) {
      return new NextResponse("Too Many Requests", {
        status: 429,
        headers: { "Retry-After": "60" },
      });
    }
  } else {
    rateLimitMap.set(ip, { count: 1, timestamp: now });
  }
 
  // Periodic cleanup to prevent memory leak
  if (rateLimitMap.size > 10_000) {
    const cutoff = now - windowMs;
    for (const [key, val] of rateLimitMap) {
      if (val.timestamp < cutoff) rateLimitMap.delete(key);
    }
  }
 
  return NextResponse.next();
}

import { NextRequest, NextResponse } from "next/server";
 
export function middleware(request: NextRequest) {
  const response = NextResponse.next();
  const url = request.nextUrl;
 
  // Device-based content negotiation
  const ua = request.headers.get("user-agent") ?? "";
  const isMobile = /mobile|android|iphone/i.test(ua);
  response.headers.set("x-device-type", isMobile ? "mobile" : "desktop");
 
  // Feature flags from cookie
  const flags = request.cookies.get("feature-flags")?.value;
  if (flags) {
    response.headers.set("x-feature-flags", flags);
  }
 
  // Locale detection for i18n
  const acceptLanguage = request.headers.get("accept-language") ?? "en";
  const preferredLocale = acceptLanguage.split(",")[0]?.split("-")[0] ?? "en";
  const supportedLocales = [
    "en", "tr", "de", "fr", "es", "pt", "ja", "ko", "it",
    "nl", "ru", "pl", "uk", "sv", "cs", "ar", "hi", "zh",
  ];
  const locale = supportedLocales.includes(preferredLocale)
    ? preferredLocale
    : "en";
 
  if (!url.pathname.startsWith(`/${locale}`) && !url.pathname.startsWith("/api")) {
    return NextResponse.redirect(new URL(`/${locale}${url.pathname}`, request.url));
  }
 
  return response;
}

// ❌ This pattern fundamentally doesn't work at the edge
import { Pool } from "pg";
 
const pool = new Pool({
  connectionString: process.env.DATABASE_URL,
  max: 10, // Connection pool of 10
});
 
// Each edge invocation would:
// 1. Create a new Pool (can't reuse across invocations reliably)
// 2. Open a TCP connection to your database (which is in us-east-1, not at the edge)
// 3. Do TLS handshake with the database
// 4. Run the query
// 5. Discard the connection when the isolate recycles
 
// Even with connection pooling services like PgBouncer,
// you're still paying the network latency from edge → origin database

// ✅ This works at the edge (HTTP-based database access)
// But it's still slow if the database is far from the edge node
import { neon } from "@neondatabase/serverless";
 
export const runtime = "edge";
 
export async function GET(request: Request) {
  const sql = neon(process.env.DATABASE_URL!);
  // This makes an HTTP request to your Neon database
  // Works, but latency depends on distance to the database region
  const posts = await sql`SELECT * FROM posts WHERE published = true LIMIT 10`;
  return Response.json(posts);
}

// ❌ These will exceed CPU time limits at the edge
export const runtime = "edge";
 
export async function POST(request: Request) {
  const data = await request.json();
 
  // Image processing — CPU intensive
  // (Also can't use sharp because it's a native module)
  const processed = heavyImageProcessing(data.image);
 
  // PDF generation — CPU intensive + needs Node.js APIs
  const pdf = generatePDF(data.content);
 
  // Large data transformation
  const result = data.items // 100,000 items
    .map(transform)
    .filter(validate)
    .sort(compare)
    .reduce(aggregate, {});
 
  return Response.json(result);
}

// ❌ These packages won't work at the edge
import bcrypt from "bcrypt";            // Native C++ binding
import sharp from "sharp";              // Native C++ binding
import puppeteer from "puppeteer";      // Needs filesystem + child_process
import nodemailer from "nodemailer";    // Needs net module
import { readFile } from "fs/promises"; // Node.js filesystem API
import mongoose from "mongoose";         // TCP connections + Node.js APIs
 
// ✅ Edge-compatible alternatives
import { hashSync } from "bcryptjs";    // Pure JS implementation (slower)
// For images: use a separate service or API
// For email: use an HTTP-based email API (Resend, SendGrid REST)
// For database: use HTTP-based clients

// ❌ Barrel file imports can pull in way too much
import { validateEmail } from "@/lib/utils";
// If utils.ts re-exports from 20 other modules, all of them get bundled
 
// ✅ Import directly from the source
import { validateEmail } from "@/lib/validators/email";

// middleware.ts — always runs at the edge, no opt-in needed
import { NextRequest, NextResponse } from "next/server";
 
export function middleware(request: NextRequest) {
  // This runs before every matching request
  // Keep it fast — no database calls, no heavy computation
  return NextResponse.next();
}
 
export const config = {
  // Only run on specific paths
  matcher: [
    "/((?!_next/static|_next/image|favicon.ico|robots.txt|sitemap.xml).*)",
  ],
};

// app/api/hello/route.ts
export const runtime = "edge"; // This one line changes the runtime
 
export async function GET(request: Request) {
  return Response.json({
    message: "Hello from the edge",
    region: process.env.VERCEL_REGION ?? "unknown",
    timestamp: Date.now(),
  });
}

// app/dashboard/page.tsx
export const runtime = "edge";
 
export default async function DashboardPage() {
  // Remember: no Node.js APIs here
  // Any data fetching must use fetch() or edge-compatible clients
  const data = await fetch("https://api.example.com/dashboard", {
    headers: { Authorization: `Bearer ${process.env.API_KEY}` },
    next: { revalidate: 60 },
  }).then((r) => r.json());
 
  return (
    <main>
      <h1>Dashboard</h1>
      {/* render data */}
    </main>
  );
}

// ✅ Available at the edge
fetch()                          // HTTP requests
Request / Response               // Web standard request/response
Headers                          // HTTP headers
URL / URLSearchParams            // URL parsing
TextEncoder / TextDecoder        // String encoding
crypto.subtle                    // Crypto operations (signing, hashing)
crypto.randomUUID()              // UUID generation
crypto.getRandomValues()         // Cryptographic random numbers
structuredClone()                // Deep cloning
atob() / btoa()                  // Base64 encoding/decoding
setTimeout() / setInterval()     // Timers (but remember CPU limits)
console.log()                    // Logging
ReadableStream / WritableStream  // Streaming
AbortController / AbortSignal    // Request cancellation
URLPattern                       // URL pattern matching
 
// ❌ NOT available at the edge
require()                        // CommonJS (use import)
fs / path / os                   // Node.js built-in modules
process.exit()                   // Process control
Buffer                           // Use Uint8Array instead
__dirname / __filename           // Use import.meta.url
setImmediate()                   // Not a web standard

// lib/edge-auth.ts — Edge-compatible auth utilities
import { jwtVerify, SignJWT, importSPKI, importPKCS8 } from "jose";
 
const PUBLIC_KEY_PEM = process.env.JWT_PUBLIC_KEY!;
const ISSUER = "https://auth.myapp.com";
const AUDIENCE = "https://myapp.com";
 
export interface TokenPayload {
  sub: string;
  email: string;
  role: "user" | "admin" | "moderator";
  iat: number;
  exp: number;
}
 
export async function verifyToken(token: string): Promise<TokenPayload | null> {
  try {
    const publicKey = await importSPKI(PUBLIC_KEY_PEM, "RS256");
    const { payload } = await jwtVerify(token, publicKey, {
      algorithms: ["RS256"],
      issuer: ISSUER,
      audience: AUDIENCE,
      clockTolerance: 30, // 30 seconds of clock skew tolerance
    });
 
    return payload as unknown as TokenPayload;
  } catch {
    return null;
  }
}
 
export function isTokenExpiringSoon(payload: TokenPayload): boolean {
  const now = Math.floor(Date.now() / 1000);
  const fiveMinutes = 5 * 60;
  return payload.exp - now < fiveMinutes;
}

// middleware.ts — The auth middleware
import { NextRequest, NextResponse } from "next/server";
import { verifyToken, isTokenExpiringSoon } from "./lib/edge-auth";
 
const PUBLIC_PATHS = ["/", "/login", "/register", "/api/auth/login"];
const ADMIN_PATHS = ["/admin"];
 
export async function middleware(request: NextRequest) {
  const { pathname } = request.nextUrl;
 
  // Skip auth for public paths
  if (PUBLIC_PATHS.some((p) => pathname === p || pathname.startsWith("/api/public"))) {
    return NextResponse.next();
  }
 
  // Extract token
  const token = request.cookies.get("auth-token")?.value;
  if (!token) {
    return NextResponse.redirect(new URL("/login", request.url));
  }
 
  // Verify token (pure crypto — no database call)
  const payload = await verifyToken(token);
  if (!payload) {
    const response = NextResponse.redirect(new URL("/login", request.url));
    response.cookies.delete("auth-token");
    return response;
  }
 
  // Role-based access control
  if (ADMIN_PATHS.some((p) => pathname.startsWith(p)) && payload.role !== "admin") {
    return NextResponse.redirect(new URL("/unauthorized", request.url));
  }
 
  // Pass verified user info to the origin as trusted headers
  const response = NextResponse.next();
  response.headers.set("x-user-id", payload.sub);
  response.headers.set("x-user-email", payload.email);
  response.headers.set("x-user-role", payload.role);
 
  // Signal if token needs refresh
  if (isTokenExpiringSoon(payload)) {
    response.headers.set("x-token-refresh", "true");
  }
 
  return response;
}

// app/api/profile/route.ts — Origin server reads trusted headers
export async function GET(request: Request) {
  // These headers were set by edge middleware after JWT verification
  // They're trusted because they come from our own infrastructure
  const userId = request.headers.get("x-user-id");
  const userRole = request.headers.get("x-user-role");
 
  if (!userId) {
    return Response.json({ error: "Unauthorized" }, { status: 401 });
  }
 
  // Now we can hit the database — we're on the origin server,
  // right next to the database, with a connection pool
  const user = await db.user.findUnique({ where: { id: userId } });
 
  return Response.json(user);
}

// app/api/products/route.ts
export const runtime = "edge";
 
export async function GET(request: Request) {
  const url = new URL(request.url);
  const category = url.searchParams.get("category") ?? "all";
 
  const products = await fetch(
    `${process.env.ORIGIN_API}/products?category=${category}`,
  ).then((r) => r.json());
 
  return Response.json(products, {
    headers: {
      // Cache on CDN for 60 seconds
      // Serve stale while revalidating for up to 5 minutes
      // Client can cache for 10 seconds
      "Cache-Control": "public, s-maxage=60, stale-while-revalidate=300, max-age=10",
 
      // Vary by these headers so different variants get different cache entries
      Vary: "Accept-Language, Accept-Encoding",
 
      // CDN-specific cache tag for targeted invalidation
      "Cache-Tag": `products,category-${category}`,
    },
  });
}

import { get } from "@vercel/edge-config";
import { NextRequest, NextResponse } from "next/server";
 
export async function middleware(request: NextRequest) {
  // Edge Config reads are extremely fast (~1ms) because
  // the data is replicated to every edge location
  const maintenanceMode = await get<boolean>("maintenance_mode");
 
  if (maintenanceMode) {
    return NextResponse.rewrite(new URL("/maintenance", request.url));
  }
 
  // Feature flags
  const features = await get<Record<string, boolean>>("feature_flags");
  if (features?.["new_pricing_page"] && request.nextUrl.pathname === "/pricing") {
    return NextResponse.rewrite(new URL("/pricing-v2", request.url));
  }
 
  // Dynamic redirects (update redirects without redeploy)
  const redirects = await get<Array<{ from: string; to: string; permanent: boolean }>>(
    "redirects",
  );
 
  if (redirects) {
    const match = redirects.find((r) => r.from === request.nextUrl.pathname);
    if (match) {
      return NextResponse.redirect(
        new URL(match.to, request.url),
        match.permanent ? 308 : 307,
      );
    }
  }
 
  return NextResponse.next();
}

// ✅ PERFECT for edge
// - Middleware (auth, redirects, rewrites, headers)
// - Geolocation logic
// - A/B test assignment
// - Bot detection / WAF rules
// - Cache-friendly API responses
// - Feature flag checks
// - CORS preflight responses
// - Static data transformations (no DB)
// - Webhook signature verification
 
// ❌ KEEP on Node.js regional
// - Database CRUD operations
// - File uploads / processing
// - Image manipulation
// - PDF generation
// - Email sending (use HTTP API, but still regional)
// - WebSocket servers
// - Background jobs / queues
// - Anything using native npm packages
// - SSR pages with database queries
// - GraphQL resolvers that hit databases
 
// 🤔 IT DEPENDS
// - Authentication (edge for JWT, regional for session-DB)
// - API routes (edge if no DB, regional if DB)
// - Server-rendered pages (edge if data comes from cache/fetch, regional if DB)
// - Real-time features (edge for initial auth, regional for persistent connections)