Edge Computing

CDN serves static files. Edge computing runs logic at the edge. This is the next level.

---

CDN vs Edge Computing

	CDN	Edge Computing
What it does	Caches static files (HTML, JS, images)	Runs code at edge nodes
Dynamic content	❌ Must go to origin	✅ Can compute at edge
Latency	Low for cache hits	Low even for dynamic responses
Examples	CloudFront, Fastly	Cloudflare Workers, Lambda@Edge, Deno Deploy

---

🧠 Edge Functions

Code that runs in the CDN network, geographically close to users.

Runtimes:

• Cloudflare Workers — V8 isolates, JS/WASM, 0ms cold start
• Lambda@Edge — AWS, triggered by CloudFront, higher cold starts
• Vercel Edge Functions — built on Cloudflare Workers
• Deno Deploy — TypeScript-native

Constraints:

• No filesystem access
• Limited CPU time (1–50ms budgets)
• Limited memory (~128MB)
• No long-running processes
• Some have no Node.js APIs (use Web APIs only)

---

🔑 Use Cases

1. Authentication & Authorization at the Edge

User → Edge Node → check JWT → valid? → forward to origin
                              invalid? → return 401 immediately

• Block unauthenticated requests before they hit your servers
• No round-trip to origin for auth validation
• Reduces load on origin by rejecting bad requests early

2. Personalization

User request hits edge →
  Read cookie / geo / device header →
  Rewrite URL or inject headers →
  Serve personalized variant

• A/B testing without origin involvement
• Locale-based content (language, currency)
• Device-based content (mobile vs desktop)

3. Bot Detection & Rate Limiting

• Block bots at the edge before they consume origin resources
• Cloudflare's Bot Management runs entirely at edge

4. Request/Response Transformation

• Add security headers (CSP, HSTS) to every response
• Strip sensitive headers before forwarding
• Rewrite URLs for legacy compatibility

5. Geo-Based Redirects

// Cloudflare Worker example
export default {
  async fetch(request) {
    const country = request.cf.country;
    if (country === "DE") {
      return Response.redirect("https://de.example.com" + request.url);
    }
    return fetch(request);
  },
};

---

🗺️ Edge vs Origin Decision Tree

Is the response the same for all users?
  YES → CDN cache it (static)
  NO  → Does personalization need server state/DB?
          NO  → Edge function (compute from request headers/cookies)
          YES → Origin server (but edge can cache origin response per variant)

---

📦 Edge KV Storage

Some edge platforms offer distributed key-value stores accessible from edge:

• Cloudflare KV — globally replicated, eventual consistency, low-latency reads
• Cloudflare Durable Objects — strongly consistent, single-region, great for coordination

Use for:

• Feature flags
• Rate limit counters (approximate)
• Session data
• Config that changes infrequently

---

⚖️ Trade-offs

✅ Pros

• Sub-10ms response for many use cases
• Reduces load and cost on origin servers
• No infrastructure to manage (serverless)
• Global presence instantly

❌ Cons

• No access to databases or internal services directly
• Limited runtime environment (no full Node.js)
• Debugging is harder (distributed, remote)
• Cold start exists (minimal but nonzero for Lambda@Edge)
• Vendor lock-in (Cloudflare Workers API ≠ Lambda@Edge API)

⚖️ When to use

• Auth validation, bot blocking, geo-redirects
• A/B testing, personalization headers
• Performance-critical responses that don't need DB

When NOT to use

• Complex business logic requiring DB queries
• Long-running operations
• When you need full OS / filesystem access

---