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Gouranga Das Samrat
Gouranga Das Samrat

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Edge Computing

#backend #distributedsystems #systemdesign

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
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  • 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
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  • 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);
  },
};
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🗺️ 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)
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📦 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

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