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Edge Computing

A comprehensive skill for building, deploying, and optimizing applications that run at the network edge - close to end users rather than in centralized data centers. This covers the full edge stack: writing Cloudflare Workers and Deno Deploy functions, configuring CDN cache rules and invalidation, implementing geo-routing and A/B testing at the edge, and systematically reducing latency through edge-side processing. The core principle is to move computation to where the user is, not the other way around.

When to use this skill

Trigger this skill when the user:

• Wants to write or debug a Cloudflare Worker, Deno Deploy function, or Vercel Edge Function
• Needs to configure CDN cache headers, cache keys, or invalidation strategies
• Is implementing geo-routing, A/B testing, or feature flags at the edge
• Wants to reduce TTFB or latency by moving logic closer to users
• Needs to transform requests or responses at the CDN layer
• Is working with edge-side KV stores, Durable Objects, or D1 databases
• Wants to implement authentication, rate limiting, or bot protection at the edge
• Is debugging cold start times or execution limits in edge runtimes

Do NOT trigger this skill for:

• General serverless architecture with traditional Lambda/Cloud Functions (use cloud-aws or cloud-gcp skill)
• Full backend API design that belongs in a centralized server (use backend-engineering skill)

Key principles

1. Edge is not a server - respect the constraints - Edge runtimes use V8 isolates, not Node.js. No filesystem access, limited CPU time (typically 10-50ms for free tiers), restricted APIs (no eval, no native modules). Design for these constraints from the start rather than porting server code and hoping it works.

2. Cache aggressively, invalidate precisely - The fastest request is one that never reaches your origin. Set long Cache-Control max-age on immutable assets, use stale-while-revalidate for dynamic content, and implement surgical cache purging by surrogate key or tag rather than full-site flushes.

3. Minimize origin round-trips - Every request back to origin adds 50-200ms of latency. Use edge KV stores for read-heavy data, coalesce multiple origin fetches with Promise.all, and implement request collapsing so concurrent identical requests share a single origin fetch.

4. Fail open, not closed - When the edge function errors or times out, fall through to the origin server rather than showing an error page. Edge logic should enhance performance, not become a single point of failure.

5. Measure from the user's perspective - TTFB measured from your data center is meaningless. Use Real User Monitoring (RUM) with geographic breakdowns to understand actual latency. Synthetic tests from a single region miss the whole point of edge.

Core concepts

V8 isolates vs containers - Edge platforms like Cloudflare Workers use V8 isolates instead of containers. An isolate starts in under 5ms (vs 50-500ms for a cold container), shares a single process with other isolates, and has hard memory limits (~128MB). This architecture enables near-zero cold starts but restricts you to Web Platform APIs only.

Edge locations and PoPs - A Point of Presence (PoP) is a physical data center in the CDN network. Cloudflare has 300+ PoPs, AWS CloudFront has 400+. Your edge code runs at whichever PoP is geographically closest to the requesting user. Understanding PoP distribution matters for cache hit ratios - more PoPs means more cache fragmentation.

Cache tiers - Most CDNs use a tiered caching architecture: L1 (edge PoP closest to user) -> L2 (regional shield/tier) -> Origin. The L2 tier reduces origin load by coalescing requests from multiple L1 PoPs. Configure cache tiers explicitly when available (Cloudflare Tiered Cache, CloudFront Origin Shield).

Edge KV and state - Edge is inherently stateless per-request, but platforms provide persistence layers: Cloudflare KV (eventually consistent, read-optimized), Durable Objects (strongly consistent, single-point coordination), D1 (SQLite at the edge), and R2 (S3-compatible object storage). Choose based on consistency requirements and read/write ratio.

Request lifecycle at the edge - Incoming request -> DNS resolution -> nearest PoP -> edge function executes -> checks cache -> (cache miss) fetches from origin -> transforms response -> caches result -> returns to client. Understanding this flow is essential for placing logic at the right phase.

Common tasks

Write a Cloudflare Worker

Basic request/response handler using the Workers API:

export default {
  async fetch(request: Request, env: Env, ctx: ExecutionContext): Promise<Response> {
    const url = new URL(request.url);

    // Route handling
    if (url.pathname === '/api/health') {
      return new Response('OK', { status: 200 });
    }

    // Fetch from origin and transform
    const response = await fetch(request);
    const html = await response.text();
    const modified = html.replace('</head>', '<script src="/analytics.js"></script></head>');

    return new Response(modified, {
      status: response.status,
      headers: response.headers,
    });
  },
};

Workers have a 10ms CPU time limit on the free plan (50ms on paid). Use ctx.waitUntil() for non-blocking async work like logging that should not block the response.

Configure cache headers for optimal CDN behavior

Set cache-control headers that balance freshness with performance:

function setCacheHeaders(response: Response, type: 'static' | 'dynamic' | 'api'): Response {
  const headers = new Headers(response.headers);

  switch (type) {
    case 'static':
      // Immutable assets with content hash in filename
      headers.set('Cache-Control', 'public, max-age=31536000, immutable');
      break;
    case 'dynamic':
      // HTML pages - serve stale while revalidating in background
      headers.set('Cache-Control', 'public, max-age=60, stale-while-revalidate=86400');
      headers.set('Surrogate-Key', 'page-content');
      break;
    case 'api':
      // API responses - short cache with revalidation
      headers.set('Cache-Control', 'public, max-age=5, stale-while-revalidate=30');
      headers.set('Vary', 'Authorization, Accept');
      break;
  }

  return new Response(response.body, { status: response.status, headers });
}

Always set Vary headers for responses that change based on request headers (e.g., Accept-Encoding, Authorization). Missing Vary headers cause cache poisoning where one user gets another's personalized response.

Implement geo-routing at the edge

Route users to region-specific content or origins based on their location:

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    const country = request.headers.get('CF-IPCountry') ?? 'US';
    const continent = request.cf?.continent ?? 'NA';

    // Route to nearest regional origin
    const origins: Record<string, string> = {
      EU: 'https://eu.api.example.com',
      AS: 'https://ap.api.example.com',
      NA: 'https://us.api.example.com',
    };
    const origin = origins[continent] ?? origins['NA'];

    // GDPR compliance - block or redirect EU users to compliant flow
    if (continent === 'EU' && new URL(request.url).pathname.startsWith('/track')) {
      return new Response('Tracking disabled in EU', { status: 451 });
    }

    const url = new URL(request.url);
    url.hostname = new URL(origin).hostname;
    return fetch(url.toString(), request);
  },
};

Use edge KV for read-heavy data

Store configuration, feature flags, or lookup tables in Cloudflare KV:

interface Env {
  CONFIG_KV: KVNamespace;
}

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    // Re