Use this skill when deploying edge functions, writing Cloudflare Workers, configuring CDN cache logic, optimizing latency with edge-side processing, or building serverless-at-the-edge architectures. Triggers on edge functions, CDN rules, Cloudflare Workers, Deno Deploy, Vercel Edge Functions, Lambda@Edge, cache headers, geo-routing, and any task requiring computation close to the user.
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Use this skill when deploying edge functions, writing Cloudflare Workers, configuring CDN cache logic, optimizing latency with edge-side processing, or building serverless-at-the-edge architectures. Triggers on edge functions, CDN rules, Cloudflare Workers, Deno Deploy, Vercel Edge Functions, Lambda@Edge, cache headers, geo-routing, and any task requiring computation close to the user.
When this skill is activated, always start your first response with the 🧢 emoji.
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
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.
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.
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.
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.
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:
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:
functionsetCacheHeaders(response: Response, type: 'static' | 'dynamic' | 'api'): Response {
const headers = newHeaders(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;
}
returnnewResponse(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:
Split traffic without client-side JavaScript or origin involvement:
exportdefault {
asyncfetch(request: Request, env: Env): Promise<Response> {
const url = newURL(request.url);
// Sticky assignment via cookielet variant = getCookie(request, 'ab-variant');
if (!variant) {
variant = Math.random() < 0.5 ? 'control' : 'experiment';
}
// Rewrite to variant-specific origin pathif (variant === 'experiment' && url.pathname === '/pricing') {
url.pathname = '/pricing-v2';
}
const response = awaitfetch(url.toString(), request);
const newResponse = newResponse(response.body, response);
// Set sticky cookie so user stays in same variant
newResponse.headers.append('Set-Cookie', `ab-variant=${variant}; Path=/; Max-Age=86400`);
// Vary on cookie to prevent cache mixing variants
newResponse.headers.set('Vary', 'Cookie');
return newResponse;
},
};
functiongetCookie(request: Request, name: string): string | null {
const cookies = request.headers.get('Cookie') ?? '';
const match = cookies.match(newRegExp(`${name}=([^;]+)`));
return match ? match[1] : null;
}
Optimize cold starts and execution time
Minimize startup cost and stay within CPU limits:
// Hoist expensive initialization outside the fetch handler// This runs once per isolate, not per requestconst decoder = newTextDecoder();
const encoder = newTextEncoder();
constSTATIC_CONFIG = { version: '1.0', maxRetries: 3 };
exportdefault {
asyncfetch(request: Request, env: Env, ctx: ExecutionContext): Promise<Response> {
const start = Date.now();
// Use streaming to reduce memory pressure and TTFBconst originResponse = awaitfetch('https://api.example.com/data');
const { readable, writable } = newTransformStream();
// Non-blocking: pipe transform in background
ctx.waitUntil(transformStream(originResponse.body!, writable));
// Log timing without blocking response
ctx.waitUntil(
Promise.resolve().then(() => {
console.log(`Request processed in ${Date.now() - start}ms`);
})
);
returnnewResponse(readable, {
headers: { 'Content-Type': 'application/json' },
});
},
};
asyncfunctiontransformStream(input: ReadableStream, output: WritableStream): Promise<void> {
const reader = input.getReader();
const writer = output.getWriter();
try {
while (true) {
const { done, value } = await reader.read();
if (done) break;
await writer.write(value);
}
} finally {
await writer.close();
}
}
Anti-patterns / common mistakes
Mistake
Why it's wrong
What to do instead
Using Node.js APIs in edge functions
Edge runtimes are V8 isolates, not Node.js - fs, path, Buffer global are unavailable
Use Web Platform APIs: fetch, Request, Response, TextEncoder, crypto.subtle
Caching personalized responses without Vary
User A sees User B's dashboard; cache poisoning at scale
Always set Vary: Cookie or Vary: Authorization on personalized responses
Storing mutable state in KV for counters
KV is eventually consistent - concurrent increments lose writes silently
Use Durable Objects for counters, locks, and any read-modify-write patterns
Catching all errors silently at the edge
Origin never sees the request; debugging becomes impossible
Fail open - on error, pass request through to origin and log the error via ctx.waitUntil
Putting entire app logic in a single Worker
Hits CPU time limits; becomes unmaintainable; defeats the purpose of edge (simple, fast)
Keep edge logic thin: routing, caching, auth checks, transforms. Heavy logic stays at origin
Ignoring cache key design
Default cache keys cause low hit rates for URLs with query params or headers
Explicitly define cache keys to strip unnecessary query params and normalize URLs
Gotchas
ctx.waitUntil() is required for async work after Response is returned - Any await after you return a Response in a Cloudflare Worker is silently dropped. Logging, analytics calls, and cache writes that happen post-response must be wrapped in ctx.waitUntil(promise) or they never execute.
Cloudflare KV has ~60 second eventual consistency - don't use it for flags that must take effect immediately - A KV write to disable a feature or block a user may take up to a minute to propagate across all PoPs. If you need instant effect (rate limiting, auth revocation), use Durable Objects, not KV.
Vary: Cookie on cached responses causes catastrophic cache fragmentation - Setting Vary: Cookie tells CDNs to cache a separate copy for every unique Cookie header value. Most users have unique session cookies, effectively making your cache useless. Instead, strip the cookie from the cache key and use a separate Vary value that identifies the variant (e.g., a normalized A/B bucket cookie).
Edge functions can't use Node.js built-ins even if they're in node_modules - A library that uses require('crypto'), require('buffer'), or require('path') will fail at runtime in a V8 isolate even though the import succeeds at build time. Audit all dependencies for Node.js API usage before deploying to edge.
A/B test cookie without Vary: Cookie on the response causes cache mixing - If you set an ab-variant cookie but don't set Vary: Cookie (or a more targeted Vary), CDN caches may serve one variant's cached response to users assigned the other variant. Always pair sticky cookies with appropriate Vary headers.
References
Load the relevant reference file only when the current task requires it:
references/cloudflare-workers.md - Cloudflare Workers API reference, wrangler CLI,
bindings (KV, R2, D1, Durable Objects), and deployment patterns
references/cdn-caching.md - Cache-Control directives, surrogate keys, cache tiers,
invalidation strategies, and CDN-specific headers across providers
references/latency-optimization.md - TTFB reduction techniques, connection reuse,
edge-side includes, streaming responses, and RUM measurement
Companion check
On first activation of this skill in a conversation: check which companion skills are installed by running ls ~/.claude/skills/ ~/.agent/skills/ ~/.agents/skills/ .claude/skills/ .agent/skills/ .agents/skills/ 2>/dev/null. Compare the results against the recommended_skills field in this file's frontmatter. For any that are missing, mention them once and offer to install:
