CDN & Edge Computing

What Is a CDN?

A Content Delivery Network is a globally distributed network of edge servers that cache and serve content from locations close to end users. Instead of every request traveling to your origin server in Virginia, a user in Tokyo gets served from a nearby edge node.

How CDN Requests Work

Flow:

1. User Request — Browser requests an image
2. DNS Resolution — Resolves to nearest PoP
3. Edge Cache Check — Is the asset cached?
4. Cache HIT — Return from edge (fast!)
5. Cache MISS — Fetch from origin server
6. Cache & Serve — Store at edge, return to user

With CDN vs Without CDN

Without CDN	With CDN
All requests hit origin server	90%+ requests served from edge
High latency for distant users	Low latency worldwide
Origin bears full bandwidth cost	Origin bandwidth reduced dramatically
Single point of failure	DDoS protection at the edge
No edge caching	Automatic failover to other PoPs

What to Cache on a CDN

Content Type	Cacheable?	TTL Recommendation
Static images, CSS, JS	Yes	Long (1 year with cache-busting hash)
HTML pages	Sometimes	Short (60s) or stale-while-revalidate
API responses (public)	Sometimes	Short (10-60s) with Vary headers
API responses (personalized)	No	Do not cache — user-specific data
Video/audio streaming	Yes	Long, chunked delivery

Cache-Control Headers

# Cache for 1 year (immutable content with hash in filename)
Cache-Control: public, max-age=31536000, immutable

# Cache for 60 seconds, serve stale while revalidating
Cache-Control: public, max-age=60, stale-while-revalidate=30

# Never cache (personalized API responses)
Cache-Control: private, no-store

# Cache at CDN but not browser
Cache-Control: public, s-maxage=300, max-age=0

Edge Computing

Edge computing takes CDNs further by running application logic at edge nodes, not just serving cached assets. Cloudflare Workers, AWS Lambda@Edge, and Vercel Edge Functions let you execute code in 200+ locations worldwide.

• A/B testing — route users to different variants at the edge
• Authentication — validate JWTs before hitting origin
• Geo-routing — redirect users to region-specific content
• Image optimization — resize and convert formats on the fly

TIP: A good CDN cache hit ratio target is 95%+. If your ratio is below 80%, review your Cache-Control headers and URL structure — query parameters often cause unnecessary cache misses.

Key Takeaways

1. CDNs reduce latency by serving content from geographically closer nodes.
2. Use long TTLs with cache-busting hashes for static assets.
3. Never cache personalized or authenticated responses at the CDN.
4. Edge computing enables running logic at the edge, not just caching.