HLD · Edge

CDN & Content Delivery

Serve static and cacheable content from edge locations close to users.

1. What is a CDN?

A Content Delivery Network is a geographically distributed set of edge servers (PoPs) that cache content from an origin server.

Example: User in Mumbai requests cdn.site.com/logo.png → served from Mumbai PoP (20ms) instead of US origin (300ms).

2. What belongs on a CDN

• Images, videos, CSS, JS bundles
• Static HTML (marketing pages)
• Cacheable API responses (public product catalog) with correct headers

Don't CDN: Personalized authenticated API responses unless carefully keyed and short-TTL.

3. Cache control headers

ETag / Last-Modified: Conditional requests — 304 Not Modified saves bandwidth.

4. Pull vs push CDN

• Pull (most common): CDN fetches from origin on first request, then caches. Origin = S3, your web server.
• Push: You upload content to CDN proactively. Live streaming, large file distribution.

5. Invalidation

When you deploy new JS bundle, old cached version is stale.

• Cache busting: Filename hashing — app.a1b2c3.js (best practice)
• Purge API: Cloudflare/Fastly purge by URL or tag — use sparingly (propagation delay)

6. CDN + dynamic origin pattern

1. Static assets → CDN → S3 origin
2. API → LB → app servers (not CDN-cached)
3. Hybrid: CDN caches GET /api/v1/products with 60s TTL at edge

7. DDoS and TLS

CDNs absorb volumetric attacks at edge. TLS often terminates at CDN; origin can use private network or mTLS.

8. Example: video streaming (YouTube-like)

Upload → origin storage → transcoding → segments to CDN. Playback requests HLS chunks from nearest PoP. Popular videos stay hot in cache.

9. Providers

Cloudflare, Akamai, Fastly, AWS CloudFront, Google Cloud CDN.