Content Delivery Networks Explained

A CDN distributes content to edge servers globally, delivering it from the location closest to the user.

Why CDN?

Without CDN:
User (Mumbai) → Server (US-East) = ~200ms

With CDN:
User (Mumbai) → CDN Edge (Mumbai) = ~5ms

How CDN Works

Cache Miss (First Request)

User → CDN Edge → Origin Server
                 ↓
           CDN caches response
           Return to user

Cache Hit (Subsequent Requests)

User → CDN Edge → Return cached response

Cache Control Headers

Cache-Control: public, max-age=86400, stale-while-revalidate=3600
ETag: "abc123"

• max-age: Cache for 24 hours
• stale-while-revalidate: Serve stale while fetching fresh
• ETag: Content fingerprint for validation

Cache Invalidation

1. URL versioning: /app.v2.js (recommended)
2. API invalidation: CDN API to purge specific URLs
3. TTL: Wait for expiry

CDN for Dynamic Content

• Edge Side Includes (ESI): Cache fragments of pages
• Cloudflare Workers: Run logic at the edge
• Lambda@Edge: AWS edge functions

Push vs Pull CDN

Pull CDN: Fetches content from origin on demand (most common)
Push CDN: Content pre-uploaded to CDN nodes (large files, assets)

Major CDN Providers

Provider	Strength
Cloudflare	Security + performance
AWS CloudFront	Deep AWS integration
Akamai	Enterprise, largest network
Fastly	Real-time purging, edge compute

Conclusion

CDNs are essential for global performance. Design assets with long cache TTLs + URL versioning, and use edge compute for dynamic personalization.