CDN in Architecture

A Content Delivery Network (CDN) caches content at edge locations close to users.

CDN Architecture

                    ┌─────────────┐
                    │   Origin    │
                    │   Server    │
                    └──────┬──────┘
                           │
                    ┌──────▼──────┐
                    │   CDN Edge  │
                    │   (PoP)     │
                    └──────┬──────
                           │
              ┌────────────┼────────────┐
              │            │            │
       ┌──────▼──┐  ┌──────▼──┐  ┌──────▼──┐
       │  Edge   │  │  Edge   │  │  Edge   │
       │ Tokyo   │  │ London  │  │  NY     │
       └────┬────┘  └────┬────┘  └────┬────┘
            │            │            │
       ┌────▼──┐   ┌────▼──┐   ┌────▼──┐
       │ User  │   │ User  │   │ User  │
       │ Japan │   │  UK   │   │  US   │
       └───────┘   └───────┘   └───────┘

CDN Flow

1. User requests content
   User → DNS → CDN Edge (closest)

2. Cache HIT (content at edge)
   Edge → User (fast, < 10ms)

3. Cache MISS (content not at edge)
   Edge → Origin → Edge → User
   Edge caches for next time

CDN Benefits

Benefit	Description
Lower latency	Content served from nearby edge
Reduced bandwidth	Origin serves less traffic
Better availability	Edge handles origin failures
DDoS protection	Distributed architecture
Cost savings	Less origin traffic

CDN Providers

Major CDNs:
- Cloudflare (free tier available)
- AWS CloudFront
- Akamai
- Fastly
- Azure CDN
- Google Cloud CDN

Edge locations are data centers distributed globally where CDN caches content.

Edge Location Network

Global Edge Locations:

North America: 100+ locations
Europe: 100+ locations
Asia Pacific: 50+ locations
South America: 20+ locations
Africa: 10+ locations

Total: 300+ edge locations worldwide

How Edge Locations Work

1. DNS Resolution
   User in Tokyo → DNS → Edge Tokyo (closest)

2. Content Delivery
   Edge Tokyo serves content
   Latency: 5ms (vs 160ms from US origin)

3. Cache Management
   Edge stores hot content
   Evicts cold content
   Refreshes from origin as needed

Edge vs Origin

Aspect	Edge	Origin
Location	Global, close to users	Central, few locations
Content	Cached, hot content	Complete, all content
Latency	Low (5-20ms)	Higher (50-200ms)
Capacity	Distributed	Limited
Cost	Higher per request	Lower per request

Content Types at Edge

Static Content (cacheable):
- Images (JPG, PNG, GIF, WebP)
- CSS files
- JavaScript files
- Fonts
- Videos

Semi-Dynamic:
- API responses (with short TTL)
- Personalized content (variant caching)

Dynamic Content (not cached):
- User-specific data
- Real-time data
- Forms

Edge Computing

Modern CDNs support edge computing:

- Cloudflare Workers
- AWS Lambda@Edge
- Fastly Compute@Edge

Run code at the edge:
- A/B testing
- Authentication
- Image transformation
- Personalization

Choosing the right CDN strategy depends on content type and update frequency.

Push vs Pull CDN

Push CDN:
- You push content to CDN
- CDN caches immediately
- You control when content is cached

Pull CDN:
- CDN pulls content on first request
- Auto-caches on cache miss
- Simpler to manage

Push CDN

When to use:
- Content updates infrequently
- You want control over caching
- Small number of large files

Example:
- Software downloads
- Video files
- Static website assets

Flow:
You → Push to CDN → CDN caches → Users access

Pull CDN

When to use:
- Content updates frequently
- Large number of files
- Don't want to manage CDN

Example:
- E-commerce products
- News articles
- Dynamic websites

Flow:
User → Request → CDN → Cache miss → Origin → CDN caches → User

CDN Configuration

# Nginx CDN configuration
location ~* \\.(jpg|jpeg|png|gif|ico|css|js)$ {
    expires 30d;
    add_header Cache-Control "public, immutable";
    add_header CDN-Cache-Control "max-age=31536000";
}

# API caching
location /api/ {
    proxy_cache my_cache;
    proxy_cache_valid 200 5m;
    proxy_cache_use_stale error timeout;
}

Cache Invalidation

Strategies:

1. TTL (Time-To-Live)
   Cache-Control: max-age=3600
   Content expires after 1 hour

2. Versioned URLs
   /style.v2.css
   New version = new URL

3. Purge API
   CDN purge /path/to/file
   Immediate invalidation

4. Tag-based
   Purge by tag: "product-123"

CDN Best Practices

1. Cache static assets aggressively: Long TTLs for CSS, JS, images
2. Use versioned URLs: For cache busting
3. Set appropriate TTLs: Based on update frequency
4. Monitor cache hit ratio: Aim for > 90%
5. Use CDN for API responses: Cache semi-dynamic content