Content Delivery Networks (CDN)

A Content Delivery Network (CDN) is a globally distributed network of servers that caches content closer to users. CDNs dramatically reduce latency and improve availability for static and cacheable content.

Why CDNs Matter

Without a CDN:

• Users far from your servers experience high latency
• Your origin servers handle all traffic
• Geographic redundancy is expensive
• Traffic spikes can overwhelm your infrastructure

With a CDN:

• Content served from the nearest edge location
• Origin servers protected from direct traffic
• Global availability built-in
• Traffic spikes absorbed by distributed cache

How CDNs Work

User in Tokyo → CDN Edge (Tokyo) → Cache hit → Return content
                                 → Cache miss → Origin (US) → Cache → Return

Subsequent requests from Tokyo region → Cache hit → Fast response

Key Concepts

CDN Providers

What to Cache

Always Cache

• Static assets (JS, CSS, images)
• Public content that doesn't change (documentation, marketing pages)
• Media files (videos, podcasts)
• API responses that are the same for all users

Sometimes Cache

• Semi-static content with appropriate TTL
• Public API responses (with short TTL)
• Personalized content (using edge computing)

Never Cache

• User-specific private data
• Authentication tokens
• Real-time data (stock prices, live scores)
• POST/PUT/DELETE responses

Cache Control Headers

Control caching behavior via HTTP headers:

Cache-Control: public, max-age=86400

Common Patterns

# Static assets with cache busting (file.abc123.js)
Cache-Control: public, max-age=31536000, immutable

# API response that can be cached briefly
Cache-Control: public, max-age=60

# User-specific content
Cache-Control: private, max-age=0, no-store

# Content that might update but can serve stale while revalidating
Cache-Control: public, max-age=60, stale-while-revalidate=300

Cache Invalidation

When content changes, you need to update the cache.

TTL-Based

Wait for cache to expire naturally.

Pros: Simple, no extra complexity Cons: Stale content until expiration

Purge/Invalidation

Actively clear cached content.

# CloudFront invalidation
aws cloudfront create-invalidation \
  --distribution-id EXAMPLE \
  --paths "/images/*" "/index.html"

Pros: Immediate updates Cons: Takes time to propagate, may have costs

Cache Busting

Include version/hash in filename.

# Old version
<script src="/app.js">

# With cache busting
<script src="/app.abc123.js">  # abc123 is content hash

Pros: Instant update, no invalidation needed Cons: Requires build system support

CDN Architecture Patterns

Static Content Delivery

Browser → CDN → S3 bucket (origin)

Most common pattern. CDN serves static assets from blob storage.

Full Site Acceleration

Browser → CDN → Application servers

CDN caches dynamic pages too. Useful for mostly-static sites.

API Acceleration

Browser → CDN → API servers

Cache API responses at the edge. Short TTLs for freshness.

Edge Computing

Browser → CDN (runs code at edge) → Origin (sometimes)

Run code at CDN edge locations. Examples: A/B testing, personalization, authentication.

Technologies: Cloudflare Workers, CloudFront Lambda@Edge, Fastly Compute

Level-Based Expectations

Performance Metrics

Cache Hit Ratio

Hit Ratio = Cache Hits / (Cache Hits + Cache Misses)

Target: >90% for static content, >70% for dynamic content.

Improve by:

• Longer TTLs
• More consistent URLs (avoid unnecessary query params)
• Warming popular content

Time To First Byte (TTFB)

Time from request to first byte of response.

CDN impact: Edge serving can reduce TTFB by 100-500ms for distant users.

Common Interview Scenarios

"How do you serve images globally with low latency?"

"I'd store images in S3 and serve them through a CDN like CloudFront. The CDN caches images at edge locations worldwide. Users download from the nearest edge—say, Tokyo for Asian users, Frankfurt for European users. I'd use cache busting with content hashes in filenames and a long max-age, so images are cached until they actually change."

"How do you handle a traffic spike?"

"CDNs are excellent for absorbing traffic spikes. Static content is served from edge cache without hitting our origin. For the origin traffic that does come through, I'd combine the CDN with auto-scaling application servers and rate limiting. The CDN also provides DDoS protection to filter malicious traffic."

"How do you update content that's cached?"

"For static assets, I use cache busting—content hash in the filename. When the file changes, the name changes, and users get the new version. For other content, I have two options: short TTL so it refreshes automatically, or explicit invalidation when content changes. I'd prefer short TTL for simplicity unless freshness requirements are strict."

What's Next

CDNs handle content delivery at the edge. Next, we'll look at rate limiters, which protect your services from abuse and overload.