Content Delivery Network (CDN)

// CDN request flow
//
//  User (Mumbai) → DNS → CDN Edge (Mumbai PoP)
//     │
//     ▼
//  Cache HIT? ──yes──→ Serve from edge (< 20ms)
//     │
//     no
//     │
//     ▼
//  Fetch from Origin (us-east-1) → Cache at edge → Serve user
//
// Subsequent requests from Mumbai region → served from edge cache

// Cache-Control headers control CDN behaviour
Cache-Control: public, max-age=86400, s-maxage=604800
//  max-age=86400    → browser caches for 1 day
//  s-maxage=604800  → CDN caches for 7 days
//  public           → CDN is allowed to cache this

// Invalidation
// POST /invalidation { paths: ["/images/*", "/css/main.css"] }

// Pull CDN (most common — CloudFront, Cloudflare)
// 1. User requests asset
// 2. Edge checks cache → miss → pulls from origin
// 3. Edge caches + serves
// Good for: websites, APIs, assets that change often

// Push CDN (e.g. S3 + CloudFront with origin groups)
// 1. You upload/deploy assets to CDN storage
// 2. CDN distributes to edges proactively
// 3. User requests → always served from edge
// Good for: large videos, software downloads, predictable content

// CloudFront distribution (Terraform)
resource "aws_cloudfront_distribution" "app" {
  origin {
    domain_name = aws_s3_bucket.static.bucket_regional_domain_name
    origin_id   = "s3-static"
  }
  default_cache_behavior {
    target_origin_id       = "s3-static"
    viewer_protocol_policy = "redirect-to-https"
    allowed_methods        = ["GET", "HEAD"]
    cached_methods         = ["GET", "HEAD"]
    compress               = true
    default_ttl            = 86400
  }
  price_class = "PriceClass_200"  # edge locations in US, EU, Asia
}

// Cloudflare Worker — edge-side A/B testing
addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request))
})

async function handleRequest(request) {
  const cookie = request.headers.get('Cookie') || ''
  let variant = cookie.includes('ab=B') ? 'B' : 'A'

  // New users: random assignment
  if (!cookie.includes('ab=')) {
    variant = Math.random() < 0.5 ? 'A' : 'B'
  }

  const url = variant === 'B'
    ? 'https://origin.example.com/landing-v2'
    : 'https://origin.example.com/landing-v1'

  const response = await fetch(url, request)
  const newResponse = new Response(response.body, response)
  newResponse.headers.set('Set-Cookie', `ab=${variant}; Path=/; Max-Age=86400`)
  return newResponse
}