CDN

Definition

A CDN distributes content via global edge locations to reduce latency and improve reliability. For SEO, it helps improve TTFB/LCP, increases availability, and reduces origin load during traffic spikes and crawling.

Why it matters

• Dramatically reduces global TTFB — users connect to nearest edge, latency drops from 500ms to 50ms
• Improves LCP and Core Web Vitals — faster resource delivery means higher Google scores
• Absorbs traffic spikes — protects origin from crashing during viral events or campaigns
• Improves crawler efficiency — Googlebot doesn't overload origin, crawl budget stays healthy
• DDoS protection — edge network absorbs malicious traffic, origin stays safe
• SSL/TLS termination — edge handles encryption, reducing origin CPU load
• Reduces bandwidth costs — cache hits don't consume origin bandwidth, significant long-term savings

How to implement

• Choose appropriate CDN (Cloudflare, Fastly, AWS CloudFront, Vercel Edge)
• Configure correct caching: long cache for static assets, SWR or short TTL for HTML
• Enable automatic compression (gzip/Brotli) to reduce transfer size
• Set Cache-Control and s-maxage to manage CDN and browser cache separately
• Monitor Cache Hit Ratio — target > 90%
• Set up purge/invalidate mechanism to clear old cache when content updates
• Use Real User Monitoring (RUM) to track actual user TTFB and LCP

Examples

# Cloudflare Pages has CDN built-in
# Just configure caching in public/_headers

# public/_headers
/*
  Cache-Control: public, max-age=0, must-revalidate
  X-Robots-Tag: all

/assets/*
  Cache-Control: public, max-age=31536000, immutable

# wrangler.toml (optional: set cache TTL)
[vars]
  ASSETS_BROWSER_TTL = 31536000

# Verify CDN caching (curl)
curl -I https://your-site.pages.dev/assets/bundle.js
# Look for cf-cache-status: HIT to confirm cache hit

// CloudFront Distribution config summary
{
  "Origins": [{
    "DomainName": "your-origin.example.com",
    "OriginProtocolPolicy": "https-only"
  }],
  "DefaultCacheBehavior": {
    "ViewerProtocolPolicy": "redirect-to-https",
    "CachePolicyId": "658327ea-f89d-4fab-a63d-7e88639e58f6",
    // CachingOptimized policy: TTL 86400s, gzip/br enabled
    "Compress": true
  },
  "PriceClass": "PriceClass_100",
  // Use only NA+EU edge locations (lower cost)
  "HttpVersion": "http2and3"
}