Content delivery networks (CDNs)

Light in fibre is fast, but a round trip between continents still takes ~150 ms, and a page needs many round trips. A content delivery network fixes that by keeping copies of your content in points of presence (PoPs) around the world, so most requests are answered a few milliseconds from the user.

An origin shield is an extra cache layer between the edges and your origin, so a cache miss in 50 PoPs becomes one request to the origin rather than 50.

Pull vs push

Controlling freshness

• Cache-Control: public, max-age=31536000, immutable: cache for a year, for files whose name changes when their content does.
• Cache-Control: max-age=60: cache briefly, for things that change.
• Cache-Control: no-store: never cache private or per-user responses.
• ETag and Last-Modified let caches revalidate cheaply (304 Not Modified) instead of downloading again.

What else a CDN gives you

• Lower origin load and cost: most bandwidth is served from the edge.
• Spike and DDoS absorption: huge distributed capacity in front of your servers.
• TLS termination near users: faster handshakes.
• Edge compute: small functions at the PoP for redirects, A/B tests or auth checks.
• Dynamic acceleration: even uncached API calls benefit from the CDN’s optimised network paths back to the origin.