Art of Codeartofcode
Lesson 15 of 16

26. CDNs

1
The picture to keep in your head
2
26.1 Why use a CDN?
3
26.2 What are edge locations (PoPs)?
4
26.3 Pull vs push CDN: what's the difference?
5
26.4 How do cache keys and headers work?
6
26.5 How does purging work?
7
26.6 What is an origin shield?
8
26.7 Can the CDN handle dynamic content?
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26.8 How do CDNs deliver video (HLS, DASH)?
10
26.9 What happens when the CDN fails?
11
Memory card and where the analogy breaks
12
Practice: five levels
13
Interview questions
14
Solutions
15
Sources
16
Code lab
26. CDNs

Sources

3 min read+10 XP

Source notes

  • HTTP caching (Cache-Control, s-maxage, private, no-store, validators, Vary): IETF RFC 9111, "HTTP Caching", 2022. stale-while-revalidate and stale-if-error: IETF RFC 5861. immutable: IETF RFC 8246.
  • Fastly outage of 8 June 2021 (a valid customer configuration change triggered a bug introduced in a 12 May deployment; 85% of the network returned errors; detected within a minute; 95% of the network normal within 49 minutes): Fastly blog, "Summary of June 8 outage", checked 2026-10-08.
  • HLS 6-second target segment duration: Apple, "HTTP Live Streaming (HLS) authoring specification for Apple devices", checked 2026-10-08. HLS: IETF RFC 8216. MPEG-DASH: ISO/IEC 23009-1.
  • Origin shield and request collapsing: Amazon CloudFront documentation ("Origin Shield"); Fastly documentation ("Shielding").
  • Edge compute: Cloudflare Workers, CloudFront Functions and Lambda@Edge documentation.
  • Round-trip times, traffic volumes, page sizes and bitrate ladder are illustrative assumptions; arithmetic computed 2026-10-08.
  • Level 3 and Level 4 code: tested 2026-10-08 (code/ch26_cdn.py, all checks pass).