What Is a CDN

The Distance Problem

 speed of light is the enemy:
   user in Manila ──► origin in Virginia ≈ 14,000 km
   round trips accumulate: TCP handshake + TLS + request/response
   even perfect servers can't beat geography: ~200ms+ floor

 CDN solution: move COPIES of content to ~300 cities worldwide:

 Manila user ──20ms──► [Manila edge] ─(occasional miss)─► origin

What a CDN Physically Is

 three components:

 EDGES          servers in datacenters/ISPs worldwide;
                cache + terminate traffic close to users
 SHIELD/MID     intermediate tier protecting origin from
                simultaneous edge misses
 ORIGIN         your servers — the source CDN pulls from

 routing magic (anycast): ONE IP announced globally;
 BGP delivers each user to their nearest edge.
 no client configuration; DNS/CNAME points your domain at it.

What CDNs Deliver

Capability	Mechanism
Latency	Proximity: 200ms → 15ms for far users
Origin offload	90–99% hit rates on static content
Scale absorption	Viral spikes served from edges
DDoS shielding	Attack traffic absorbed/dropped at edge
TLS termination	Edges handle handshakes; keep key management centralized
Modern protocol	HTTP/2/3 to users even if origin speaks HTTP/1.1

The Economics

 without CDN:
   every user-byte crosses your cloud's egress (expensive)
   origin fleet sized for GLOBAL peak simultaneously

 with CDN:
   origin sees misses only → smaller fleet
   CDN egress typically cheaper per GB than cloud egress
   regional spikes flatten into distributed hits

 CDNs usually PAY FOR THEMSELVES in bandwidth alone
 before counting performance.

Static vs Dynamic Content

 STATIC (images, JS, video): cached everywhere; the classic win
 SEMI-STATIC (API GETs): short TTLs at edge — increasingly common
 DYNAMIC/personalized: not cacheable BUT still proxied through
   edges → benefit from persistent connections, TLS reuse,
   route optimization back to origin ("acceleration" mode)

 modern CDNs are full application platforms:
   edge compute (own lessons), security products, analytics.

Integration Mechanics

 simplest path:
   cdn.yourapp.com CNAME → yourcdn.net        (asset domain)
   
 full-site proxy mode:
   www.yourapp.com → CDN → origin             (everything proxied;
   CDN decides per-response what to cache)     config-driven rules)

 purging: by URL or tag when content changes unexpectedly
          (bad image uploaded, price error published)

Interview Framing

“Global product, images are slow for Asia-Pacific users” = CDN question. Scored shape: name proximity as root fix, quantify latency delta, cover offload economics, mention anycast routing and purge strategy, then extend to semi-static API caching as bonus depth. CDNs should appear in nearly EVERY design answer’s diagram — omitting them reads as inexperience.