CDN and Multi-CDN | DevOps Peach

A CDN caches content at edge locations close to users, cutting latency and origin load; cache keys, TTLs and invalidation control freshness, and multi-CDN spreads traffic across providers for resilience, performance and cost.

A Content Delivery Network (CDN) is a global network of cache servers (edge locations, also called points of presence). Users fetch content from the nearest edge instead of your origin.

flowchart LR
  U1["User in Hanoi"] --> E1["Edge Singapore"]
  U2["User in Paris"] --> E2["Edge Frankfurt"]
  E1 -->|"miss only"| O["Origin: S3 / load balancer"]
  E2 -->|"miss only"| O
  DNS["DNS / traffic steering"] -.-> E1
  DNS -.-> E2
  %% step U1: DNS sends each user to a nearby edge location.
  %% step E1: The edge serves cached content in a few milliseconds.
  %% step O: Only cache misses travel to the origin, which sees a fraction of the traffic.
  %% step DNS: With multi-CDN, a steering layer picks between CDN providers per region or by health.

What a CDN gives you

Latency: content is served from tens of milliseconds away instead of hundreds.
Offload: the origin serves misses only — often under 10% of requests.
Resilience: absorbs spikes and DDoS; can serve stale content when the origin is down.
Security and TLS: TLS terminates at the edge; WAF and bot rules run there.

Freshness

Control	Use
`Cache-Control: max-age=31536000, immutable`	Versioned files (`app.4f9a.js`) — cache forever
`Cache-Control: max-age=60`	HTML or API responses that change often
`s-maxage`	TTL for shared caches (CDN) only
Invalidation / purge	Remove a path now (costs time and sometimes money)
Cache key	Which URL parts, headers or cookies make a different object — keep it small

Best practice: version file names instead of invalidating; short TTLs for HTML.

Headers like X-Cache: Hit from cloudfront or cf-cache-status: HIT and Age tell you whether a response came from the edge.

Multi-CDN

Single CDN: all content from one provider. Multi-CDN: content is served by two or more providers, steered by DNS or a traffic manager.

Benefit	Why
Resiliency	One CDN outage does not take the site down
Performance	Pick the fastest CDN per country or network, using real-user measurements
Cost flexibility	Send more traffic to the cheaper provider; better negotiating position
Scale	Combined capacity for extreme events (big launches, live sport)

Costs: same configuration in several providers (cache rules, TLS certificates, WAF), more complex logs and invalidation, steering logic to run.

Traps: caching personalised pages under a shared key (users see each other's data); cache key including every cookie (hit ratio near zero); invalidating /* on every deploy; forgetting to restrict the origin so users can bypass the CDN.