Z3 · Caching & Delivery

CDN (content delivery network)

Bronze

A CDN keeps copies of static content on servers close to users, so most requests for images and assets never reach your origin — and arrive sooner.

Use when: Images, video and JS / CSS bundles · Users far from the origin · Spikes of requests for the same files
Avoid when: Personalized responses — mark them private so the shared cache never keeps them · Content that must never be served stale
Trade-offs: Freshness vs time to live — version file names instead of purging · Misses and uncacheable requests still reach the origin
Requires: Cache basics: hit ratio, TTL, eviction, Object storage

3 objectives · 8 misconceptions

predict Predict the load on the origin for a given CDN hit ratio.
explain Explain why answering from an edge near the user cuts latency.
choose Choose what goes behind a CDN and how long it may be cached (TTL, versioned file names).

Not true: A CDN speeds up every request, including personalized API responses.
Not true: Behind a CDN the origin no longer needs any capacity.
Not true: Changing a file updates every CDN copy at once.
Not true: A CDN only matters for apps with users all over the world.
Not true: Once a file is cached, every request for it is a hit.
Not true: A CDN only makes responses arrive sooner — the origin still answers every request.
Not true: A CDN only protects the origin from load; it doesn't make anything arrive sooner.
Not true: Edge servers answer sooner because they are faster machines than the origin.

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