The CDN

Serving content close to users: the origin and edge servers, cache lifetime, purging, versioned file names, and the pitfalls of a shared cache.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

CDN: what is it?
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A network of servers spread worldwide that keep copies of content close to users.
Latency drops and the origin receives far fewer requests.
The site is hosted in Paris and users in Japan find images very slow. What do you do?
- Serve images through a CDN
- A more powerful server in Paris
- A database index
- More RAM
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Serve images through a CDN
The problem is distance; only a copy close to users fixes it.
Origin server: what is it?
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The server holding the reference version of the content, which the CDN asks when it lacks it.
Everything the CDN cannot serve from cache falls back on it.
The origin fails. What do visitors see?
- Content already cached can still be served; the rest fails
- Everything fails at once
- The CDN rebuilds pages by itself
- Nothing changes
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Content already cached can still be served; the rest fails
Some CDNs can even serve an expired copy when the origin errors, if configured.