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In 2004, if your website suddenly got popular, often the webserver wouldn’t be able to handle all the requests.

person 1: I want cat picture!
person 2: me too!
person 3: me 300,000!

server, on fire:
web host: now you owe me $1000 for bandwidth
you: how will I pay for this?

A CDN (content delivery network) can make your site faster and save you money by caching your site and handling most requests itself.

20 million requests for 1 cute cat picture -> CDN (many powerful computers) ->
just 1 request: hey send me that cat picture?
server: here you go!

Today, there are many free or cheap CDN services available, which means if your site gets popular suddenly you can easily keep it running!

This is great but caching can cause problems too!

I updated my site yesterday but people are still seeing the old site!
(Cache-Control header)

French users are seeing the English site?!? Why?
(Vary header)

Next, we’ll explain the HTTP headers your CDN or browser uses to decide how to do caching.

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