What a CDN Does, and Why Sites Hide Behind One

A content delivery network keeps copies of a site close to its visitors and stands between the site and the open internet.

The basic job

A content delivery network is a geographically distributed set of servers that deliver content from a location near the user. The reference is Wikipedia's article on content delivery networks. A visitor in Taipei gets files from a nearby node instead of from a server on another continent.

What gets cached

Images, stylesheets, scripts and often whole pages that do not change per user. Personalised content still comes from the origin server, sometimes through the same network.

The protective role

• Absorbing floods. Because traffic reaches the network first, it can soak up or filter a denial-of-service attack before it reaches the origin.
• Hiding the origin. Visitors see the network's addresses, not the server's.
• Filtering. Rules can block known bad traffic or challenge suspicious visitors.

Why you sometimes see a check page

The "checking your browser" screen is the network deciding whether you are a person. It is more likely on shared networks, VPNs and unusual browsers.

Trade-offs

• A stale cache can show old content after an update until it is cleared.
• The network operator sits in the middle of the traffic, so the site owner is trusting it.
• An outage at a large network can take many unrelated sites down at once.

For anyone running a small site

A CDN is usually worth having. Learn how to purge its cache, and know which settings are doing the protecting, before the day you need them.