CDN – What are they?

lucy-hogg .

A Content Delivery Network, called CDN for short, is a network of servers that sit throughout the world that deliver content faster to web site visitors.

Due to the global nature of the Internet a web site can receive visitors from all over the Internet. If your web site is hosted physically close to your visitor the download times will be very good. However if a web site visits from the other side of the world the content may take some time to download.

The content delivery network will have servers in each continent or region in the globe. Some CDNs will have multiple servers in regions to improve performance and redundancy should something go wrong or they need to performance maintenance. When a visitor views a web site with a CDN the DNS server will automatically determine the closest CDN server to use. As this CDN server will be physically closer to the web site visitor the downtimes will be much faster.

Each of these CDN servers will hold cached copies of the static content of the web sites including images, CSS, javascript and raw html pages. This cache content will have a set expiry time where it will be deleted and a fresh copy of the web site from what we call the origin server. The origin server is where the original copy of the web site hosted and updated.

In some cases CDNs can cache the complete website, including dynamic content, which will typically have a low expiry time. This is particularly useful if you are expecting a large influx of traffic and the web site owner would prefer the websites to see slightly delayed content rather the web site going down.

Adding a CDN to your web site is fairly cost effective as the majority traffic is low cost local bandwidth.  As CDN providers have vast global networks they can absorb DDOS attacks without much impact so they usually include a form of DDOS, distributed denial of service, protection as standard.

To summarise the key benefits of a CDN are: –

  • Improved download times

  • Improve capacity including some DDOS protection

  • Improved SEO and user experience

  • Easy implementation and low cost