A CDN (Content Delivery Network) is simply a network of data centers spread across the globe which serve static content to a user from the nearest available data center. Thus a request for a static file by a user in Australia
would be served from a nearby AsiaPac data center instead of the US (where the main site may to served from). This reduces the latency and therefore boosts performance.
A secondary factor boosting performance is that typically browsers can only open up 2 simultaneous connects to a host, therefore if your web page will require 20 file requests and all static files are on the same server the requests are
queued two at a time. If a CDN is used, the browser can open up more simultaneous file requests and so boost performance.
Which CDN ?
There are numerous CDNs to choose from, personally I use the Rackspace CloudFiles. Rackspace recently partnered with Akamai and so the CDN network is top notch – although the tools are quite weak. It is not possible to FTP a file
to a CDN but the FireUploader utility for Firefox is satisfactory (note that this had some issues with Firefox 4.0 when I tested it and as such I am still using it with Firefox 3.6).
I won’t list all available CDN options as I have not tried that many, but I also had a good experience with MaxCDN, although their pricing structure is quite unusual and so you should carefully examine this to guage your costs.
One thing to note, Amazon’s S3 storage solution is NOT a CDN. All files served from S3 are served from a single location and not from an array of data centers. Amazon does have a CDN offering – CloudFront, however I would not recommend this as it did not perform well in any of my testing. The Azure CDN offering had a similar performance to the CloudFront product in my experience and so I wouldn’t recommend it.
How to Use a CDN
Using a CDN is very simple, just upload your static files to the CDN and then set the src attribute of the element (such as an <img> tag).
When a file is requested for the first time it will first be served from the central data center and then cached at the edge location (ie worldwide data center nearest to the requesting user). All subsequent requests will be served from the cache of the edge location until the TTL (Time To Live) value has expired. This can be a gotcha if you are updating the files. This typically happens a lot with css files. Therefore if you update your site’s css file, then upload it to your CDN, site users around the worldwide may be served a cached version of the old file. To get around this you can specify version numbers for your css files, in this way each file is unique and when you push an update the CDN will request the new css file from the central store before caching it.
By way of example if you look at the source for this SharePoint article you will see that most of the static files are served from a CDN.