CDNs Explained for People Who Do Not Care About CDNs
Your website lives on one server in one city. Every visitor's request travels to that city and back. A content delivery network puts copies of your files in dozens of cities so the trip is short. That is the whole concept.
What you get
- Speed for distant visitors. The further someone is from your server, the bigger the improvement. A visitor across the country can save hundreds of milliseconds on every file.
- Resilience under load. Traffic spikes hit the network instead of your server, which matters during a promotion, a news mention, or a seasonal rush.
- Security in front of the site. Most CDNs include protection against denial-of-service traffic and a basic web application firewall, which is often the more valuable half.
- Bandwidth savings, since the network serves the repeat requests your host would otherwise bill you for.
When it is not worth the complexity
If your customers are all within a hundred miles of your server, your site is small, and traffic is steady, a CDN adds a layer to debug for a modest gain. Fix your images and enable page caching first; those changes deliver more improvement than a CDN will on a local audience.
The complications to know about
- Caching confusion. You update a page and still see the old version, because a copy is cached at the edge. Learn where the purge button is before you need it.
- Certificates. The CDN usually terminates SSL, so certificate renewal moves to them. Two certificates that disagree cause strange errors.
- DNS. Enabling a CDN typically means moving your DNS, which touches your email records. Copy them carefully.
- Dynamic pages. Carts, logged-in views, and forms must be excluded from caching or visitors see each other's pages. Any decent setup handles this; verify it rather than assuming.
The pragmatic answer
If your host includes one, turn it on and confirm the exclusions are right. If not, add one when you have a national audience, a traffic spike to survive, or a security requirement. Otherwise, spend the effort on image weight and caching, where the return is larger.
