What Is a CDN and How Does It Work?

CDN - you keep seeing the acronym. Maybe in URLs, maybe on landing pages, but it never quite clicked - what are Content Delivery Networks, what do they do exactly?

We'll explain in this overview article, and demonstrate on two popular ones in followup posts.

CDN Basics

A CDN is a network of computers that delivers content.

More specifically, it's a bunch of servers geographically positioned between the origin server of some web content, and the user requesting it, all with the purpose of delivering the content faster by reducing latency. This is their primary purpose.

These geographically closer servers, also called PoPs or Points of Presence, also cache the cacheable content which removes a lot of the load from the origin server. There are different types of CDNs offering different kinds of services, and they can have differing network topology: scattered CDNs aim to have as many servers scattered around the world as possible. Akamai is one such CDN. Consolidated CDNs have fewer points, but bigger ones built for network performance, throughput, and DDoS resistance.

Types of CDNs

We said that their primary purpose was to reduce latency and speed up rendering. But in the modern world of 2MB images and 500kb JavaScript libraries that take 3 minutes to boot up on websites, this latency matters little. But there are other purposes to CDNs, too, which evolved over time.

Content-oriented CDNs

Initially, CDNs were just for static content (JS, CSS, HTML). You had to push content to them as you created/uploaded it (they didn't know they needed to update their cache with your content, not even as someone requested it).

Then, they added origin pulling, making things more automatic - this meant that a user requested the CDN's URL, and then the CDN requested the origin website's URL automatically, caching what ever it got back. Additionally, availability became an important factor. Many CDNs now cache a website's "last alive" state so that if the origin goes down, the CDNed content is still accessible to users, creating the illusion of stability until things return to normal.

Additionally, modern CDNs often offer auto-optimization layers which will automagically resize images and save them for future use based on the image size requested. This means what if your site has a 2MB header image and someone requests it on a 300px wide screen, the CDN will make a copy that's 30kb in size and 300px wide and serve that in the future to all mobile users, automatically making the site faster.

Security-oriented CDNs

The final layer of practicality added to CDNs was DDoS and bot protection. CDNs like Incapsula specialize in this.

As the CDN is the outermost layer of a website's infrastructure and the first recipient of traffic, it can detect DDoS attacks early and block them with special DDoS protection servers called scrubbers without them ever reaching the origin server and crashing it.

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