What Is a CDN?

A content delivery network acts as a distributor. It moves the job of distributing content that would be served by your web server (or more likely server farm) to more efficient servers that are geographically dispersed and closer to the user visiting your site.

Content is algorithmically directing a more efficient infrastructure for delivering it. It is more efficient than traversing the same network to the same old web server and downloading files.

The CDN directs a client (browser) to download files from a more local source within the CDN network. This has the effect of dramatically reducing the page load time.

The CDN copies the content placed in it to the geographically dispersed nodes of its CDN network. When a server in Boston, Massachusetts, gets a visitor from Paris, France, latency becomes an issue. If they were to download all of the resources from the site in Boston, the performance might not be optimal. With a CDN and caching in place, the French visitor can load the content placed in the CDN from a more local server, dramatically improving load time. This means more optimized web performance and a generally happier end user.

Types of CDNs

The origin push type of CDN requires a manual push or upload of content into the CDN. That is to say, the user (you) manually pushes content into the CDN and then links to it on your site. This allows more control over what is uploaded and minimizes uploaded content. An origin pull CDN doesn't require a manual upload of the content to the content network. Instead, the CDN automatically loads files without user intervention. That is to say, it tests to see whether it has the requested content, and if not, it automatically loads it from the original server. Origin pull has the benefit of automating the propagation of your content into the CDN and therefore is optimal for caching and CDN configuration. Herein I will describe an origin pull CDN in Amazon CloudFront. In this case, your CDN will also automatically rewrite URLs to point to those in the CDN and not on the original web server.

Baseline Your Website

Taking a baseline of your site performance before you start is vital. In the interest of limiting the scope of this article, I'll use Google PageSpeed Insights [4], which you can use to test any site for which you would like to improve the performance (Figure 1). With your page speed results, Google provides copious detail on what you can improve upon [5]. These details give clear tips on what you need to work on to improve your site performance.

Figure 1: My Pre-optimized Google PageSpeed Insights site scores: desktop 75/100; mobile 62/100.