So far in our series on Web Performance Optimization, we've focused on how to reduce the number of requests between client and server through caching, and how to make requests more efficient by managing server resources. Another strategy in Web optimization is intelligent distribution of resources across the Internet, which can greatly reduce request latency by locating redundant copies of Web content on multiple servers spread across the Internet. In this installment of our series, we focus on content delivery networks (CDN), a technology that increases throughput by bringing content closer to the people requesting it.
 

What is a CDN?

In the simplest Web site configuration, a single Web server services requests from multiple clients. While this is often good enough for the simplest, lowest traffic Web sites, complex Web sites that need to scale to thousands or millions of visitors require more processing power. This is why many sites have resorted to using Web server farms, which are clusters of multiple Web servers offering redundant copies of a site's content. Web farms use load balancing software to monitor the amount of load on any one server. They can also use this information to route requests to the server with the least load at a given point in time.

A CDN is a type of Web farm or server cluster, except that instead of using a single farm or cluster, the servers are spread out over the Internet in multiple geographical locations. These are called edge servers, because they are located at the extremes, or edges, of the Internet, instead of all being located off of a central Internet backbone link. The goal of a CDN is to decrease the time it takes to deliver content to a specific user based on that user's location.

Let's say, for example, that a company based in New York receives a request for Web content from a user in Seattle, WA. In a traditional setup, the Seattle computer's request would have to find the most efficient route on the Internet to New York, usually via a busy backbone link. In a CDN configuration, the CDN could tell the Seattle client that its nearest edge server is on a subnet in Portland, Oregon. By obtaining the content from a server closer on the network, the client greatly reduces request latency.