How can a video reach millions of people at once without the internet buckling?

The answer is to serve many copies instead of one original. That is what a content delivery network, or CDN, does: it is a geographically distributed network of proxy servers and data centres that provides high availability and performance by distributing content closer to end users. CDNs arose in the late 1990s to relieve the performance bottlenecks of an internet that was becoming a critical medium.

The business is simple to state. Content owners such as media companies and online shops pay CDN operators to deliver their content to their end users. In turn, the CDN pays internet service providers and network operators to host its servers in their data centres. Some CDNs reach thousands of nodes and tens of thousands of servers on many points of presence. Their caches store popular content where demand for it is greatest, which reduces the load on the original servers and improves response times.

The clever part is sending each visitor to the right copy. Request routing directs a client to the content source best able to serve it: the closest service node, or the one with the most capacity. It can use DNS-based request routing, or anycasting, among other methods. Akamai, a CDN company that grew out of research on consistent hashing, takes its name from a Hawaiian word meaning "clever".