What Is a CDN?

A Content Delivery Network (CDN) is a globally distributed network of proxy servers and data centers that provides high availability and performance by distributing content closer to end-users. This matters because physical distance between a user and a server creates internet lag, impacting user experience and site performance.

How a CDN Works

At its core, a CDN works by creating a global network of local servers, often called "edge servers" or "Points of Presence" (PoPs). When a user requests content, the CDN routes that request to the nearest available edge server.

Edge Servers and Anycast Routing

Edge servers are strategically located worldwide to minimize the physical distance between users and content. To ensure users connect to the closest server, CDNs often employ Anycast routing. Anycast is a network addressing and routing method where data is routed to the nearest node in a group of potential receivers sharing the same destination IP address. This means a request from Tokyo will be directed to a Tokyo PoP, rather than a distant origin server in Virginia.

Caching Content at the Edge

When an edge server receives a request for content that it doesn't have locally (a "cache miss"), it fetches the content from the origin server. Once retrieved, the edge server saves a local copy of that content for a specified duration, determined by its Time To Live (TTL). Subsequent requests for the same content from users in that region will then be served directly from the edge server's cache (a "cache hit"), providing instant delivery and significantly reducing load times.

Handling Dynamic and Static Content

CDNs are highly effective for static content (like images, CSS, and JavaScript files) which can be aggressively cached. For dynamic content, which changes frequently and cannot be cached, CDNs still offer benefits. While dynamic requests typically bypass the cache, they still leverage the CDN's optimized network connections to reach the origin server faster and more securely than a direct connection.

Key Takeaways

• CDNs bring web content physically closer to users.
• They use a global network of edge servers and Anycast routing.
• Caching static content at the edge dramatically reduces latency.
• CDNs improve website load times and user experience.
• They reduce the load on origin servers and enhance security.