What is a CDN?

A Content Delivery Network (CDN) is a geographically distributed network of servers that cache and deliver content to users based on their location. Instead of every request traveling to your origin server, users get content from a server nearby.

But that definition is abstract. Let's make it concrete with an analogy.

Imagine a massive central library with every book ever written. It's in one location—let's say Amsterdam. When someone in Tokyo wants a book, they have to request it, wait for it to be shipped across the world, and only then can they read it.

Now imagine that library opens local branches in every major city. Each branch keeps copies of the most popular books. When someone in Tokyo wants a popular book, they just walk to their local branch—no international shipping required.

That's exactly what a CDN does:

• Central library = Your origin server
• Local branches = CDN edge servers (Points of Presence)
• Popular books = Cached content
• Requesting a book = HTTP request

Without a CDN, every user request travels to your origin server—potentially thousands of miles away. This creates three problems:

Physics is unforgiving. Light in fiber optic cables travels at about 200,000 km/s. A round trip from Tokyo to Amsterdam is roughly 18,000 km, adding at least 90ms of latency—and that's before your server even starts processing the request.

With a CDN edge server in Tokyo, that same request might travel 50km, adding less than 1ms. The difference is perceptible to users.

If your origin server goes down, your entire site goes down. If the network path to your origin has issues, users in affected regions can't reach you.

CDNs provide redundancy. If one edge server fails, traffic routes to another. If your origin goes down, cached content can still be served (depending on configuration).

When your site goes viral or gets hit with a traffic spike, your origin server has to handle every request. Servers have limits—CPU, memory, bandwidth—and exceeding them means slow responses or crashes.

With a CDN, most requests never reach your origin. The edge handles them. Your origin only serves unique or expired content, dramatically reducing load.

Understanding what CDNs don't do is just as important as understanding what they do:

Here's the key insight that will help you throughout this course:

To give you a sense of how central CDNs are to the modern internet:

• CDNs deliver an estimated 50-70% of all internet traffic
• Major providers operate hundreds of PoPs across every continent
• Single CDNs handle tens of terabits per second of traffic
• Response times are measured in single-digit milliseconds

When you stream Netflix, scroll Instagram, or visit almost any major website—you're using a CDN, whether you know it or not.