What is a CDN?

A CDN is a network of servers across the globe. The full name is Content Delivery Network. These servers cache content and serve it to users near them. So users get content from a server nearby. Most requests no longer travel to your origin.

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

Imagine a huge central library. It holds every book ever written. It sits in one place, say Amsterdam. When someone in Tokyo wants a book, they request it. They wait for it to be shipped across the world. Only then can they read it.

Now it opens local branches in each big city. Each branch keeps copies of the most popular books. When someone in Tokyo wants a popular book, they walk to their local branch. No international shipping is needed.

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, each request goes to your origin server. That server may be 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. It adds at least 90ms of latency. And that's before your server starts processing the request.

With a CDN edge server in Tokyo, the request might travel 50km. It adds less than 1ms. Users can notice the difference.

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

CDNs provide redundancy. If one edge server fails, traffic routes to another. If your origin goes down, cached content may still be served. It depends on your setup.

When a site goes viral, the origin gets all requests. Servers have limits. CPU, memory, and bandwidth can run out. Past those limits, you see slow pages or crashes.

With a CDN, most requests never reach your origin. The edge handles them. Only new or expired content hits the origin. That cuts load a lot.

Know what CDNs don't do. It matters as much as 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 move most of the internet's public traffic
• Big providers run from dozens to thousands of PoPs. They span every continent.
• Single CDNs handle hundreds of terabits per second at peak
• Response times are short. Often in single-digit milliseconds.

When you stream Netflix or scroll Instagram, you're using a CDN. You may not know it.