System Design 101: Understanding Content Delivery Networks (CDNs)

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In our last episode, we discussed Database Sharding—how Spotify and Zoom split data across servers to handle massive scale. We saw how sharding enables lightning-fast searches and global performance.

Now, let’s dive into CDNs, the systems that bring content physically closer to you to eliminate lag and slowness.

What is a CDN?

When you click “Play” on Netflix, your show starts in seconds—even in 4K. This isn’t magic. It’s a CDN.

A Content Delivery Network (CDN) is a global network of servers (called edge servers) that store copies of videos, images, and other static content. Instead of fetching data from a single central server (which could be continents away), CDNs serve it from the server closest to you.

Example: If you’re in Tokyo watching Stranger Things, Netflix’s CDN delivers it from a server in Japan—not from California.

Why CDNs Matter

Without CDNs, streaming Netflix would look like this:

• Click “Play” → Request goes to a central server in the U.S.

• Video data travels 8,000 km across oceans → Buffering hell

• Every user in Asia, Europe, etc., competes for the same server

This is why CDNs exist. For Netflix, which delivers 250+ million hours of video daily, CDNs:

• Reduce latency (delay) by 50-90%

• Cut bandwidth costs

• Prevent crashes during peak hours (like Squid Game release nights)

How Netflix Uses CDNs: A Behind-the-Scenes Look

1. Netflix’s CDN is called Open Connect. Here’s how it works:

   1. Pre-Storage: Netflix uploads popular shows (The Crown, Bridgerton) to edge servers worldwide before they trend.

   2. Your Request: When you hit “Play,” Netflix’s DNS directs you to the nearest edge server.

   3. Edge Delivery: The server in your region streams the video. No cross-continent data haul.

   4. Adaptive Quality: CDNs enable Netflix to downgrade/upgrade video quality based on your internet speed—without buffering.

By the Numbers:

• Netflix has 15,000+ edge servers in 175+ countries.

• 95% of Netflix’s traffic flows through its CDN.

Key Components of a CDN

1. Edge Servers: Local servers that cache content (e.g., Tokyo, Berlin, São Paulo).

2. Origin Server: The central source of truth (Netflix’s main data centers).

3. Load Balancers: Direct traffic to the fastest edge server.

4. Caching Algorithms: Decide what content to store (e.g., keep trending shows, remove less-watched titles).

Why This Matters to Engineers

CDNs aren’t just for video. They’re used for:

• Software updates (Windows, iOS)

• Game downloads (Call of Duty, Fortnite)

• E-commerce product images (Amazon)

Tradeoffs:
✔️ Pros: Faster load times, reduced server load, scalability.
❌ Cons: Cost (CDNs are expensive to maintain), cache invalidation challenges.

Real-World Impact: Netflix vs. Buffering

In 2012, Netflix struggled with ISP bottlenecks during peak hours. Their fix? Build their own CDN (Open Connect).

• Partnered with ISPs to install edge servers inside ISP networks.

• Result: Video quality improved by 4x, and buffering dropped by 66%.

CDNs are why you can stream Wednesday in 4K without a single buffer—even if millions are watching simultaneously. They’re a cornerstone of modern system design, turning global scale into local speed.

Next week, we’ll explore Encryption at Scale—how platforms like WhatsApp and Signal protect billions of messages daily.