A Content Delivery Network (CDN) is a highly-distributed platform of servers that helps minimize delays in loading web page content by reducing the physical distance between the server and the user.

Think of a Global Pizza Chain:

1. You have one main kitchen in Italy (The Origin Server).
2. If a customer in New York wants a pizza, it takes 8 hours to fly it there (High Latency).
3. Instead, you build small Local Kitchens in New York, London, and Tokyo.
4. You pre-deliver the dough and tomato sauce to these kitchens.
5. When a New York customer orders, they get their pizza from the New York kitchen in 15 minutes.
6. The CDN is that network of local kitchens.

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Edge Servers vs. Origin Server

• Origin Server: The main server where your original website/app lives.
• Edge Server (POP - Points of Presence): Servers distributed globally that store cached copies of your data.

Push vs. Pull CDN

• Pull CDN: The CDN automatically "pulls" an image from your origin server the first time a user requests it, then caches it for future users. (Most common).
• Push CDN: You manually "upload" content to the CDN whenever you update it. This is best for large files or infrequent updates.

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What to cache on a CDN?

• Static Content: Images, Videos, CSS files, JavaScript files.
• Dynamic Content: Historically no, but modern CDNs (like Cloudflare) can even cache dynamic responses for a few seconds to handle viral traffic.

The Power of "Nearness"

Lower latency is not the only benefit. A CDN also:

• Reduces Bandwidth Costs: Traffic is served from the CDN, not your expensive origin server.
• Protects against DDoS: The massive CDN network absorbs the attack before it reaches your small server.

Popular Tooling

• Cloudflare, Akamai, Amazon CloudFront, Fastly.

Key Term: TTFB (Time To First Byte)

CDNs significantly reduce TTFB because the user's browser connects to a server just a few miles away instead of across an ocean. 🌎