CDNs and the edge

Light travels about 200,000 km per second, and that sets a minimum delay between users and servers. A content delivery network (CDN) moves content, and increasingly compute, to hundreds of locations close to users. It also absorbs attacks, cuts the bandwidth bill and shields your origin from traffic spikes. Nearly every large website and app sits behind one.

A CDN is a geographically distributed cache with a lot of networking attached. If you already know caching, most of the ideas here are the same ones applied to the edge of the network.

The pages below go in order. First, why distance matters and how a CDN routes requests. Then how to control what gets cached, how to change it, what it costs and what the edge can do.

Concepts on this topic

Step	Concept	What it answers
1	Speed of light and round trips	Why does distance set a latency floor?
2	CDN routing and cache hierarchy	How does a user reach a nearby PoP, and how do misses stay cheap?
3	Cache control and cache keys	What does the CDN store, and for how long?
4	Purging and versioned URLs	How do you change content already in the cache?
5	CDN hit ratio and cost	What does a hit ratio save you, in dollars and origin load?
6	Edge compute and security	What can run at the edge, and how does it defend you?

Common mistakes

Serving only static files. Put everything behind the CDN, including APIs. Even uncached requests get TLS termination near users and reused connections.
Purging everything at peak. It sends all traffic to the origin at once.
Letting personal data be cached publicly. Use private or no-store for anything per-user.
Ignoring hit ratio. A deploy that adds a header to the cache key can drop hit ratio from 95% to 30% and quadruple origin load. Watch it as a first-class metric.

Design rule of thumb

In a read-heavy design, such as a URL shortener, an image feed or video, the CDN shows up early. Say what goes through it, what its TTLs and keys are, how you invalidate, and roughly what hit ratio to expect. Then use that hit ratio to size the origin.

Key takeaways

Distance sets a latency floor, and a CDN is how you move content closer.
Tiers and an origin shield keep a new object's origin fetches to roughly one per object.
Cache keys and Cache-Control headers decide hit ratio, and a missing key part can leak private data.
Fingerprinted URLs and tag purges let content stay cached and still change quickly.
Egress and origin load both scale with the miss rate.