CDNs and edge caching

What you take away

• A CDN keeps copies at points of presence near users, so most requests never reach your origin.
• Pull CDNs fetch on the first miss; push CDNs are loaded ahead of time. Pull is the default for almost everything.
• The cache key is whatever you make it — vary on a per-user cookie and the hit ratio collapses to zero.
• Purging propagates over seconds to minutes; versioned URLs avoid needing it at all.

How the class is laid out

1. The problem
2. The simple version
3. Pull, push, and what actually goes in the key
4. Keeping it fresh
5. Check your understanding