At a glance
- Did you know
- A CDN keeps copies at edge locations near users, so a cache hit skips the trip to your origin.
- Rule of thumb
- Purge by path or tag. Purging everything sends every request to the origin at once.
- Watch out
- Miss storms after a purge can overload the origin.
Problems that start with it
- YouTube end to end 20,000 requests/s routed by path: watch, search, comments and uploads. Metadata cache in front of Vitess MySQL, recommendations, view events into Kafka, uploads to storage and a transcoding queue.
- Breaking news A news article goes viral: traffic ramps to 4× over the run. Pages are cacheable at the edge, but the CDN is tiny.
- Breaking news on a tiny CDN A story goes viral: traffic ramps to 4×. The CDN has 256 MB of edge storage for half a million pages, and the origin API renders pages at 6 ms of CPU each on 2 vCPUs per replica.
- Instagram Stories: views reach the origin Stories are photos and short videos viewed millions of times, then they expire. The CDN is too small, so most views go back to the origin.
- Netflix: a new season at midnight A new season drops: traffic jumps 4× at 10 s for 15 seconds. The CDN is undersized, so the origin sees a large share of the spike.
- CDN purge at peak At 12 s a deploy purges the whole CDN while 2,500 requests a second are flowing. Every request now goes to an origin sized for the usual few misses.
Full lesson coming soon
Lessons are being added one component at a time: how it works, its settings, a live exercise to break and fix it, interview questions and a short check. See the Circuit breaker lesson for what one looks like.