Redis dies mid flash sale. Don't fall back to the database.

transcript

Flash sale. 600,000 requests a second. And then Redis dies. The obvious fallback is the database. That's how you lose the site.

With Redis up, 99 percent of reads hit the cache. The database sees about 6,000 a second. Redis gone? All 600,000 a second head for the database. It was never sized for that. The database fails. The site is down.

So a circuit breaker spots the failure in under a second, and opens. Requests stop trying Redis. They check a local in-memory cache, in each process. A hit is served straight from memory. A miss gets a default. Product temporarily unavailable. No database query.

Personalisation shows temporarily unavailable. Flash sale purchases are blocked for a while. No overselling. After 30 seconds, the breaker tests Redis, then restores traffic gradually.

A few product views look degraded for a while. The rest of the site stays up. That's the trade.

Full breakdown in the lecture.