Great Stack to Doesn't Work #3 — Redis: "99% Cache Hit Ratio, System Down"

A survival guide for when everything goes wrong in production.

Your Redis dashboard looks perfect. Hit ratio: 99.2%. Latency: sub-millisecond. Memory usage: 60% of available. Every metric says healthy.

Then at 2:47 PM, your API starts returning 500s. Response times spike to 30 seconds. Users can't log in. The dashboard still shows 99% hit ratio because the cache is working — it's serving cached errors to everyone equally fast.

Redis is doing exactly what you told it to do. The problem is what you told it to do.

Why Single-Threaded Is Fast (Until It Isn't)