Amazon DynamoDB stores hundreds of trillions of items and handles tens of millions of requests per second at peak. Netflix uses it to keep track of what you were watching. Airbnb uses it for availability calendars. Discord uses it for message storage. What do all of these have in common? They all need to store and retrieve data by a simple key — blazingly fast, at global scale, with near-zero downtime.

Imagine you are the infrastructure engineer at a hot social media platform. Your system is humming along with 4 cache servers, each holding about 25% of your data. Life is good. Then your platform goes viral overnight. You urgently add a 5th server. You restart everything. And suddenly, your database is on fire — every single cache server is getting a tsunami of cache misses. Users experience 10× slower page loads.