8.1 Design a Rate Limiter

How many requests may one API key send in a minute, and who keeps count when fifty stateless servers share that key's traffic? A rate limiter answers this question for every request. It must therefore stay correct under heavy concurrency, add almost no latency, and never become the cause of an outage.

This walkthrough follows one running example: a public API that allows 100 requests per minute per API key. This lesson makes two decisions in order: the algorithm and the atomic implementation in Redis