Building a Distributed Rate Limiter for FastAPI with Redis (Sliding Window Algorithm)

Building a Distributed Rate Limiter for FastAPI with Redis Every API eventually runs into the same problem. A bot, scraper, or even a buggy client suddenly starts sending thousands of requests per second. When that happens, your server slows down, your database struggles, and real users start seeing errors. This is exactly the kind of situation rate limiting is meant to prevent. Recently I built RateGuard , a small Python library that adds distributed rate limiting to FastAPI using Redis. In this post I want to walk through how it works and the design decisions behind it. Why I Built RateGuard While working with FastAPI, I looked at a few rate limiting libraries. Most of them had at least one issue. Some only support in-memory limits , which means they break once your API runs on multiple servers. Others work in distributed setups but require more infrastructure than I wanted. So I decided to build something simple with a few goals in mind: easy to plug into FastAPI works across multip