How to Implement Rate Limiting in FastAPI with SlowAPI

In this article I will show you how to implement request rate limiting in FastAPI.

Module Introduction

We will use SlowAPI , a library that enables rate limiting in FastAPI applications. It helps developers restrict how often clients can call the API, protecting server resources and improving stability. SlowAPI supports IP‑based or custom variable (e.g., token) rate‑limit strategies.

For IP‑based limiting, SlowAPI uses the client’s IP address as the identifier, limiting each IP’s request count within a time window. For token‑ or variable‑based limiting, developers can provide a custom function (e.g., get_token ) to extract information from request headers such as Authorization and use that as the limiting key.

Install SlowAPI with:

<code>pip install slowapi</code>

Combined with FastAPI, we can implement two types of rate limiting:

IP‑based rate limiting

Token or variable‑based rate limiting

Specific Implementation

IP‑Based Rate Limiting

This is the default method of SlowAPI.

<code>from fastapi import FastAPI
from slowapi import Limiter, _rate_limit_exceeded_handler
from slowapi.util import get_remote_address
from slowapi.errors import RateLimitExceeded

limiter = Limiter(key_func=get_remote_address)
app = FastAPI()
app.state.limiter = limiter
app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)

@app.get("/home")
@limiter.limit("5/minute")
async def homepage(request: Request):
    return PlainTextResponse("test")

@app.get("/mars")
@limiter.limit("5/minute")
async def mars_endpoint(request: Request, response: Response):
    return {"key": "value"}
</code>

Token or Variable‑Based Rate Limiting

If you prefer to use a custom variable instead of the default IP address, you can define your own key function.

<code>from slowapi import Limiter, _rate_limit_exceeded_handler
from starlette.requests import Request
from slowapi.errors import RateLimitExceeded

def get_token(request: Request) -> str:
    """Return token from request headers"""
    token = request.headers.get("Authorization")
    if not token:
        return "127.0.0.1"
    return token

limiter = Limiter(key_func=get_token)
app = FastAPI()
app.state.limiter = limiter
app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)

@app.get("/home")
@limiter.limit("5/minute")
async def homepage(request: Request):
    return PlainTextResponse("test")

@app.get("/mars")
@limiter.limit("5/minute")
async def mars_endpoint(request: Request, response: Response):
    return {"key": "value"}
</code>

Conclusion

We have detailed how to use SlowAPI to implement request rate limiting in FastAPI, whether through simple IP‑based limits or more complex token/variable strategies. Proper configuration protects server resources from abuse and enhances overall stability and user experience.

Hope this guide helps you quickly get started with SlowAPI and apply rate limiting in real projects. For further questions or extensions, refer to the official SlowAPI documentation and community resources. Remember, safeguarding server resources is a key responsibility for every developer, and rate limiting is a crucial tool to achieve that.