SlowAPI for rate limiting

 SlowAPI is a Python library specifically designed to add rate limiting to FastAPI and Starlette applications. It is heavily inspired by (and based on) flask-limiter, adapting its battle-tested logic for the modern, asynchronous nature of FastAPI.

Its primary job is to prevent your API from being overwhelmed by too many requests—whether from a single user, a bot, or a malicious actor—by restricting how many times an endpoint can be called within a specific timeframe (e.g., "5 requests per minute").

Key Features

 * Decorator Support: Apply limits to specific routes using @limiter.limit().

 * Global Limits: Apply a default limit to every route in your application.

 * Flexible Identification: Rate limit based on IP address, headers, or even custom authentication tokens.

 * Storage Backends: Supports in-memory tracking (default), Redis, or Memcached for distributed systems.

Basic Example: FastAPI + SlowAPI

To use SlowAPI, you first need to install it:

pip install slowapi

Below is a complete implementation that limits a specific endpoint to 5 requests per minute per IP address.

from fastapi import FastAPI, Request

from slowapi import Limiter, _rate_limit_exceeded_handler

from slowapi.util import get_remote_address

from slowapi.errors import RateLimitExceeded

# 1. Initialize the Limiter

# 'key_func' determines how to identify the user (here, by their IP address)

limiter = Limiter(key_func=get_remote_address)

app = FastAPI()

# 2. Attach the limiter to the app state

app.state.limiter = limiter

# 3. Add the exception handler to return a 429 error when limits are hit

app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)

@app.get("/ping")

# 4. Apply the limit to a specific route

@limiter.limit("5/minute")

async def ping(request: Request):

    return {"message": "pong"}

@app.get("/unlimited")

async def unlimited(request: Request):

    return {"message": "I have no limits!"}

Important Usage Notes:

 * The Request object: Every route you want to rate limit must explicitly accept the request: Request argument, even if you don't use it in your code. SlowAPI needs this object to extract the client's identity (like the IP).

 * Order of Decorators: If you are using other decorators (like @app.get), ensure the @limiter.limit decorator is placed below the FastAPI route decorator.

Comparison: Rate Limiting Strategies

| Strategy | Description | Best For |

|---|---|---|

| Fixed Window | Resets at the start of every minute/hour. | Simple, predictable apps. |

| Sliding Window | Calculates usage over a rolling time window. | High-precision accuracy. |

| Token Bucket | Allows "bursts" of requests while maintaining an average rate. | APIs that expect occasional spikes. |

Would you like me to show you how to set up Global Rate Limits so you don't have to add a decorator to every single route?

Quick and Easy Rate Limiting for FastAPI

This video provides a practical walkthrough of setting up SlowAPI in a FastAPI project, including different strategies for managing traffic.

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