SlowAPI: Secure Your FastAPI App with Rate Limiting

Have you ever experienced a sudden surge in API requests that slowed down your FastAPI app or even caused it to crash? Whether it's a malicious attack or an accidental overload, excessive requests can harm your application's performance and security.

That’s where rate limiting comes in! By setting limits on how often users can make requests, you can protect your API from abuse, ensure fair usage, and maintain optimal performance.

In this guide, we’ll walk you through SlowAPI, a simple yet effective way to add rate limiting to your FastAPI application. Whether you’re a beginner or an experienced developer, you’ll find step-by-step instructions to implement and customize rate limiting effortlessly.

Let’s dive in and make your API more secure and efficient! 🚀

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What is Rate Limiting, and Why Do You Need It?

Rate limiting is a technique used to control the number of requests a client can make within a specific timeframe. It's essential for:

✅ Preventing API abuse – Stops users from overwhelming your API with too many requests.
✅ Improving security – Mitigates DDoS attacks and brute-force login attempts.
✅ Enhancing performance – Ensures fair resource allocation and prevents slowdowns.
✅ Saving costs – Reduces unnecessary server load and prevents excessive bandwidth usage.

FastAPI doesn’t have built-in rate limiting, but SlowAPI provides an easy way to implement it. Now, let’s set it up step by step!

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Step 1: Install SlowAPI

To get started, install the SlowAPI package using pip:

pip install slowapi

Since SlowAPI depends on Starlette Middleware, FastAPI will work seamlessly with it.

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Step 2: Set Up the Rate Limiter

Now, in your FastAPI application, import and configure the Limiter from SlowAPI:

from fastapi import FastAPI, Request
from slowapi import Limiter
from slowapi.util import get_remote_address

# Initialize the rate limiter
limiter = Limiter(key_func=get_remote_address)

# Create the FastAPI app and add the limiter
app = FastAPI()
app.state.limiter = limiter

Here’s what’s happening:

✅ Limiter: Creates a rate limiter instance.
✅ get_remote_address: Uses the client’s IP address as the identifier for rate limiting.
✅ app.state.limiter: Stores the limiter in the app's state for global access.

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Step 3: Apply Rate Limiting to Routes

Now, let’s apply rate limits to your API endpoints.

Limit Requests on a Single Route

You can restrict a specific route using a decorator:

from slowapi.errors import RateLimitExceeded
from slowapi.middleware import SlowAPIMiddleware
from starlette.responses import JSONResponse

# Middleware to handle rate limiting
app.add_middleware(SlowAPIMiddleware)

@app.get("/limited-route")
@limiter.limit("5/minute")  # Allows only 5 requests per minute
async def limited_endpoint(request: Request):
    return {"message": "This route is rate limited!"}

Handling Rate Limit Errors

If a user exceeds the limit, they should receive a clear error message. You can define a custom error handler like this:

from starlette.requests import Request

@app.exception_handler(RateLimitExceeded)
async def rate_limit_handler(request: Request, exc: RateLimitExceeded):
    return JSONResponse(
        status_code=429,
        content={"error": "Too many requests! Please try again later."}
    )

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Step 4: Apply Global Rate Limits

To set a global limit for all routes, modify the app setup:

limiter = Limiter(key_func=get_remote_address, default_limits=["100/hour"])  # 100 requests per hour

app = FastAPI()
app.state.limiter = limiter
app.add_middleware(SlowAPIMiddleware)

Now, all routes in your API will have a global limit of 100 requests per hour unless specified otherwise.

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Step 5: Customize Rate Limits for Different Users

You might want to allow different limits based on user roles, API keys, or authentication levels. You can customize the key_func to achieve this:

def custom_key_func(request: Request):
    user = request.headers.get("X-User-ID", "anonymous")  # Use user ID if available
    return user

limiter = Limiter(key_func=custom_key_func, default_limits=["50/minute"])

This approach:
✅ Identifies users based on headers (or authentication tokens).
✅ Prevents abuse by distinguishing between authenticated and anonymous users.
✅ Allows flexible rate limits per user type.

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Final Thoughts & Next Steps

Congratulations! 🎉 You’ve successfully implemented rate limiting in your FastAPI app using SlowAPI. Now, your API is more secure, efficient, and protected from abuse.

Next Steps:

🚀 Test your API by making multiple requests to ensure rate limiting works as expected.
🔧 Adjust the rate limits based on your app’s needs.
🔑 Implement role-based rate limits for better control.

Rate limiting is just one step toward securing your FastAPI app. Want to take it further? Consider authentication, logging, and monitoring for a fully protected API.

Now, go ahead and secure your API with SlowAPI today! 🚀

If you're new to Python and want to strengthen your core knowledge, check out:
🔹 Exploring Python's Core: From Basics to Behind-the-Scenes
🔹 Mastering Python Operators