Rate limiting for Python + Flask

Arcjet rate limiting lets you define rules which limit the number of requests a client can make over a period of time.

What is Arcjet?

Arcjet is the runtime security platform that ships with your code. Enforce budgets, stop prompt injection, detect bots, and protect personal information with Arcjet's AI security building blocks.

Quick start

This guide shows you how to add a basic rate limit to your app.

1. Install Arcjet

In your project root, install the SDK:

In your project root, install the Arcjet Python SDK along with Flask:

uv add arcjet flask

If you don’t use uv, pip install arcjet flask (or your preferred package manager’s equivalent) works too.

2. Set your key

Create a free Arcjet account then follow the instructions to add a site and get a key.

Add your key to a .env file in your project root. The Flask CLI loads .env automatically when python-dotenv is installed. Arcjet does not read ARCJET_KEY automatically – your application must pass it explicitly to arcjet_sync(key=...).

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# Run Arcjet in development <https://docs.arcjet.com/environment#arcjet-env>.
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ARCJET_ENV=development
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# Arcjet key for your site (from <https://console.arcjet.com>).
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# More info: <https://docs.arcjet.com/environment#arcjet-key>.
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ARCJET_KEY=ajkey_yourkey

3. Add a rate limit to a route

The following example applies a token bucket rate limit rule to a route where we identify the user based on their ID, for example when they’re logged in. The bucket is configured with a maximum capacity of 10 tokens and refills by 5 tokens every 10 seconds. Each request consumes 5 tokens.

Create a new file at main.py with the contents:

import os

from arcjet import Mode, arcjet_sync, token_bucket
from flask import Flask, jsonify, request

app = Flask(__name__)

aj = arcjet_sync(
    key=os.environ["ARCJET_KEY"],  # Get your site key from https://console.arcjet.com
    rules=[
        # Create a token bucket rate limit. Other algorithms are supported.
        token_bucket(
            mode=Mode.LIVE,  # Blocks requests. Use Mode.DRY_RUN to log only
            characteristics=["userId"],  # track requests by a custom user ID
            refill_rate=5,  # refill 5 tokens per interval
            interval=10,  # refill every 10 seconds
            capacity=10,  # bucket maximum capacity of 10 tokens
        ),
    ],
)


@app.get("/")
def index():
    user_id = "user123"  # Replace with your authenticated user ID
    # Deduct 5 tokens from the bucket
    decision = aj.protect(request, characteristics={"userId": user_id}, requested=5)
    print("Arcjet decision", decision)

    if decision.is_denied():
        return jsonify(error="Too Many Requests"), 429

    return jsonify(message="Hello world")

4. Start server

Start the Flask dev server:

uv run flask --app main run --debug

Then make some requests to hit the rate limit:

for i in $(seq 1 5); do curl http://localhost:5000; done

After a few requests you get a 429 Too Many Requests response because each request consumes 5 tokens from a bucket with a maximum capacity of 10.

The requests also appear in the Arcjet dashboard.

FAQs

Do I need to run any infrastructure e.g. Redis?

No, Arcjet handles all the infrastructure for you so you don't need to worry about deploying global Redis clusters, designing data structures to track rate limits, or keeping security detection rules up to date.

What is the performance overhead?

Arcjet SDK tries to do as much as possible asynchronously and locally to minimize latency for each request. Where decisions can be made locally or previous decisions are cached in-memory, latency is usually <1ms.

When a call to the Cloud API is required, such as when tracking a rate limit in a serverless environment, there is some additional latency before a decision is made. The Cloud API has been designed for high performance and low latency, and is deployed to multiple regions around the world. The SDK will automatically use the closest region which means the total overhead is typically no more than 20-30ms, often significantly less.

What happens if Arcjet is unavailable?

Where a decision has been cached locally e.g. blocking a client, Arcjet will continue to function even if the service is unavailable.

If a call to the Cloud API is needed and there is a network problem or Arcjet is unavailable, the default behavior is to fail open and allow the request. You have control over how to handle errors, including choosing to fail close if you prefer. See the reference docs for details.

How does Arcjet protect me against DDoS attacks?

Network layer attacks tend to be generic and high volume, so these are best handled by your hosting platform. Most cloud providers include network DDoS protection by default.

Arcjet sits closer to your application so it can understand the context. This is important because some types of traffic may not look like a DDoS attack, but can still have the same effect. For example, a customer making too many API requests and affecting other customers, or large numbers of signups from disposable email addresses.

Network-level DDoS protection tools find it difficult to protect against this type of traffic because they don't understand the structure of your application. Arcjet can help you to identify and block this traffic by integrating with your codebase and understanding the context of the request e.g. the customer ID or sensitivity of the API route.

Volumetric network attacks are best handled by your hosting provider. Application level attacks need to be handled by the application. That's where Arcjet helps.

What next?

Rate limiting reference Full details about rate limiting.

Rate limiting algorithms Learn which algorithm to use.

Testing Write tests for your rules.

Explore

Arcjet can be used with specific rules on individual routes or as general protection on your entire application. You can set up bot protection, minimize fraudulent registrations with the signup form protection and more.

Example apps See the example apps.

Learn how Arcjet works Arcjet's architecture.

SDK reference Python SDK reference guide.

Get help

Need help with anything? Email support@arcjet.com to get support from our engineering team.