Python API Rate Limiting with Token Bucket

Overview

Rate limiting protects APIs from abuse, prevents DoS attacks, and ensures fair resource allocation. The token bucket algorithm is one of the most common approaches: it allows bursts of traffic while maintaining a steady average rate. This approach handles implementing token bucket rate limiting in Flask and FastAPI, with both in-memory and Redis-backed distributed variants.

When to Use

• For alternatives, see API Rate Limiting.

• You are building a public API and need to prevent abuse

• You have different pricing tiers with different rate limits

• You need distributed rate limiting across multiple server instances

• You want to throttle specific endpoints differently

Solution

Token bucket with Flask (in-memory)

import time
from collections import defaultdict
from functools import wraps
from flask import Flask, request, jsonify

app = Flask(__name__)

class TokenBucket:
    def __init__(self, capacity: int, refill_rate: float):
        self.capacity = capacity
        self.refill_rate = refill_rate
        self.tokens = capacity
        self.last_refill = time.monotonic()

    def consume(self, tokens: int = 1) -> bool:
        now = time.monotonic()
        elapsed = now - self.last_refill
        self.tokens = min(self.capacity, self.tokens + elapsed * self.refill_rate)
        self.last_refill = now

        if self.tokens >= tokens:
            self.tokens -= tokens
            return True
        return False


buckets: dict[str, TokenBucket] = defaultdict(lambda: TokenBucket(capacity=10, refill_rate=1.0))

def rate_limit(capacity: int = 10, refill_rate: float = 1.0):
    def decorator(func):
        @wraps(func)
        def wrapper(*args, **kwargs):
            client_ip = request.remote_addr
            bucket = buckets[client_ip]

            if not bucket.consume():
                return jsonify({
                    "error": "Rate limit exceeded",
                    "retry_after": round(bucket.capacity / bucket.refill_rate, 1)
                }), 429

            return func(*args, **kwargs)
        return wrapper
    return decorator

@app.route("/api/data")
@rate_limit(capacity=10, refill_rate=1.0)
def get_data():
    return jsonify({"data": "success"})

if __name__ == "__main__":
    app.run(debug=True)

Token bucket with FastAPI (in-memory)

import time
from collections import defaultdict
from fastapi import FastAPI, Request, HTTPException
from fastapi.responses import JSONResponse

app = FastAPI()

class TokenBucket:
    def __init__(self, capacity: int, refill_rate: float):
        self.capacity = capacity
        self.refill_rate = refill_rate
        self.tokens = capacity
        self.last_refill = time.monotonic()

    def consume(self, tokens: int = 1) -> bool:
        now = time.monotonic()
        elapsed = now - self.last_refill
        self.tokens = min(self.capacity, self.tokens + elapsed * self.refill_rate)
        self.last_refill = now

        if self.tokens >= tokens:
            self.tokens -= tokens
            return True
        return False


buckets: dict[str, TokenBucket] = {}

def get_bucket(key: str, capacity: int, refill_rate: float) -> TokenBucket:
    if key not in buckets:
        buckets[key] = TokenBucket(capacity, refill_rate)
    return buckets[key]

@app.middleware("http")
async def rate_limit_middleware(request: Request, call_next):
    if request.url.path.startswith("/api/"):
        client_ip = request.client.host
        bucket = get_bucket(client_ip, capacity=100, refill_rate=10.0)

        if not bucket.consume():
            return JSONResponse(
                status_code=429,
                content={
                    "error": "Rate limit exceeded",
                    "retry_after": round(bucket.capacity / bucket.refill_rate, 1)
                }
            )

    return await call_next(request)

@app.get("/api/data")
async def get_data():
    return {"data": "success"}

Redis-backed distributed rate limiting

import time
import redis
from functools import wraps
from flask import Flask, request, jsonify

app = Flask(__name__)
r = redis.Redis(host="localhost", port=6379, db=0, decode_responses=True)

def redis_token_bucket(key: str, capacity: int, refill_rate: float) -> bool:
    lua_script = """
        local key = KEYS[1]
        local capacity = tonumber(ARGV[1])
        local refill_rate = tonumber(ARGV[2])
        local now = tonumber(ARGV[3])
        local requested = tonumber(ARGV[4])

        local bucket = redis.call("HMGET", key, "tokens", "last_refill")
        local tokens = tonumber(bucket[1])
        local last_refill = tonumber(bucket[2])

        if tokens == nil then
            tokens = capacity
            last_refill = now
        end

        local elapsed = now - last_refill
        tokens = math.min(capacity, tokens + elapsed * refill_rate)

        if tokens >= requested then
            tokens = tokens - requested
            redis.call("HMSET", key, "tokens", tokens, "last_refill", now)
            redis.call("EXPIRE", key, math.ceil(capacity / refill_rate))
            return 1
        else
            redis.call("HMSET", key, "tokens", tokens, "last_refill", now)
            redis.call("EXPIRE", key, math.ceil(capacity / refill_rate))
            return 0
        end
    """

    now = time.time()
    result = r.eval(lua_script, 1, key, capacity, refill_rate, now, 1)
    return bool(result)

def rate_limit(capacity: int = 10, refill_rate: float = 1.0):
    def decorator(func):
        @wraps(func)
        def wrapper(*args, **kwargs):
            client_ip = request.remote_addr
            key = f"rate_limit:{func.__name__}:{client_ip}"

            if not redis_token_bucket(key, capacity, refill_rate):
                return jsonify({
                    "error": "Rate limit exceeded",
                    "retry_after": round(capacity / refill_rate, 1)
                }), 429

            return func(*args, **kwargs)
        return wrapper
    return decorator

@app.route("/api/expensive")
@rate_limit(capacity=5, refill_rate=0.5)
def expensive_operation():
    return jsonify({"result": "computed"})

Sliding window rate limiter with FastAPI

import time
from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
import redis

app = FastAPI()
r = redis.Redis(host="localhost", port=6379, db=0, decode_responses=True)

def sliding_window_limit(key: str, limit: int, window: int) -> bool:
    lua_script = """
        local key = KEYS[1]
        local limit = tonumber(ARGV[1])
        local window = tonumber(ARGV[2])
        local now = tonumber(ARGV[3])

        redis.call("ZREMRANGEBYSCORE", key, 0, now - window)
        local count = redis.call("ZCARD", key)

        if count < limit then
            redis.call("ZADD", key, now, now .. "-" .. math.random())
            redis.call("EXPIRE", key, window)
            return 1
        else
            return 0
        end
    """

    now = time.time()
    result = r.eval(lua_script, 1, key, limit, window, now)
    return bool(result)

@app.middleware("http")
async def sliding_window_middleware(request: Request, call_next):
    if request.url.path.startswith("/api/"):
        client_ip = request.client.host
        key = f"sliding_window:{client_ip}"

        if not sliding_window_limit(key, limit=60, window=60):
            return JSONResponse(
                status_code=429,
                content={"error": "Too many requests", "window": "60 seconds", "limit": 60}
            )

    return await call_next(request)

Per-endpoint rate limits with FastAPI dependency

from fastapi import FastAPI, Depends, HTTPException, Request
import time

app = FastAPI()

class RateLimiter:
    def __init__(self, capacity: int, refill_rate: float):
        self.capacity = capacity
        self.refill_rate = refill_rate
        self._buckets: dict[str, tuple[float, float]] = {}

    def check(self, key: str) -> None:
        now = time.monotonic()
        if key in self._buckets:
            tokens, last_refill = self._buckets[key]
            elapsed = now - last_refill
            tokens = min(self.capacity, tokens + elapsed * self.refill_rate)
        else:
            tokens = self.capacity

        if tokens < 1:
            raise HTTPException(
                status_code=429,
                detail=f"Rate limit exceeded. Capacity: {self.capacity}, refill: {self.refill_rate}/s"
            )

        self._buckets[key] = (tokens - 1, now)

def create_limiter(capacity: int, refill_rate: float):
    limiter = RateLimiter(capacity, refill_rate)
    def dependency(request: Request):
        limiter.check(request.client.host)
    return Depends(dependency)

@app.get("/api/free")
async def free_endpoint():
    return {"message": "no rate limit"}

@app.get("/api/limited", dependencies=[create_limiter(capacity=10, refill_rate=1.0)])
async def limited_endpoint():
    return {"message": "rate limited"}

@app.get("/api/strict", dependencies=[create_limiter(capacity=3, refill_rate=0.5)])
async def strict_endpoint():
    return {"message": "strictly limited"}

Explanation

The token bucket algorithm works by maintaining a bucket of tokens that refills at a constant rate. Each request consumes one token. If the bucket is empty, the request is rejected.

Key concepts:

• Capacity: Maximum burst size. A bucket with capacity 10 allows 10 requests in quick succession.
• Refill rate: Tokens added per second. A refill rate of 1.0 means 1 token per second.
• Distributed: Redis-backed implementations use Lua scripts for atomicity. The script reads, updates, and writes in a single Redis operation, preventing race conditions.
• Sliding window: Instead of fixed windows, the sliding window tracks individual request timestamps in a sorted set. It counts requests within the last N seconds, providing smoother limits.

The Lua script in the Redis variant is important: without atomic execution, concurrent requests could read the same token count and all pass the limit check.

Variants

Approach	Algorithm	Storage	Use When
In-memory	Token bucket	Process memory	Single instance, low traffic
Redis	Token bucket	Redis	Multiple instances, distributed
Redis	Sliding window	Redis sorted set	Strict per-second limits
Redis	Fixed window	Redis INCR	Simple, approximate limits
Database	Token bucket	SQL/Postgres	When Redis is unavailable

Guidelines

• Use Redis-backed limits in production. In-memory limits do not work across multiple instances.
• Return 429 Too Many Requests with a Retry-After header.
• Set different limits for different endpoints based on cost.
• Use API keys or user IDs as the bucket key, not just IP addresses.
• Monitor rate limit hits. A sudden spike may indicate a misconfigured client or an attack.
• Set EXPIRE on Redis keys to prevent memory growth from inactive clients.

Common Mistakes

• Using IP address as the only key. Multiple users behind a NAT share one IP and hit limits unfairly.
• Not using atomic operations in Redis. Concurrent requests can race and bypass the limit.
• Setting capacity too low. Legitimate users get blocked during normal usage bursts.
• Not returning Retry-After. Clients cannot implement backoff without it.
• Forgetting to clean up Redis keys. Inactive client keys accumulate and consume memory.