sliding-rate-limiter-redis 1.0.1

Redis-backed sliding-window rate limiter for Python

sliding-rate-limiter-redis

A Redis-backed sliding window rate limiter for Python.

Why sliding window?

Most rate limiters use a fixed window (e.g. "100 requests per minute, resetting on the clock"). This creates a burst problem: a client can make 100 requests at 00:59 and another 100 at 01:00 — 200 requests in two seconds.

The sliding window log algorithm solves this by tracking the exact timestamp of every request in a Redis sorted set. Only requests within the last window seconds are counted, so the limit is enforced continuously rather than in discrete buckets.

All Redis operations execute atomically via a Lua script, so check-and-increment is race-condition free under high concurrency.

Installation

pip install sliding-rate-limiter-redis

Requires redis-py >= 4.0.0 and a running Redis instance.

Quick start

import redis
from rate_limiter import SlidingWindowLimiter, RateLimitError

r = redis.Redis()
limiter = SlidingWindowLimiter(r, limit=100, window=60, key_prefix="rl")

# Non-throwing: inspect the result yourself
result = limiter.check("user:42")
print(result)
# RateLimitResult(allowed=True, remaining=99, retry_after=None, reset_at=..., limit=100)

# Throwing: raises RateLimitError if the limit is exceeded
try:
    limiter.consume("user:42")
except RateLimitError as e:
    print(f"Rate limited. Retry after {e.retry_after}s")

FastAPI middleware example

from fastapi import Request
from fastapi.responses import JSONResponse

@app.middleware("http")
async def rate_limit(request: Request, call_next):
    try:
        result = limiter.consume(request.client.host)
        response = await call_next(request)
        response.headers["X-RateLimit-Remaining"] = str(result.remaining)
        return response
    except RateLimitError as e:
        return JSONResponse(
            status_code=429,
            content={"error": "Too Many Requests"},
            headers={"Retry-After": str(e.retry_after)},
        )

API

SlidingWindowLimiter(redis, *, limit, window, key_prefix="rl")

Parameter	Type	Description
redis	redis.Redis	Connected redis-py client
limit	int	Maximum requests per window
window	int	Window duration in seconds
key_prefix	str	Redis key prefix. Default: "rl"

check(identifier) → RateLimitResult

Records the request and returns a result. Does not raise on limit exceeded.

consume(identifier) → RateLimitResult

Same as check() but raises RateLimitError if the limit is exceeded.

RateLimitResult

Field	Type	Description
allowed	bool	Whether the request was allowed
remaining	int	Requests remaining in the current window
retry_after	int | None	Seconds until next request allowed; None if allowed
reset_at	datetime	UTC datetime when the window resets
limit	int	The configured limit

RateLimitError

Raised by consume() when the limit is exceeded. Carries retry_after, reset_at, and limit.

Full documentation

See the GitHub repository for architecture details, Node.js package, benchmarks, and more.

License

MIT