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🪟 Fixed Window Rate Limiting

1. What is Fixed Window Rate Limiting?

  • Concept:
    Time is split into fixed intervals (e.g., 1 minute). Each client can make up to a set number of requests per interval. When the interval resets, so does the counter.

  • Example:
    If the limit is 5 requests per minute, a client can make 5 requests between 12:00:00 and 12:00:59. At 12:01:00, the counter resets.

2. Usage

Single Limiter Example

from fastapicap import RateLimiter
from fastapi import Depends

limiter = RateLimiter(limit=5, minutes=1)

@app.get("/limited", dependencies=[Depends(limiter)])
async def limited_endpoint():
    return {"message": "You are within the rate limit!"}

Multiple Limiters Example

limiter_1s = RateLimiter(limit=1, seconds=1)
limiter_30m = RateLimiter(limit=30, minutes=1)

@app.get("/strict", dependencies=[Depends(limiter_1s), Depends(limiter_30m)])
async def strict_endpoint():
    return {"message": "You passed both rate limits!"}

3. Available Configuration Options

You can customize the Fixed Window limiter using the following parameters:

Parameter Type Description Default
limit int Required. Maximum number of requests allowed per window. Must be positive. —
seconds int Number of seconds in the window. Can be combined with minutes, hours, or days. 0
minutes int Number of minutes in the window. 0
hours int Number of hours in the window. 0
days int Number of days in the window. 0
key_func Callable Function to extract a unique key from the request (e.g., by IP, user ID, etc.). By default, uses client IP and path.
on_limit Callable Function called when the rate limit is exceeded. By default, raises HTTP 429.
prefix str Redis key prefix for all limiter keys. "cap"

Note:
- The window size is calculated as the sum of all time units provided (seconds, minutes, hours, days). - At least one time unit must be positive, and limit must be positive.

Example: 

# 100 requests per hour, with a custom Redis key prefix
limiter = RateLimiter(limit=100, hours=1, prefix="myapi")

4. How Fixed Window Works

Suppose you set a limit of 5 requests per minute:

  • At 12:00:00, the window starts.
  • The client makes 5 requests between 12:00:00 and 12:00:59 — all are allowed.
  • A 6th request at 12:00:30 is blocked with a 429 error.
  • At 12:01:00, the window resets. The client can make 5 more requests.

Visualization:

Time Request # Allowed? Reason
12:00:01 1 ✅ Within limit
12:00:10 2 ✅ Within limit
12:00:20 3 ✅ Within limit
12:00:30 4 ✅ Within limit
12:00:50 5 ✅ Within limit
12:00:55 6 ❌ Limit exceeded
12:01:01 7 ✅ New window, allowed

5. Notes, Pros & Cons

Pros: - Simple to understand and implement. - Low memory usage. - Good for aggressive, clear-cut limits.

Cons: - Can allow bursts at window boundaries (e.g., 5 requests at 12:00:59 and 5 more at 12:01:00).

Use Fixed Window for simple, aggressive limits where bursts at window boundaries are acceptable.