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Fixed Window

This algorithm divides time into fixed durations/windows. For example each window is 10 seconds long. When a new request comes in, the current time is used to determine the window and a counter is increased. If the counter is larger than the set limit, the request is rejected.

In fixed & sliding window algorithms, the reset time is based on fixed time boundaries (which depend on the period), not on when the first request was made. So two requests made right before the window ends still count toward the current window, and limits reset at the start of the next window.

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Pros

  • Very cheap in terms of data size and computation
  • Newer requests are not starved due to a high burst in the past

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Cons

  • Can cause high bursts at the window boundaries to leak through
  • Causes request stampedes if many users are trying to access your server, whenever a new window begins

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Usage

from upstash_ratelimit import Ratelimit, FixedWindow
from upstash_redis import Redis

ratelimit = Ratelimit(
    redis=Redis.from_env(),
    limiter=FixedWindow(max_requests=10, window=10),
)

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Sliding Window

Builds on top of fixed window but instead of a fixed window, we use a rolling window. Take this example: We have a rate limit of 10 requests per 1 minute. We divide time into 1 minute slices, just like in the fixed window algorithm. Window 1 will be from 00:00:00 to 00:01:00 (HH:MM:SS). Let’s assume it is currently 00:01:15 and we have received 4 requests in the first window and 5 requests so far in the current window. The approximation to determine if the request should pass works like this:

limit = 10

# 4 request from the old window, weighted + requests in current window
rate = 4 * ((60 - 15) / 60) + 5 = 8

return rate < limit # True means we should allow the request

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Pros

  • Solves the issue near boundary from fixed window.

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Cons

  • More expensive in terms of storage and computation
  • It’s only an approximation because it assumes a uniform request flow in the previous window

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Usage

from upstash_ratelimit import Ratelimit, SlidingWindow
from upstash_redis import Redis

ratelimit = Ratelimit(
    redis=Redis.from_env(),
    limiter=SlidingWindow(max_requests=10, window=10),
)

reset field in the limit method of sliding window does not provide an exact reset time. Instead, the reset time is the start time of the next window.

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Token Bucket

Consider a bucket filled with maximum number of tokens that refills constantly at a rate per interval. Every request will remove one token from the bucket and if there is no token to take, the request is rejected.

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Pros

  • Bursts of requests are smoothed out and you can process them at a constant rate.
  • Allows setting a higher initial burst limit by setting maximum number of tokens higher than the refill rate

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Cons

  • Expensive in terms of computation

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Usage

from upstash_ratelimit import Ratelimit, TokenBucket
from upstash_redis import Redis

ratelimit = Ratelimit(
    redis=Redis.from_env(),
    limiter=TokenBucket(max_tokens=10, refill_rate=5, interval=10),
)