Why does a fixed-window counter allow 2× the rate limit?
If the limit is 100 requests per minute, a caller can send 100 requests at 0:59 and 100 more at 1:01—both inside separate minute windows, yet 200 requests arrived in 2 seconds. This boundary burst is the fundamental flaw in fixed-window rate limiting and why APIs like Stripe moved to token buckets.
Answered in
Token Buckets: Bounding Rate Limits at the EdgeFixed-window counters leak at boundaries. Token buckets refill steadily, absorb bursts, and bound the sustained rate strictly—the algorithm Stripe uses.
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