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Rate Limiter Algorithm Comparison — Same Requests, Real Different Decisions

Three real rate-limiting algorithms — Fixed Window Counter, Sliding Window Log, and Token Bucket — run against the exact same request timeline, so you see genuinely different allow/deny decisions on identical input, not a conceptual comparison chart. The default scenario reproduces Fixed Window Counter's classic boundary-burst flaw: a burst split across a window boundary sails through at roughly double the configured limit, while Sliding Window Log correctly catches it.

How to use Rate Limiter Algorithm Comparison

  1. Pick a request scenario — boundary burst, sudden burst, or steady traffic.
  2. Adjust the limit.
  3. Compare which requests each algorithm allows (green) or denies (red).
  4. Read why the boundary-burst scenario breaks Fixed Window specifically.

Frequently asked questions

What is the Fixed Window boundary-burst flaw?
Fixed Window Counter resets its count at fixed clock boundaries with no memory of the previous window — a burst of near-limit requests right before a boundary, followed by another near-limit burst right after, gets counted as two separate compliant windows even though the real elapsed time between them was tiny, letting through roughly double the intended rate.
Why does Sliding Window Log fix that?
It tracks the actual timestamp of every recent request and only counts ones within the true trailing window from right now, with no artificial boundary to straddle — at the cost of storing a timestamp per request instead of a single counter.
What does Token Bucket do differently?
It allows a burst up to the bucket's capacity instantly, then only replenishes tokens at a steady refill rate — so it tolerates legitimate burstiness while still enforcing a real long-run average rate, which is why it is the common production default.
Which algorithm should I actually use in production?
Token Bucket is the most common real-world choice — a good balance of burst tolerance and long-run rate enforcement without per-request memory; Sliding Window Log is used when precision matters more than memory cost; plain Fixed Window is mostly seen where simplicity matters more than the boundary flaw.

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