FA-73390 / Rate limiter algorithms / Member archive

Generic cell rate algorithm limiter: retry-after ignores the burst tolerance · case 05

Denied clients are told to wait far longer than necessary.

Member previewVariant 5 · 3 implementations · 6 checks per implementation

Case contract

Input {period_ms T, burst B, requests [[t, quantity]]}. State TAT starts at 0. A quantity above B is ["never", 0, 0]. Otherwise tat = max(TAT, t) and new_tat = tat + q*T; the request is allowed iff new_tat - t <= B*T, in which case TAT = new_tat and remaining = floor((B*T - (TAT - t))/T). A denied request leaves TAT unchanged and reports retry_after = new_tat - B*T - t and remaining floor((B*T - (tat - t))/T). Return [[decision, retry_after, remaining]].

Why this case matters

GCRA limiters store a single theoretical-arrival timestamp per key; every decision and header value is derived from it, so small formula errors leak bursts or starve clients.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
burst of five	`[["allow", 0, 4], ["allow", 0, 3], ["allow", 0, 2], ["allow", 0, 1], ["allow", 0, 0], ["deny", 600, 0], ["allow", 0, 0], ["allow", 0, 0]]`	`[["allow", 0, 4], ["allow", 0, 3], ["allow", 0, 2], ["allow", 0, 1], ["allow", 0, 0], ["deny", 100, 0], ["allow", 0, 0], ["allow", 0, 0]]`	Failed

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