FA-73427 / Rate limiter algorithms / Member archive

Fixed-window request counter: remaining reported before charging · case 02

The last permitted request is advertised as leaving one more.

Member previewVariant 2 · 3 implementations · 6 checks per implementation

Case contract

Input {limit, window_ms, requests [[t, key]]}. Windows are epoch-aligned: index t div W. Each (case-sensitive) key keeps [window, count]; a request in a later window resets the count, while a late request from an earlier window is charged to the current window. Allow while count < limit and report remaining = limit - count after charging; denials are not counted. reset = ms until the end of the key's current window. Return [[decision, remaining, reset]].

Why this case matters

Fixed windows are the simplest API quota; alignment, reset and key-identity mistakes show up as double bursts, stuck quotas or cross-tenant throttling.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
window boundary resets	`[["allow", 2, 10], ["allow", 1, 5], ["deny", 0, 1], ["allow", 2, 1000], ["allow", 1, 498]]`	`[["allow", 1, 10], ["allow", 0, 5], ["deny", 0, 1], ["allow", 1, 1000], ["allow", 0, 498]]`	Failed

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