FA-73705 / Rate limiter algorithms / Member archive

Distributed limiter with leased quota chunks: returned lease not credited · case 05

Quota returned by draining nodes is lost for the rest of the window.

Member previewVariant 5 · 3 implementations · 6 checks per implementation

Case contract

Input {quota, window_s, chunk, events [[t, node, "req"|"return"]]} with nondecreasing t. Each epoch window starts with the full global quota and voids all node leases. A node with an empty lease fetches min(chunk, remaining) from the coordinator; each request spends one leased unit or is denied when none is available. "return" credits the node's unused lease back and reports the new remaining. Return [results, remaining, sorted [node, lease]].

Why this case matters

Large-scale limiters hand out quota in chunks to avoid a coordinator round trip per request; lease accounting errors oversubscribe the global limit or strand quota.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
return credits leftover	`[["allow", 2, "allow", "allow", "deny"], 0, [["b", 0], ["c", 0]]]`	`[["allow", 5, "allow", "allow", "allow"], 0, [["b", 2], ["c", 0]]]`	Failed

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