FA-74274 / Feature flag rollout bucketing / Member archive

Traffic then variant allocation: The variant split reuses the traffic bucket · case 04

At 20 percent traffic every allocated user lands in the first arm.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Stage 1: t = crc32(salt + ":traffic:" + user) % 100; users with t >= traffic (1..100) are not in the experiment. Stage 2 uses an independent v = crc32(salt + ":variant:" + user) % 100 and walks weights ([arm, percent of allocated users], summing to 100) cumulatively, half-open. Return [in_experiment, arm or None].

Why this case matters

Separating traffic allocation from the variant split keeps arms balanced when traffic changes.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
low traffic still fills every arm	`[true, "control"]`	`[true, "treatment"]`	Failed

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