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Percentage split and variant hash collide when segment key equals feature key

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Evaluación

Dificultad
3/5
Tiempo estimado
1-2 días
Aptitud para principiantes
78/100
Tipo de issue
Error
Claridad
Bien especificado
Estado de actividad
Activo
Stack tecnológico
python
Área
backend

Línea de trabajo

Start in segments/evaluator.py at the PERCENTAGE_SPLIT handling and variant selection, then trace the get_evaluation_result entry point used by the reproduction. Verify that percentage splits and multivariate variants no longer collide when their keys match, and rerun the 1,000-identity example to confirm both variants are served.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Bug

A percentage split segment and a multivariate feature hash the same inputs when the segment key equals the feature key:

  • the split hashes [segment_key, identity_key] (segments/evaluator.py, PERCENTAGE_SPLIT)
  • the variant hashes [feature_key, identity_key] (segments/evaluator.py, variant selection)

Both give the same number. An identity is only in a 50% split when that number is 50 or below, so it always lands in the first variant. The other variants are never served to anyone in the segment.

Segment and feature keys are ids from different tables, so they can be equal by chance.

Reproduction

from flag_engine.utils.hashing import get_hashed_percentage_for_object_ids as h

identity = "user-1"
assert h(["95", identity]) == h(["95", identity])  # split == variant

End to end: a segment with a 50% split on $.identity.key, overriding a feature with two 50/50 variants, evaluated for 1000 identities:

from collections import Counter

from flag_engine.engine import get_evaluation_result


def variants(segment_key: str, feature_key: str) -> Counter[str]:
    context = {
        "environment": {"key": "env", "name": "env"},
        "segments": {
            segment_key: {
                "key": segment_key,
                "name": "rollout",
                "rules": [{"type": "ALL", "conditions": [
                    {"property": "$.identity.key", "operator": "PERCENTAGE_SPLIT", "value": "50"},
                ]}],
                "overrides": [{
                    "key": feature_key, "name": "checkout", "enabled": True, "value": "control",
                    "variants": [
                        {"value": "a", "weight": 50, "priority": 0, "key": "variant-a"},
                        {"value": "b", "weight": 50, "priority": 1, "key": "variant-b"},
                    ],
                }],
            }
        },
        "features": {"checkout": {"key": "1", "name": "checkout", "enabled": False, "value": None}},
    }
    counts: Counter[str] = Counter()
    for i in range(1000):
        context["identity"] = {"identifier": f"user-{i}", "key": f"user-{i}"}
        flag = get_evaluation_result(context)["flags"]["checkout"]
        if flag["enabled"]:
            counts[flag["variant"]] += 1
    return counts


print(variants("95", "96"))  # Counter({'variant-b': 260, 'variant-a': 254})
print(variants("95", "95"))  # Counter({'variant-a': 514})

Impact

Any segment override with variants on a percentage split segment whose key equals the feature key serves only one variant to the whole segment. For an experiment rollout, the experiment has a single arm. Seen in Flagsmith's API tests in Flagsmith/flagsmith#8646.

Lenguaje dominante
Python
Estrellas
10
Forks
5
Sin PR fusionados en 30 d

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Primeros pasos

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  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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