chore(): small improvements

src/backend/apps/conflicts/services.py

@@ -1,3 +1,4 @@
+import hashlib
 from uuid import UUID
 
 from django.core.exceptions import ValidationError
@@ -12,6 +13,17 @@ from apps.conflicts.selectors import domain_active_experiments
 from apps.experiments.models import ACTIVE_STATUSES, Experiment
 
 
+def _subject_winner_index(
+    *,
+    subject_id: str,
+    domain_id: UUID,
+    size: int,
+) -> int:
+    seed = f"{domain_id}:{subject_id}".encode()
+    digest = hashlib.sha256(seed).digest()
+    return int.from_bytes(digest[:8], byteorder="big") % size
+
+
 @transaction.atomic
 def conflict_domain_create(
     *,
@@ -167,7 +179,15 @@ def resolve_domain_conflict(
         return True
 
     if domain.policy == ConflictPolicy.MUTUAL_EXCLUSION:
-        winner = active_memberships[0]
+        if subject_id:
+            winner_idx = _subject_winner_index(
+                subject_id=subject_id,
+                domain_id=domain_id,
+                size=len(active_memberships),
+            )
+            winner = active_memberships[winner_idx]
+        else:
+            winner = active_memberships[0]
         return str(winner.experiment_id) == str(experiment_id)
 
     if domain.policy == ConflictPolicy.PRIORITY:
@@ -187,7 +207,19 @@ def resolve_domain_conflict(
         tied = [m for m in active_memberships if m.priority == top_priority]
         if len(tied) <= 1:
             return True
-        winner = min(tied, key=lambda m: m.experiment.created_at)
+        if subject_id:
+            ordered_tied = sorted(
+                tied,
+                key=lambda m: str(m.experiment_id),
+            )
+            winner_idx = _subject_winner_index(
+                subject_id=subject_id,
+                domain_id=domain_id,
+                size=len(ordered_tied),
+            )
+            winner = ordered_tied[winner_idx]
+        else:
+            winner = min(tied, key=lambda m: m.experiment.created_at)
         return str(winner.experiment_id) == str(experiment_id)
 
     return True

src/backend/apps/conflicts/tests/test_conflicts.py

@@ -285,7 +285,7 @@ class ResolveDomainConflictTest(TestCase):
         exp = experiment_approve(experiment=exp, approver=self.approver)
         return experiment_start(experiment=exp, user=self.experimenter)
 
-    def test_mutual_exclusion_winner_is_first(self) -> None:
+    def test_mutual_exclusion_deterministic_per_subject(self) -> None:
         domain = make_domain(
             suffix="_me",
             policy=ConflictPolicy.MUTUAL_EXCLUSION,
@@ -293,10 +293,17 @@ class ResolveDomainConflictTest(TestCase):
         )
         exp1 = self._make_and_start("_me1", domain)
         exp2 = self._make_and_start("_me2", domain)
-        winner = resolve_domain_conflict(exp1.pk, domain.pk, "u1")
-        self.assertTrue(winner)
-        loser = resolve_domain_conflict(exp2.pk, domain.pk, "u1")
-        self.assertFalse(loser)
+        exp1_u1 = resolve_domain_conflict(exp1.pk, domain.pk, "u1")
+        exp2_u1 = resolve_domain_conflict(exp2.pk, domain.pk, "u1")
+        self.assertNotEqual(exp1_u1, exp2_u1)
+        self.assertEqual(
+            exp1_u1,
+            resolve_domain_conflict(exp1.pk, domain.pk, "u1"),
+        )
+        self.assertEqual(
+            exp2_u1,
+            resolve_domain_conflict(exp2.pk, domain.pk, "u1"),
+        )
 
     def test_priority_higher_wins(self) -> None:
         domain = make_domain(
@@ -309,7 +316,7 @@ class ResolveDomainConflictTest(TestCase):
         self.assertTrue(resolve_domain_conflict(exp_high.pk, domain.pk, "u1"))
         self.assertFalse(resolve_domain_conflict(exp_low.pk, domain.pk, "u1"))
 
-    def test_priority_tie_first_created_wins(self) -> None:
+    def test_priority_tie_deterministic_per_subject(self) -> None:
         domain = make_domain(
             suffix="_tie",
             policy=ConflictPolicy.PRIORITY,
@@ -317,8 +324,17 @@ class ResolveDomainConflictTest(TestCase):
         )
         exp1 = self._make_and_start("_tie1", domain, priority=5)
         exp2 = self._make_and_start("_tie2", domain, priority=5)
-        self.assertTrue(resolve_domain_conflict(exp1.pk, domain.pk, "u1"))
-        self.assertFalse(resolve_domain_conflict(exp2.pk, domain.pk, "u1"))
+        exp1_u1 = resolve_domain_conflict(exp1.pk, domain.pk, "u1")
+        exp2_u1 = resolve_domain_conflict(exp2.pk, domain.pk, "u1")
+        self.assertNotEqual(exp1_u1, exp2_u1)
+        self.assertEqual(
+            exp1_u1,
+            resolve_domain_conflict(exp1.pk, domain.pk, "u1"),
+        )
+        self.assertEqual(
+            exp2_u1,
+            resolve_domain_conflict(exp2.pk, domain.pk, "u1"),
+        )
 
     def test_single_experiment_always_wins(self) -> None:
         domain = make_domain(suffix="_single")