Add stability and redistribution tests

Author: betatim

binderhub/tests/test_utils.py

@@ -12,12 +12,45 @@ def test_rendezvous_rank():
 
 
 def test_rendezvous_independence():
-    # check that the relative ranking of two buckets doesn't depend on the
-    # presence of a third bucket
+    # check that the relative ranking of 80 buckets doesn't depend on the
+    # presence of 20 extra buckets
     key = "k1"
-    two_buckets = utils.rendezvous_rank(["b1", "b3"], key)
-    three_buckets = utils.rendezvous_rank(["b1", "b2", "b3"], key)
-    # remove "b2" and check ranking
-    three_buckets.remove("b2")
+    eighty_buckets = utils.rendezvous_rank(["b%i" % i for i in range(80)], key)
+    hundred_buckets = utils.rendezvous_rank(["b%i" % i for i in range(100)], key)
 
-    assert two_buckets == three_buckets
+    for i in range(80, 100):
+        hundred_buckets.remove("b%i" % i)
+
+    assert eighty_buckets == hundred_buckets
+
+
+def test_rendezvous_redistribution():
+    # check that approximately a third of keys move to the new bucket
+    # when one is added
+    n_keys = 3000
+
+    # counnt how many keys were moved and which bucket were they moved from
+    n_moved = 0
+    from_b1 = 0
+    from_b2 = 0
+
+    for i in range(n_keys):
+        key = f"key-{i}"
+        two_buckets = utils.rendezvous_rank(["b1", "b2"], key)
+        three_buckets = utils.rendezvous_rank(["b1", "b2", "b3"], key)
+
+        if two_buckets[0] != three_buckets[0]:
+            n_moved += 1
+            if two_buckets[0] == "b1":
+                from_b1 += 1
+            if two_buckets[0] == "b2":
+                from_b2 += 1
+            # should always move to the newly added bucket
+            assert three_buckets[0] == "b3"
+
+    # because of statistical fluctuations we have to leave some room when
+    # making this comparison
+    assert 0.31 < n_moved / n_keys < 0.35
+    # keys should move from the two original buckets with approximately
+    # equal probability
+    assert abs(from_b1 - from_b2) < 10