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Type: Build Failure
Resolution: Done
Priority: Unknown
Fix Version/s: None
Affects Version/s: None
Component/s: None
Labels:
- greenerbuild

Confidence Status:
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Assigned Teams:

Python Drivers

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1. What would you like to communicate to the user about this feature? 2. Would you like the user to see examples of the syntax and/or executable code and its output? 3. Which versions of the driver/connector does this apply to?

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Goal Name(s):
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Name of Failure:

test.test_server_selection_in_window.TestProse.test_load_balancing

Link to task:

https://spruce.corp.mongodb.com/task/mongo_python_driver_mongodb_v4.4_test_server_version_python3.14t_sync_auth_ssl_sharded_cluster_bb7a81f1e01d00d9534da0c114f9e7b9f9f76b7c_26_06_30_13_42_02/tests?execution=0&sorts=STATUS%3AASC

Context of when and why the failure occurred:

Historically flaky test that was not made entirely deterministic by ~~PYTHON-3689~~. Possibly a free-threading issue with the event publishing performed by the test.

Stack trace:



[2026/06/30 07:03:17.913] FAILURE: AssertionError: 99 != 100 ()
[2026/06/30 07:03:17.913] self = <test.test_server_selection_in_window.TestProse testMethod=test_load_balancing>
[2026/06/30 07:03:17.913]     @client_context.require_failCommand_appName
[2026/06/30 07:03:17.913]     @client_context.require_multiple_mongoses
[2026/06/30 07:03:17.913]     @unittest.skipIf(
[2026/06/30 07:03:17.913]         sys.platform == "darwin" and platform.machine() == "arm64" and "CI" in os.environ,
[2026/06/30 07:03:17.913]         "PYTHON-5861: Load balancing frequency assertion is timing-sensitive on macOS ARM64 CI",
[2026/06/30 07:03:17.913]     )
[2026/06/30 07:03:17.913]     def test_load_balancing(self):
[2026/06/30 07:03:17.913]         listener = OvertCommandListener()
[2026/06/30 07:03:17.913]         cmap_listener = CMAPListener()
[2026/06/30 07:03:17.913]         # PYTHON-2584: Use a large localThresholdMS to avoid the impact of
[2026/06/30 07:03:17.913]         # varying RTTs.
[2026/06/30 07:03:17.913]         client = self.rs_client(
[2026/06/30 07:03:17.913]             client_context.mongos_seeds(),
[2026/06/30 07:03:17.913]             appName="loadBalancingTest",
[2026/06/30 07:03:17.913]             event_listeners=[listener, cmap_listener],
[2026/06/30 07:03:17.913]             localThresholdMS=30000,
[2026/06/30 07:03:17.913]             minPoolSize=10,
[2026/06/30 07:03:17.913]         )
[2026/06/30 07:03:17.913]         wait_until(lambda: len(client.nodes) == 2, "discover both nodes")
[2026/06/30 07:03:17.913]         # Wait for both pools to be populated.
[2026/06/30 07:03:17.913]         cmap_listener.wait_for_event(ConnectionReadyEvent, 20)
[2026/06/30 07:03:17.913]         # Delay find commands on only one mongos.
[2026/06/30 07:03:17.913]         delay_finds = {
[2026/06/30 07:03:17.913]             "configureFailPoint": "failCommand",
[2026/06/30 07:03:17.913]             "mode": {"times": 10000},
[2026/06/30 07:03:17.913]             "data": {
[2026/06/30 07:03:17.913]                 "failCommands": ["find"],
[2026/06/30 07:03:17.913]                 "blockConnection": True,
[2026/06/30 07:03:17.913]                 "blockTimeMS": 500,
[2026/06/30 07:03:17.913]                 "appName": "loadBalancingTest",
[2026/06/30 07:03:17.913]             },
[2026/06/30 07:03:17.913]         }
[2026/06/30 07:03:17.913]         coll = client.test.test
[2026/06/30 07:03:17.913]         N_TASKS = 10
[2026/06/30 07:03:17.913]         with self.fail_point(delay_finds):
[2026/06/30 07:03:17.913]             nodes = client_context.client.nodes
[2026/06/30 07:03:17.913]             self.assertEqual(len(nodes), 1)
[2026/06/30 07:03:17.913]             delayed_server = next(iter(nodes))
[2026/06/30 07:03:17.913]             # Start background tasks to build up op_count on the delayed server.
[2026/06/30 07:03:17.913]             # This ensures the measurement phase sees a stable op_count imbalance
[2026/06/30 07:03:17.913]             # rather than a 50/50 random distribution from equal initial counts.
[2026/06/30 07:03:17.913]             background_tasks = [FinderTask(coll, 1) for _ in range(N_TASKS)]
[2026/06/30 07:03:17.913]             for task in background_tasks:
[2026/06/30 07:03:17.913]                 task.start()
[2026/06/30 07:03:17.913]             # Wait until all background finds are dispatched so the delayed
[2026/06/30 07:03:17.913]             # server's finds are in-flight (each blocked for 500ms).
[2026/06/30 07:03:17.913]             wait_until(
[2026/06/30 07:03:17.913]                 lambda: len(listener.started_events) >= N_TASKS,
[2026/06/30 07:03:17.913]                 "background tasks to dispatch finds",
[2026/06/30 07:03:17.913]             )
[2026/06/30 07:03:17.913]             # Measure distribution while the delayed server is busy.
[2026/06/30 07:03:17.913]             listener.reset()
[2026/06/30 07:03:17.913] >           freqs = self.frequencies(client, listener)
[2026/06/30 07:03:17.913]                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[2026/06/30 07:03:17.913] test/test_server_selection_in_window.py:193: 
[2026/06/30 07:03:17.913] _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
[2026/06/30 07:03:17.913] test/test_server_selection_in_window.py:131: in frequencies
[2026/06/30 07:03:17.913]     self.assertEqual(len(events), n_finds * N_TASKS)
[2026/06/30 07:03:17.913] E   AssertionError: 99 != 100

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