Rename `[scheduler] max_threads` to `[scheduler] parsing_processes` (#12605)

From Airflow 2.0, `max_threads` config under `[scheduler]` section has been renamed to `parsing_processes`.

This is to align the name with the actual code where the Scheduler launches the number of processes defined by
`[scheduler] parsing_processes` to Parse DAG files, calculates next DagRun date for each DAG,
serialize them and store them in the DB.

UPDATING.md

@@ -52,6 +52,14 @@ assists users migrating to a new version.
 
 ## Master
 
+### `[scheduler] max_threads` config has been renamed to `[scheduler] parsing_processes`
+
+From Airflow 2.0, `max_threads` config under `[scheduler]` section has been renamed to `parsing_processes`.
+
+This is to align the name with the actual code where the Scheduler launches the number of processes defined by
+`[scheduler] parsing_processes` to Parse DAG files, calculates next DagRun date for each DAG,
+serialize them and store them in the DB.
+
 ### Unify user session lifetime configuration
 
 In previous version of Airflow user session lifetime could be configured by

airflow/config_templates/config.yml

@@ -1732,10 +1732,10 @@
       version_added: 2.0.0
       type: boolean
       default: ~
-    - name: max_threads
+    - name: parsing_processes
       description: |
-        The scheduler can run multiple threads in parallel to schedule dags.
-        This defines how many threads will run.
+        The scheduler can run multiple processes in parallel to parse dags.
+        This defines how many processes will run.
       version_added: ~
       type: string
       example: ~

airflow/config_templates/default_airflow.cfg

@@ -866,9 +866,9 @@ use_row_level_locking = True
 # Default: True
 # schedule_after_task_execution =
 
-# The scheduler can run multiple threads in parallel to schedule dags.
-# This defines how many threads will run.
-max_threads = 2
+# The scheduler can run multiple processes in parallel to parse dags.
+# This defines how many processes will run.
+parsing_processes = 2
 
 # Turn off scheduler use of cron intervals by setting this to False.
 # DAGs submitted manually in the web UI or with trigger_dag will still run.

airflow/config_templates/default_test.cfg

@@ -105,7 +105,7 @@ job_heartbeat_sec = 1
 schedule_after_task_execution = False
 scheduler_heartbeat_sec = 5
 scheduler_health_check_threshold = 30
-max_threads = 2
+parsing_processes = 2
 catchup_by_default = True
 scheduler_zombie_task_threshold = 300
 dag_dir_list_interval = 0

airflow/configuration.py

@@ -163,6 +163,7 @@ class AirflowConfigParser(ConfigParser):  # pylint: disable=too-many-ancestors
         ('metrics', 'statsd_datadog_enabled'): ('scheduler', 'statsd_datadog_enabled'),
         ('metrics', 'statsd_datadog_tags'): ('scheduler', 'statsd_datadog_tags'),
         ('metrics', 'statsd_custom_client_path'): ('scheduler', 'statsd_custom_client_path'),
+        ('scheduler', 'parsing_processes'): ('scheduler', 'max_threads'),
     }
 
     # A mapping of old default values that we want to change and warn the user

airflow/utils/dag_processing.py

@@ -510,10 +510,10 @@ class DagFileProcessorManager(LoggingMixin):  # pylint: disable=too-many-instanc
         self._async_mode = async_mode
         self._parsing_start_time: Optional[datetime] = None
 
-        self._parallelism = conf.getint('scheduler', 'max_threads')
+        self._parallelism = conf.getint('scheduler', 'parsing_processes')
         if 'sqlite' in conf.get('core', 'sql_alchemy_conn') and self._parallelism > 1:
             self.log.warning(
-                "Because we cannot use more than 1 thread (max_threads = "
+                "Because we cannot use more than 1 thread (parsing_processes = "
                 "%d ) when using sqlite. So we set parallelism to 1.",
                 self._parallelism,
             )

docs/faq.rst

@@ -205,8 +205,11 @@ This means ``explicit_defaults_for_timestamp`` is disabled in your mysql server
 How to reduce airflow dag scheduling latency in production?
 -----------------------------------------------------------
 
-- ``max_threads``: Scheduler will spawn multiple threads in parallel to schedule dags. This is controlled by ``max_threads`` with default value of 2. User should increase this value to a larger value (e.g numbers of cpus where scheduler runs - 1) in production.
-- If you're using Airflow 1.10.x, consider moving to Airflow 2, which has reduced dag scheduling latency dramatically, and allows for running multiple schedulers.
+- ``parsing_processes``: Scheduler will spawn multiple threads in parallel to parse dags.
+  This is controlled by ``parsing_processes`` with default value of 2.
+  User should increase this value to a larger value (e.g numbers of cpus where scheduler runs + 1) in production.
+- If you're using Airflow 1.10.x, consider moving to Airflow 2, which has reduced dag scheduling latency dramatically,
+  and allows for running multiple schedulers.
 
 Why next_ds or prev_ds might not contain expected values?
 ---------------------------------------------------------

scripts/in_container/airflow_ci.cfg

@@ -53,4 +53,4 @@ _test_only_string = this is a test
 [scheduler]
 job_heartbeat_sec = 1
 scheduler_heartbeat_sec = 5
-max_threads = 2
+parsing_processes = 2

tests/utils/test_dag_processing.py

@@ -238,7 +238,7 @@ class TestDagFileProcessorManager(unittest.TestCase):
         file processors until the next zombie detection logic is invoked.
         """
         test_dag_path = os.path.join(TEST_DAG_FOLDER, 'test_example_bash_operator.py')
-        with conf_vars({('scheduler', 'max_threads'): '1', ('core', 'load_examples'): 'False'}):
+        with conf_vars({('scheduler', 'parsing_processes'): '1', ('core', 'load_examples'): 'False'}):
             dagbag = DagBag(test_dag_path, read_dags_from_db=False)
             with create_session() as session:
                 session.query(LJ).delete()