[GitHub] [airflow] ephraimbuddy commented on issue #11858: Xcom_pull results order is not deterministic

[GitBox <gi...@apache.org>]

ephraimbuddy commented on issue #11858:
URL: https://github.com/apache/airflow/issues/11858#issuecomment-716747899


   This example dag below gives inconsistent results:
   
   ```
   #
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   # or more contributor license agreements.  See the NOTICE file
   # distributed with this work for additional information
   # regarding copyright ownership.  The ASF licenses this file
   # to you under the Apache License, Version 2.0 (the
   # "License"); you may not use this file except in compliance
   # with the License.  You may obtain a copy of the License at
   #
   #   http://www.apache.org/licenses/LICENSE-2.0
   #
   # Unless required by applicable law or agreed to in writing,
   # software distributed under the License is distributed on an
   # "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   # KIND, either express or implied.  See the License for the
   # specific language governing permissions and limitations
   # under the License.
   
   """Example DAG demonstrating the usage of XComs."""
   from airflow import DAG
   from airflow.operators.python import PythonOperator
   from airflow.utils.dates import days_ago
   
   dag = DAG(
       'example_xcom',
       schedule_interval="@once",
       start_date=days_ago(2),
       default_args={'owner': 'airflow'},
       tags=['example']
   )
   
   value_1 = [1, 2, 3]
   value_2 = {'a': 'b'}
   
   
   def push(**kwargs):
       """Pushes an XCom without a specific target"""
       kwargs['ti'].xcom_push(key='value from pusher 1', value=value_1)
   
   
   def push_by_returning(**kwargs):
       """Pushes an XCom without a specific target, just by returning it"""
       return value_2
   
   
   def puller(**kwargs):
       """Pull all previously pushed XComs and check if the pushed values match the pulled values."""
       ti = kwargs['ti']
   
       # get value_1
       pulled_value_1 = ti.xcom_pull(key=None, task_ids='push')
       if pulled_value_1 != value_1:
           raise ValueError(f'The two values differ {pulled_value_1} and {value_1}')
   
       # get value_2
       pulled_value_2 = ti.xcom_pull(task_ids='push_by_returning')
       if pulled_value_2 != value_2:
           raise ValueError(f'The two values differ {pulled_value_2} and {value_2}')
   
       # get both value_1 and value_2
       pulled_value_1, pulled_value_2 = ti.xcom_pull(key=None, task_ids=['push', 'push_by_returning'])
       if pulled_value_1 != value_1:
           raise ValueError(f'The two values differ {pulled_value_1} and {value_1}')
       if pulled_value_2 != value_2:
           raise ValueError(f'The two values differ {pulled_value_2} and {value_2}')
   
   
   push1 = PythonOperator(
       task_id='push',
       dag=dag,
       python_callable=push,
   )
   
   push2 = PythonOperator(
       task_id='push_by_returning',
       dag=dag,
       python_callable=push_by_returning,
   )
   
   pull = PythonOperator(
       task_id='puller',
       dag=dag,
       python_callable=puller,
   )
   
   pull << [push1, push2]
   ```
   I checked the code, the xcom_pull are ordered both with execution_date and timestamp.
   However, for the above DAG, the tasks have the same execution_date and may also have the same timestamp. In this case, the order of the xcom_pull result is not consistent.


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