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Subject [GitHub] [airflow] JonnyIncognito commented on a change in pull request #6210: [AIRFLOW-5567] BaseAsyncOperator
Date Wed, 16 Oct 2019 23:26:50 GMT
JonnyIncognito commented on a change in pull request #6210: [AIRFLOW-5567] BaseAsyncOperator
URL: https://github.com/apache/airflow/pull/6210#discussion_r335752238
 
 

 ##########
 File path: airflow/models/base_async_operator.py
 ##########
 @@ -0,0 +1,161 @@
+# -*- coding: utf-8 -*-
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# 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.
+
+"""
+Base Asynchronous Operator for kicking off a long running
+operations and polling for completion with reschedule mode.
+"""
+
+from abc import abstractmethod
+from typing import Dict, List, Optional, Union
+
+from airflow.models import SkipMixin, TaskReschedule
+from airflow.models.xcom import XCOM_EXTERNAL_RESOURCE_ID_KEY
+from airflow.sensors.base_sensor_operator import BaseSensorOperator
+from airflow.utils.decorators import apply_defaults
+
+
 
 Review comment:
   > XCOM data appears to be cleared at start of each task. [See here](https://github.com/apache/airflow/blob/1a4c16432b8718189269e63c3afcd2709eed7379/airflow/models/taskinstance.py#L913).
   > 
   > So when task restarts after reschedule, we lose the resource id.
   
   Ouch, that is quite a limitation for using XCom. Is there anywhere else to store task state?
If not, some options:
   
   1. Use `mode='poke'`, at the expense of using up a task slot for long-running tasks. Not
ideal, but gets "correct" behaviour. It'd then be "atomic" rather than "async" behaviour.
   
   1. Enhance `TaskInstance` to make clearing XCom data conditional by delegating it to the
task/operator. There could be a new function like `BaseOperator.pre_execute_clear_state()`
which can be overridden by implementers. `BaseOperator` is already aware of / coupled with
`TaskInstance`, so I don't think we'd be breaking separation of concerns any more than it
already is?
   
   1. There might be enough justification to say that for rescheduled tasks (i.e. transition
from `State.UP_FOR_RESCHEDULE` to `State.RUNNING`) then `TaskInstance` shouldn't clear XCom.
The call to `run()` -> `_check_and_change_state_before_execution()` does know about this
state change, but I see in the code that there are places which bypass the call to `run()`
and go directly to `_run_raw_task()` (e.g. the CLI).
   
   Seems that some form of #2 is the least risky way to get the desired outcome

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