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Subject [GitHub] [airflow] kaxil commented on a change in pull request #6515: [AIRFLOW-XXX] GSoD: How to make DAGs production ready
Date Mon, 25 Nov 2019 14:50:02 GMT
kaxil commented on a change in pull request #6515: [AIRFLOW-XXX] GSoD: How to make DAGs production

 File path: docs/best-practices.rst
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+ .. 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
+ ..
+ .. Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+Best Practices
+Running Airflow in production is seamless. It comes bundled with all the plugins and configs
+necessary to run most of the DAGs. However, you can come across certain pitfalls, which can
cause occasional errors.
+Let's take a look at what you need to do at various stages to avoid these pitfalls, starting
from writing the DAG 
+to the actual deployment in the production environment.
+Writing a DAG
+Creating a new DAG in Airflow is quite simple. However, there are many things that you need
to take care of
+to ensure the DAG run or failure does not produce unexpected results.
+Creating a task
+You should treat tasks in Airflow equivalent to transactions in a database. It implies that
you should never produce
+incomplete results from your tasks. An example is not to produce incomplete data in ``HDFS``
or ``S3`` at the end of a task.
+Airflow can retry a task if it fails. Thus, the tasks should produce the same outcome on
every re-run.
+Some of the ways you can avoid producing a different result -
+* Do not use INSERT during a task re-run, an INSERT statement might lead to duplicate rows
in your database.
+  Replace it with UPSERT.
+* Read and write in a specific partition. Never read the latest available data in a task.

+  Someone may update the input data between re-runs, which results in different outputs.

+  A better way is to read the input data from a specific partition. You can use ``execution_date``
as a partition. 
+  You should follow this partitioning method while writing data in S3/HDFS, as well.
+* The python datetime ``now()`` function gives the current datetime object. 
+  This function should never be used inside a task, especially to do the critical computation,
as it leads to different outcomes on each run. 
+  It's fine to use it, for example, to generate a temporary log.
+.. tip::
+    You should define repetitive parameters such as ``connection_id`` or S3 paths in ``default_args``
rather than declaring them for each task.
+    The ``default_args`` help to avoid mistakes such as typographical errors.
+Deleting a task
+Never delete a task from a DAG. In case of deletion, the historical information of the task
disappears from the Airflow UI. 
+It is advised to create a new DAG in case the tasks need to be deleted.
+Airflow executes tasks of a DAG on different servers in case you are using :doc:`Kubernetes
executor <../executor/kubernetes>` or :doc:`Celery executor <../executor/celery>`.

+Therefore, you should not store any file or config in the local filesystem as the next task
is likely to run on a different server without access to it — for example, a task that downloads
the data file that the next task processes.
+In the case of :class:`Local executor <airflow.executors.local_executor.LocalExecutor>`,

+storing a file on disk can make retries harder e.g., your task requires a config file that
is deleted by another task in DAG.
+If possible, use ``XCom`` to communicate small messages between tasks and a good way of passing
larger data between tasks is to use a remote storage such as S3/HDFS. 
+For example, if we have a task that stores processed data in S3 that task can push the S3
path for the output data in ``Xcom``,
+and the downstream tasks can pull the path from XCom and use it to read the data.
+The tasks should also not store any authentication parameters such as passwords or token
inside them. 
+Where at all possible, use :ref:`Connections <concepts-connections>` to store data
securely in Airflow backend and retrieve them using a unique connection id.
+You should avoid usage of Variables outside an operator's ``execute()`` method or Jinja templates
if possible, 
+as Variables create a connection to metadata DB of Airflow to fetch the value, which can
slow down parsing and place extra load on the DB.
+Airflow parses all the DAGs in the background at a specific period.
+The default period is set using ``processor_poll_interval`` config, which is by default 1
second. During parsing, Airflow creates a new connection to the metadata DB for each DAG.
+It can result in a lot of open connections.
+The best way of using variables is via a Jinja template which will delay reading the value
until the task execution. The template synaxt to do this is:
+.. code::
+    {{ var.value.<variable_name> }}
+or if you need to deserialize a json object from the variable :
+.. code::
+    {{ var.json.<variable_name> }}
+.. note::
+    In general, you should not write any code outside the tasks. The code outside the tasks
runs every time Airflow parses the DAG, which happens every second by default.
+Testing a DAG
+Airflow users should treat DAGs as production level code. The DAGs should have various tests
to ensure that it produces expected results.
+You can write a wide variety of tests for a DAG. Let's take a look at some of them.
+DAG Loader Test
+This test should ensure that your DAG does not contain a piece of code that raises error
while loading.
+No additional code needs to be written by the user to run this test.
+.. code::
+ python
+Running the above command without any error ensures your DAG does not contain any uninstalled
dependency, syntax errors, etc. 
+You can look into :ref:`Testing a DAG <testing>` for details on how to test individual
+Unit tests
+Unit tests ensure that there is no incorrect code in your DAG. You can write a unit test
for your tasks as well as your DAG.
+**Unit test for loading a DAG:**
+.. code::
 Review comment:
   .. code:: python

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