Overview
Airflow has a lot of great features and is a fast moving project. As such, there are some common pitfalls that are worth noting.
We do support more than one DAG definition per python file, but it is not recommended as we would like better isolation between DAGs from a fault and deployment perspective and multiple DAGs in the same file goes against that. For now, make sure that the dag object is in the global namespace : you can use the globals dict as in globals()[dag_id] = DAG(...)
Configuring parallelism in airflow.cfg
- parallelism = number of physical python processes the scheduler can run
- dag_concurrency = the number of TIs to be allowed to run PER-dag at once
- max_active_runs_per_dag = number of dag runs (per-DAG) to allow running at once
Understanding the execution date
- Airflow was developed as a solution for ETL needs. In the ETL world, you typically summarize data. So, if I want to summarize data for
2016-02-19, I would do it at 2016-02-20 midnight GMT, which would be right after all data for 2016-02-19 becomes available. - This date is available to you in both Jinja and a Python callable's context in many forms as documented here. As a note
ds refers to date_string, not date start as may be confusing to some.
Run your entire Airflow infrastructure in UTC. Airflow was developed at Airbnb, where every system runs on UTC (GMT). As a result, various parts of Airflow assume that the system (and database) timezone is UTC (GMT). This includes:
- Webserver
- Metadata DB
- Scheduler
- Workers (possibly)
When setting a schedule, align the start date with the schedule. If a schedule is to run at 2am UTC, the start-date should also be at 2am UTC
Bash Operator - Jinja templating and the bash commands
- Described here : see below. You need to add a space after the script name in cases where you are directly calling a bash scripts in the
bash_command attribute ofBashOperator - this is because the Airflow tries to apply a Jinja template to it, which will fail.
t2 = BashOperator(
task_id='sleep',
bash_command="/home/batcher/test.sh", // This fails with `Jinja template not found` error
#bash_command="/home/batcher/test.sh ", // This works (has a space after)
dag=dag)
- When needing to change your start_date and , change the name of the dag (a.k.a. dag_id) - I follow the convention :
my_dag_v1, my_dag_v2, my_dag_v3, my_dag_v4, etc...- Changing schedule interval always requires changing the dag_id, because previously run TaskInstances will not align with the new schedule interval
- Changing start_date without changing schedule_interval is safe, but changing to an earlier start_date will not create any new DagRuns for the time between the new start_date and the old one, so tasks will not automatically backfill to the new dates. If you manually create DagRuns, tasks will be scheduled, as long as the DagRun date is after both the task start_date and the dag start_date.
- When talking to MySQL, please use
mysqlclient, not pymysql - Using a start_date of datetime.now() can lead to unpredictable behavior, and your DAG never starting. See this post for details. It's recommended to subtract a timespan to force the scheduler to recognize the start_date. ***
- Solving MySQL Server has gone away
- you may have an issue with your connection pool keeping connections open too long and checking out an old connection for you. To get a fresh connection, you can set the sql_alchemy_pool_recycle option in your configuration. Set this sql_alchemy_pool_recycle to the number of seconds of time in the connection pool between checkouts that you want a new connection to be created instead of an existing connection to be returned.
- If you intend to use extended ASCII or Unicode characters in Airflow, you have to provide a proper connection string to Airflow MySQL database, which defines charset explicitly, ex.:
sql_alchemy_conn = mysql://airflow@localhost:3306/airflow?charset=utf8 Otherwise you will experience exceptions thrown by WTForms templating and other Airflow modules:UnicodeDecodeError: 'ascii' codec can't decode byte 0xae in position 506: ordinal not in range(128)
Other useful Tips and Tricks: