# Airflow Gotcha's

Airflow is an integral part of our data platform. ETL processes, forecasts and various analyses are scheduled and monitored through Airflow. Our Airflow instance is hosted at workflow.telemetry.mozilla.org (WTMO).

## DAGs are automatically generated for the most part

Airflow DAGs for our ETL processes get automatically generated as part of bigquery-etl. The process for scheduling queries is to specify a DAG as part of its metadata. How to schedule queries is described in detail this guide to creating a derived dataset. Generated DAGs are prefixed with bqetl_.

Some DAGs, for example for custom machine learning tasks or to schedule running custom tools, need to be defined manually. These DAGs need to be created in the telemetry-airflow repository.

A separate script syncs generated bigquery-etl DAGs every 10 minutes to our Airflow instance. DAGs that live in telemetry-airflow get deployed via CircleCI whenever a change is pushed to main.

## New DAGs need to be unpaused manually

After adding a new DAG either through bigquery-etl or telemetry-airflow, it will take about 10 minutes until the new DAG gets deployed. After deployment, the DAG is by default disabled. It is necessary to manually unpause the DAG on WTMO

## External task dependencies are managed via ExternalTaskSensors

Tasks are distributed across different Airflow DAGs. Usually, each DAG contains tasks that are closely related to a specific use case or for generating a set of related datasets. In many cases, tasks depend on other tasks that are running as part of a different DAG. For example, a lot of tasks depend on the copy_deduplicate_main_ping task.

External upstream dependencies are expressed using ExternalTaskSensors. These sensors ensure that the external upstream task is finished before the job that depends on that upstream task is executed. These sensors are usually defined like:

wait_for_bq_events = ExternalTaskSensor(
task_id="wait_for_bq_events",       # name of this wait task as it will appear in the UI
external_dag_id="copy_deduplicate", # name of the external DAG
external_task_id="bq_main_events",  # name of the external task
execution_delta=timedelta(hours=3), # delta based on differences in schedule between upstream DAG and current DAG
mode="reschedule",                  # use mode "reschedule" to unblock slots while waiting on upstream task to finish
allowed_states=ALLOWED_STATES,      # pre-defined success states
failed_states=FAILED_STATES,        # pre-defined failure states
pool="DATA_ENG_EXTERNALTASKSENSOR", # this slot pool is used for task sensors
email_on_retry=False,
dag=dag,
)



It is important to note that the execution_delta needs to be set correctly depending on the time difference between the upstream DAG schedule and the schedule of the downstream DAG. If the execution_delta is not set correctly, downstream tasks will wait indefinitely without ever getting executed.

While upstream dependencies are automatically determined between generated DAGs in bigquery-etl, if there are dependencies between DAGs in telemetry-airflow and bigquery-etl, then these dependencies need to be either added manually to the DAG definition or to the scheduling metadata of the scheduled query.

## Downstream dependencies are managed via ExternalTaskMarkers

ExternalTaskMarkers are used to indicate all downstream dependencies to a task. Whenever the task is cleared with Downstream Recursive selected, then all downstream tasks will get cleared automatically. This is extremely useful when running backfill of Airflow tasks. When clearing the tasks, a pop-up will show all the downstream tasks that will get cleared. In case a task should be cleared without its downstream dependencies running as well, deselect the Downstream Recursive option.

ExternalTaskMarkers are generally wrapped into a TaskGroup and defined like:

with TaskGroup('copy_deduplicate_all_external') as copy_deduplicate_all_external:
external_dag_id="bhr_collection",               # external downstream DAG
execution_date="{{ execution_date.replace(hour=5, minute=0).isoformat() }}",    # execution date calculated based on time differences in task schedules
)


Upstream dependencies are automatically determined between generated DAGs in bigquery-etl. If there are dependencies between DAGs in telemetry-airflow and bigquery-etl, then these dependencies need to be either added manually to the DAG definition or to the scheduling metadata of the scheduled query.

## The DAG schedules are selected based on schedules of upstream dependencies

The schedule_interval of a DAG should be set to a time that ensures that all upstream dependencies have likely finished before tasks in the DAG get executed. Airflow will send an email notification every time a task needs to be rescheduled due to upstream dependencies not having finished. To reduce the amount of notifications and avoid delays due to rescheduled tasks, the schedule_interval should be set based on when upstream tasks have finished.

## Airflow triage

To detect broken or stuck tasks, we set up an Airflow triage process that notifies tasks owners of problems with their Airflow tasks. Generally, DAGs are checked for failures or stuck tasks on a daily basis and problems are reported on Bugzilla.

In case of a failure and after merging the solution to the problem, clear the logs for the failing task to allow the DAG to run again.

## Testing Airflow DAGs

A guide on how to set up Airflow locally and test Airflow DAGs is available here.