ExternalTaskSensor

Apache Airflow Certified

Waits for a different DAG, task group, or task to complete for a specific logical date.

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Last Updated: Apr. 7, 2023

Access Instructions

Install the Apache Airflow provider package into your Airflow environment.

Import the module into your DAG file and instantiate it with your desired params.

Parameters

Documentation

Example DAGs

Parameters

external_dag_idRequiredThe dag_id that contains the task you want to wait for

external_task_idThe task_id that contains the task you want to wait for.

external_task_idsThe list of task_ids that you want to wait for. If None (default value) the sensor waits for the DAG. Either external_task_id or external_task_ids can be passed to ExternalTaskSensor, but not both.

allowed_statesIterable of allowed states, default is ['success']

failed_statesIterable of failed or dis-allowed states, default is None

execution_deltatime difference with the previous execution to look at, the default is the same logical date as the current task or DAG. For yesterday, use [positive!] datetime.timedelta(days=1). Either execution_delta or execution_date_fn can be passed to ExternalTaskSensor, but not both.

execution_date_fnfunction that receives the current execution’s logical date as the first positional argument and optionally any number of keyword arguments available in the context dictionary, and returns the desired logical dates to query. Either execution_delta or execution_date_fn can be passed to ExternalTaskSensor, but not both.

check_existenceSet to True to check if the external task exists (when external_task_id is not None) or check if the DAG to wait for exists (when external_task_id is None), and immediately cease waiting if the external task or DAG does not exist (default value: False).

Documentation

Waits for a different DAG, task group, or task to complete for a specific logical date.

If both external_task_group_id and external_task_id are None (default), the sensor waits for the DAG. Values for external_task_group_id and external_task_id can’t be set at the same time.

By default, the ExternalTaskSensor will wait for the external task to succeed, at which point it will also succeed. However, by default it will not fail if the external task fails, but will continue to check the status until the sensor times out (thus giving you time to retry the external task without also having to clear the sensor).

It is possible to alter the default behavior by setting states which cause the sensor to fail, e.g. by setting allowed_states=[State.FAILED] and failed_states=[State.SUCCESS] you will flip the behaviour to get a sensor which goes green when the external task fails and immediately goes red if the external task succeeds!

Note that soft_fail is respected when examining the failed_states. Thus if the external task enters a failed state and soft_fail == True the sensor will _skip_ rather than fail. As a result, setting soft_fail=True and failed_states=[State.SKIPPED] will result in the sensor skipping if the external task skips.

Example DAGs

external_task_marker_dag

Example DAG demonstrating setting up inter-DAG dependencies using ExternalTaskSensor and ExternalTaskMarker

Airflow Fundamentals