Airflow- how to send out email alert when sensor is set to soft fail

In the dag below, sensor A is set to soft_fail = True, because I’d like to skip B and C if A fails. The problem is I’d still like to get an email alert when A fails. But when soft_fail is true, A is marked as success when the sensor doesn’t detect anything, and no email alert would be sent out. Could someone please help to point out how to achieve this? Many thanks.

A (sensor, soft_fail = True) >> B >> C

Answer

Airflow sensor is marked as skipped (not success) when it fails and soft_fail is True.

There is no option to add email on skip not a callback. But you can create a new task from the operator EmailOperator, which run when the sensor A is marked as skipped. Unfortunately, there is no trigger rule to run a task when upstream is skipped, but you can create a new operator which check the state of A and send the email based on it.

from airflow.operators.email import EmailOperator
from airflow.utils.context import Context
from airflow.utils.state import TaskInstanceState
from airflow.utils.trigger_rule import TriggerRule


class MyNotifier(EmailOperator):
    def __int__(self, monitor_task_id: str, notify_on_state: str, *args, **kwargs):
        self.monitor_task_id = monitor_task_id
        self.notify_on_state = notify_on_state
        super().__init__(*args, **kwargs)

    def execute(self, context: Context):
        task_to_check = context["dag_run"].get_task_instance(task_id=self.monitor_task_id)
        if task_to_check.state == self.notify_on_state:
            super().execute(context)


notification_task = MyNotifier(
    task_id="sensor_skip_notifier",
    monitor_task_id="A",
    trigger_rule=TriggerRule.ALL_DONE,  # to run the task when A is done regardless the state
    notify_on_state=TaskInstanceState.SKIPPED,
    to="<email>",
    subject="<subject>",
    html_content="<content>",  # you can use jinja to add run info
)

A >> notification_task