How to retry an upstream task? - airflow

Question

task a > task b > task c
If C fails I want to retry A. Is this possible? There are a few other tickets which involve subdags, but I would like to just be able to clear A.
I'm hoping to use on_retry_callback in task C but I don't know how to call task A.
There is another question which does this in a subdag, but I am not using subdags.
I'm trying to do this, but it doesn't seem to work:
def callback_for_failures(context):
print("*** retrying ***")
if context['task'].upstream_list:
context['task'].upstream_list[0].clear()

Answer 1

As other comments mentioned, I would use caution to make sure you aren't getting into an endless loop of clearing/retries. But you can call a bash command as part of your on_failure_callback and then specify which tasks you want to clear, and if you want downstream/upstream tasks cleared etc.
from airflow import DAG
from airflow.operators.dummy_operator import DummyOperator
from airflow.operators.bash_operator import BashOperator
from datetime import datetime, timedelta
def clear_upstream_task(context):
execution_date = context.get("execution_date")
clear_tasks = BashOperator(
task_id='clear_tasks',
bash_command=f'airflow tasks clear -s {execution_date} -t t1 -d -y clear_upstream_task'
)
return clear_tasks.execute(context=context)
# Default settings applied to all tasks
default_args = {
'owner': 'airflow',
'depends_on_past': False,
'email_on_failure': False,
'email_on_retry': False,
'retries': 1,
'retry_delay': timedelta(seconds=5)
}
with DAG('clear_upstream_task',
start_date=datetime(2021, 1, 1),
max_active_runs=3,
schedule_interval=timedelta(minutes=5),
default_args=default_args,
catchup=False
) as dag:
t0 = DummyOperator(
task_id='t0'
)
t1 = DummyOperator(
task_id='t1'
)
t2 = DummyOperator(
task_id='t2'
)
t3 = BashOperator(
task_id='t3',
bash_command='exit 123',
on_failure_callback=clear_upstream_task
)
t0 >> t1 >> t2 >> t3