Can't able to run a Task in Airflow

Question

Whenever I tried to run my DAG it gets success state but it can't able to run the task. While running the first time it works perfectly

trigger_controller_dag.py:

def conditionally_trigger(context, dag_run_obj):
    c_p = context['params']['condition_param']
    if context['params']['condition_param']:
        dag_run_obj.payload = {'message': context['params']['message']}
        pp.pprint(dag_run_obj.payload)
        return dag_run_obj

dag = DAG(
    dag_id='example_trigger_controller_dag',
    default_args={
        "owner": "airflow",
        "start_date": datetime.utcnow(),
    },
    schedule_interval='@once',
)

trigger = TriggerDagRunOperator(
    task_id='test_trigger_dagrun',
    trigger_dag_id="example_trigger_target_dag",
    python_callable=conditionally_trigger,
    params={'condition_param': True, 'message': 'Hello World'},
    dag=dag,
)

trigger_target_dag.py:

args = {
    'start_date': datetime.utcnow(),
    'owner': 'airflow',
}

dag = DAG(
    dag_id='example_trigger_target_dag',
    default_args=args,
    schedule_interval=None,
)


def run_this_func(ds, **kwargs):
    print("Remotely received value of {} for key=message".
          format(kwargs['dag_run'].conf['message']))


run_this = PythonOperator(
    task_id='run_this',
    provide_context=True,
    python_callable=run_this_func,
    dag=dag,
)

While running the DAG I am getting the following error, dependency 'Task Instance State' FAILED: Task is in the 'success' state which is not a valid state for execution. The task must be cleared in order to be run

Answer 1

Your start_date variables in both DAGS are dynamic and may be causing your problem 'start_date': datetime.utcnow(). Setting them to a dynamic start is not recommended and leads to errors.

Try setting it to a static start date like 'start_date': datetime(2019, 5, 29) #year month day

Taken from Airflow FAQ

We recommend against using dynamic values as start_date, especially datetime.now() as it can be quite confusing. The task is triggered once the period closes, and in theory an @hourly DAG would never get to an hour after now as now() moves along.

Another SO question on this: why dynamic start causes issues