Apache Airflow Xcom Pull from dynamic task name

Question

I have successfully created dynamic tasks in a DAG (Bash and Docker Operators) but I'm having a hard time passing those dynamically created tasks to xcom_pull to grab data.

for i in range(0, max_tasks):
    task_scp_queue = BashOperator(task_id="scp_queue_task_{}".format(i), bash_command="""python foo""", retries=3, dag=dag, pool="scp_queue_pool", queue="foo", provide_context=True, xcom_push=True) # Pull the manifest ID from the previous task via xcom'

    task_process_queue = DockerOperator(task_id="process_task_{}".format(i), command="""python foo --queue-name={{ task_instance.xcom_pull(task_ids=scp_queue_task_{}) }}""".format(i), retries=3, dag=dag, pool="process_pool", api_version="auto", image="foo", queue="foo", execution_timeout=timedelta(minutes=5))
    task_manifest = DockerOperator(api_version="auto", task_id="manifest_task_{}".format(i), image="foo", retries=3, dag=dag, command=""" python --manifestid={{ task_instance.xcom_pull(task_ids=scp_queue_task_{}) }}""".format(i), pool="manfiest_pool", queue="d_parser")

    task_psql_queue.set_downstream(task_scp_queue)
    task_process_queue.set_upstream(task_scp_queue)
    task_manifest.set_upstream(task_process_queue)

As you can see I tried just using Python format string in the Jinja template to pass the i variable in it, however that doesn't work.

I've also tried using "task.task_id", and creating a new string with just the task_id but that doesn't work either.

Edit:

Now command looks like this

command="""python foo \ 
    --queue-name="{{ 
    task_instance.xcom_pull(task_ids='scp_queue_task_{}') }}" 
     """.format(i)

And my debug logs from Airflow look like

Using Master Queue: process_{ 
task_instance.xcom_pull(task_ids='scp_queue_task_31') }

So the string value is being populated but it's not executing the xcom_pull.

dlamblin dlamblin · Accepted Answer · 2018-10-26T10:12:45

I'm confused how this isn't working. A log of the errors you're getting would be helpful.

In brief, what you're doing looks good, if max_tasks=2 you will get:

task_psql_queue.taskid --> scp_queue_task_0 >> process_task_0 >> manifest_task_0
                       \-> scp_queue_task_1 >> process_task_1 >> manifest_task_1

I suspect you don't need the timeouts, which are really short. Because you have very long lines and randomly reorder your named params I'll reformat what you wrote:

for i in range(0, max_tasks):
    task_scp_queue = BashOperator(
        task_id="scp_queue_task_{}".format(i),
        dag=dag,
        retries=3,  # you could make it a default arg on the dag
        pool="scp_queue_pool",
        queue="foo", # you really want both queue and pool? When debugging remove them.
        bash_command="python foo",  # Maybe you snipped a multiline command
        provide_context=True,  # BashOp doesn't have this argument
        xcom_push=True,  # PUSH the manifest ID FOR the NEXT task via xcom
    )

    task_process_queue = DockerOperator(
        task_id="process_task_{}".format(i),
        dag=dag,
        retries=3,
        pool="process_pool",
        queue="foo",
        execution_timeout=timedelta(minutes=5),
        api_version="auto",
        image="foo",
        command="python foo --queue-name="
                "{{{{ task_instance.xcom_pull(task_ids=scp_queue_task_{}) }}}}".format(i),
    )

    task_manifest = DockerOperator(
        task_id="manifest_task_{}".format(i),
        retries=3,
        dag=dag,
        pool="manfiest_pool",
        queue="d_parser",
        api_version="auto",
        image="foo",
        command="python --manifestid="
                "{{{{ task_instance.xcom_pull(task_ids=scp_queue_task_{}) }}}}".format(i),
    )

    task_psql_queue >> task_scp_queue >> task_process_queue >> task_manifest

Oh, now look, you didn't pass the task_ids as strings. Try:

        command="python foo --queue-name="
                "{{{{ task_instance.xcom_pull(task_ids='scp_queue_task_{}') }}}}".format(i),
… … …
        command="python --manifestid="
                "{{{{ task_instance.xcom_pull(task_ids='scp_queue_task_{}') }}}}".format(i),

Apache Airflow Xcom Pull from dynamic task name

1 Answers