Airflow tasks get stuck at “queued” status and never gets running

Question

I'm using Airflow v1.8.1 and run all components (worker, web, flower, scheduler) on kubernetes & Docker. I use Celery Executor with Redis and my tasks are looks like:

(start) -> (do_work_for_product1)
     ├  -> (do_work_for_product2)
     ├  -> (do_work_for_product3)
     ├  …

So the start task has multiple downstreams. And I setup concurrency related configuration as below:

parallelism = 3
dag_concurrency = 3
max_active_runs = 1

Then when I run this DAG manually (not sure if it never happens on a scheduled task) , some downstreams get executed, but others stuck at "queued" status.

If I clear the task from Admin UI, it gets executed. There is no worker log (after processing some first downstreams, it just doesn't output any log).

Web server's log (not sure worker exiting is related)

/usr/local/lib/python2.7/dist-packages/flask/exthook.py:71: ExtDeprecationWarning: Importing flask.ext.cache is deprecated, use flask_cache instead.
  .format(x=modname), ExtDeprecationWarning
[2017-08-24 04:20:56,496] [51] {models.py:168} INFO - Filling up the DagBag from /usr/local/airflow_dags
[2017-08-24 04:20:57 +0000] [27] [INFO] Handling signal: ttou
[2017-08-24 04:20:57 +0000] [37] [INFO] Worker exiting (pid: 37)

There is no error log on scheduler, too. And a number of tasks get stuck is changing whenever I try this.

Because I also use Docker I'm wondering if this is related: https://github.com/puckel/docker-airflow/issues/94 But so far, no clue.

Has anyone faced with a similar issue or have some idea what I can investigate for this issue...?

Hi @Norio, we are having the similar issue that tasks in queue but scheduler seems forgot some of them. When I use airflow scheduler again, they have been picked up. I am also using 1.8.1, kubernetes and Docker, but with LocalExecutor, same issue here. — Chengzhi
@Chengzhi Thank you for the info. I utilize this shell github.com/apache/incubator-airflow/blob/… to automatically restart scheduler without relying on k8s's back off, so in my case scheduler should be periodically re-spawned but not pick some tasks forever... very weird. — Norio Akagi
super, thanks for sharing, very wired, I will keep you in loop if I found something, but it looks like this is the solution for now. — Chengzhi
I'm having a similar problem with airflow and using LocalExecutor. My setup is ismilar to the one @Chengzhi has. Did anyone find anything to try here? — fernandosjp
Just resolved my case which doesn't seem to be like yours but worth sharing. I was working with the logs/ folder and without noticing all ownership of folders changed. For this reason, Airflow was not able to write in log files and task kept getting stuck on queued state. Changing ownership of all files back to airflow user brought the application to normal. sudo chown -R airflow:airflow logs/ — fernandosjp

Bolke de Bruin Bolke de Bruin · Accepted Answer · 2017-10-22T19:52:51

Tasks getting stuck is, most likely, a bug. At the moment (<= 1.9.0alpha1) it can happen when a task cannot even start up on the (remote) worker. This happens for example in the case of an overloaded worker or missing dependencies.

This patch should resolve that issue.

It is worth investigating why your tasks do not get a RUNNING state. Setting itself to this state is first thing a task does. Normally the worker does log before it starts executing and it also reports and errors. You should be able to find entries of this in the task log.

edit: As was mentioned in the comments on the original question in case one example of airflow not being able to run a task is when it cannot write to required locations. This makes it unable to proceed and tasks would get stuck. The patch fixes this by failing the task from the scheduler.

Airflow tasks get stuck at “queued” status and never gets running

5 Answers