Why isn’t my task getting scheduled?

There are very many reasons why your task might not be getting scheduled. Here are some of the common causes:

  • Does your script “compile”, can the Airflow engine parse it and find your DAG object? To test this, you can run airflow list_dags and confirm that your DAG shows up in the list. You can also run airflow list_tasks foo_dag_id --tree and confirm that your task shows up in the list as expected. If you use the CeleryExecutor, you may want to confirm that this works both where the scheduler runs as well as where the worker runs.

  • Does the file containing your DAG contain the string “airflow” and “DAG” somewhere in the contents? When searching the DAG directory, Airflow ignores files not containing “airflow” and “DAG” in order to prevent the DagBag parsing from importing all python files collocated with user’s DAGs.

  • Is your start_date set properly? The Airflow scheduler triggers the task soon after the start_date + schedule_interval is passed.

  • Is your schedule_interval set properly? The default schedule_interval is one day (datetime.timedelta(1)). You must specify a different schedule_interval directly to the DAG object you instantiate, not as a default_param, as task instances do not override their parent DAG’s schedule_interval.

  • Is your start_date beyond where you can see it in the UI? If you set your start_date to some time say 3 months ago, you won’t be able to see it in the main view in the UI, but you should be able to see it in the Menu -> Browse ->Task Instances.

  • Are the dependencies for the task met? The task instances directly upstream from the task need to be in a success state. Also, if you have set depends_on_past=True, the previous task instance needs to have succeeded (except if it is the first run for that task). Also, if wait_for_downstream=True, make sure you understand what it means - all tasks immediately downstream of the previous task instance must have succeeded. You can view how these properties are set from the Task Instance Details page for your task.

  • Are the DagRuns you need created and active? A DagRun represents a specific execution of an entire DAG and has a state (running, success, failed, …). The scheduler creates new DagRun as it moves forward, but never goes back in time to create new ones. The scheduler only evaluates running DagRuns to see what task instances it can trigger. Note that clearing tasks instances (from the UI or CLI) does set the state of a DagRun back to running. You can bulk view the list of DagRuns and alter states by clicking on the schedule tag for a DAG.

  • Is the concurrency parameter of your DAG reached? concurrency defines how many running task instances a DAG is allowed to have, beyond which point things get queued.

  • Is the max_active_runs parameter of your DAG reached? max_active_runs defines how many running concurrent instances of a DAG there are allowed to be.

You may also want to read the Scheduler section of the docs and make sure you fully understand how it proceeds.

How do I trigger tasks based on another task’s failure?

Check out the Trigger Rules.

What’s the deal with start_date?

start_date is partly legacy from the pre-DagRun era, but it is still relevant in many ways. When creating a new DAG, you probably want to set a global start_date for your tasks using default_args. The first DagRun to be created will be based on the min(start_date) for all your task. From that point on, the scheduler creates new DagRuns based on your schedule_interval and the corresponding task instances run as your dependencies are met. When introducing new tasks to your DAG, you need to pay special attention to start_date, and may want to reactivate inactive DagRuns to get the new task onboarded properly.

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.

Previously, we also recommended using rounded start_date in relation to your schedule_interval. This meant an @hourly would be at 00:00 minutes:seconds, a @daily job at midnight, a @monthly job on the first of the month. This is no longer required. Airflow will now auto align the start_date and the schedule_interval, by using the start_date as the moment to start looking.

You can use any sensor or a TimeDeltaSensor to delay the execution of tasks within the schedule interval. While schedule_interval does allow specifying a datetime.timedelta object, we recommend using the macros or cron expressions instead, as it enforces this idea of rounded schedules.

When using depends_on_past=True, it’s important to pay special attention to start_date, as the past dependency is not enforced only on the specific schedule of the start_date specified for the task. It’s also important to watch DagRun activity status in time when introducing new depends_on_past=True, unless you are planning on running a backfill for the new task(s).

It is also important to note that the task’s start_date, in the context of a backfill CLI command, gets overridden by the backfill’s start_date commands. This allows for a backfill on tasks that have depends_on_past=True to actually start. If this were not the case, the backfill just would not start.

How can I create DAGs dynamically?

Airflow looks in your DAGS_FOLDER for modules that contain DAG objects in their global namespace and adds the objects it finds in the DagBag. Knowing this all, we need is a way to dynamically assign variable in the global namespace. This is easily done in python using the globals() function for the standard library, which behaves like a simple dictionary.

def create_dag(dag_id):
    A function returning a DAG object.

    return DAG(dag_id)

for i in range(10):
    dag_id = f'foo_{i}'
    globals()[dag_id] = DAG(dag_id)

    # or better, call a function that returns a DAG object!
    other_dag_id = f'bar_{i}'
    globals()[other_dag_id] = create_dag(other_dag_id)

What are all the airflow run commands in my process list?

There are many layers of airflow run commands, meaning it can call itself.

  • Basic airflow run: fires up an executor, and tell it to run an airflow run --local command. if using Celery, this means it puts a command in the queue for it to run remote, on the worker. If using LocalExecutor, that translates into running it in a subprocess pool.

  • Local airflow run --local: starts an airflow run --raw command (described below) as a subprocess and is in charge of emitting heartbeats, listening for external kill signals and ensures some cleanup takes place if the subprocess fails

  • Raw airflow run --raw runs the actual operator’s execute method and performs the actual work

How can my airflow dag run faster?

There are a few variables we can control to improve airflow dag performance:

  • parallelism: This variable controls the number of task instances that runs simultaneously across the whole Airflow cluster. User could increase the parallelism variable in the airflow.cfg.

  • concurrency: The Airflow scheduler will run no more than concurrency task instances for your DAG at any given time. Concurrency is defined in your Airflow DAG. If you do not set the concurrency on your DAG, the scheduler will use the default value from the dag_concurrency entry in your airflow.cfg.

  • task_concurrency: This variable controls the number of concurrent running task instances across dag_runs per task.

  • max_active_runs: the Airflow scheduler will run no more than max_active_runs DagRuns of your DAG at a given time. If you do not set the max_active_runs in your DAG, the scheduler will use the default value from the max_active_runs_per_dag entry in your airflow.cfg.

  • pool: This variable controls the number of concurrent running task instances assigned to the pool.

How can we reduce the airflow UI page load time?

If your dag takes long time to load, you could reduce the value of default_dag_run_display_number configuration in airflow.cfg to a smaller value. This configurable controls the number of dag run to show in UI with default value 25.

How to fix Exception: Global variable explicit_defaults_for_timestamp needs to be on (1)?

This means explicit_defaults_for_timestamp is disabled in your mysql server and you need to enable it by:

  1. Set explicit_defaults_for_timestamp = 1 under the mysqld section in your my.cnf file.

  2. Restart the Mysql server.

How to reduce airflow dag scheduling latency in production?

  • parsing_processes: Scheduler will spawn multiple threads in parallel to parse dags. This is controlled by parsing_processes with default value of 2. User should increase this value to a larger value (e.g numbers of cpus where scheduler runs + 1) in production.

  • scheduler_heartbeat_sec: User should consider to increase scheduler_heartbeat_sec config to a higher value (e.g 60 secs) which controls how frequent the airflow scheduler gets the heartbeat and updates the job’s entry in database.

Why next_ds or prev_ds might not contain expected values?

  • When scheduling DAG, the next_ds next_ds_nodash prev_ds prev_ds_nodash are calculated using execution_date and schedule_interval. If you set schedule_interval as None or @once, the next_ds, next_ds_nodash, prev_ds, prev_ds_nodash valueS will be set to None.

  • When manually triggering DAG, the schedule will be ignored, and prev_ds == next_ds == ds

How do I stop the sync perms happening multiple times per webserver?

Set the value of update_fab_perms configuration in airflow.cfg to False.

Why did the pause dag toggle turn red?

If pausing or unpausing a dag fails for any reason, the dag toggle will revert to its previous state and turn red. If you observe this behavior, try pausing the dag again, or check the console or server logs if the issue recurs.

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