# Scheduling & Triggers

The Airflow scheduler monitors all tasks and all DAGs, and triggers the task instances whose dependencies have been met. Behind the scenes, it monitors and stays in sync with a folder for all DAG objects it may contain, and periodically (every minute or so) inspects active tasks to see whether they can be triggered.

The Airflow scheduler is designed to run as a persistent service in an Airflow production environment. To kick it off, all you need to do is execute `airflow scheduler`. It will use the configuration specified in `airflow.cfg`.

Note that if you run a DAG on a `schedule_interval` of one day, the run stamped `2016-01-01` will be triggered soon after `2016-01-01T23:59`. In other words, the job instance is started once the period it covers has ended.

Let’s Repeat That The scheduler runs your job one `schedule_interval` AFTER the start date, at the END of the period.

The scheduler starts an instance of the executor specified in the your `airflow.cfg`. If it happens to be the `LocalExecutor`, tasks will be executed as subprocesses; in the case of `CeleryExecutor` and `MesosExecutor`, tasks are executed remotely.

To start a scheduler, simply run the command:

    airflow scheduler

## DAG Runs

A DAG Run is an object representing an instantiation of the DAG in time.

Each DAG may or may not have a schedule, which informs how `DAG Runs` are created. `schedule_interval` is defined as a DAG arguments, and receives preferably a cron expression as a `str`, or a `datetime.timedelta` object. Alternatively, you can also use one of these cron “preset”:

preset | meaning | cron
--- | --- | ---
`None` | Don’t schedule, use for exclusively “externally triggered” DAGs |
`@once` | Schedule once and only once |
`@hourly` | Run once an hour at the beginning of the hour | `0 * * * *`
`@daily` | Run once a day at midnight | `0 0 * * *`
`@weekly` | Run once a week at midnight on Sunday morning | `0 0 * * 0`
`@monthly` | Run once a month at midnight of the first day of the month | `0 0 1 * *`
`@yearly` | Run once a year at midnight of January 1 | `0 0 1 1 *`

Your DAG will be instantiated for each schedule, while creating a `DAG Run` entry for each schedule.

DAG runs have a state associated to them (running, failed, success) and informs the scheduler on which set of schedules should be evaluated for task submissions. Without the metadata at the DAG run level, the Airflow scheduler would have much more work to do in order to figure out what tasks should be triggered and come to a crawl. It might also create undesired processing when changing the shape of your DAG, by say adding in new tasks.

## Backfill and Catchup

An Airflow DAG with a `start_date`, possibly an `end_date`, and a `schedule_interval` defines a series of intervals which the scheduler turn into individual Dag Runs and execute. A key capability of Airflow is that these DAG Runs are atomic, idempotent items, and the scheduler, by default, will examine the lifetime of the DAG (from start to end/now, one interval at a time) and kick off a DAG Run for any interval that has not been run (or has been cleared). This concept is called Catchup.

If your DAG is written to handle its own catchup (IE not limited to the interval, but instead to “Now” for instance.), then you will want to turn catchup off (Either on the DAG itself with `dag.catchup = False`) or by default at the configuration file level with `catchup_by_default = False`. What this will do, is to instruct the scheduler to only create a DAG Run for the most current instance of the DAG interval series.

    """
    Code that goes along with the Airflow tutorial located at:
    https://github.com/airbnb/airflow/blob/master/airflow/example_dags/tutorial.py
    """
    from airflow import DAG
    from airflow.operators.bash_operator import BashOperator
    from datetime import datetime, timedelta

    default_args = {
        'owner': 'airflow',
        'depends_on_past': False,
        'start_date': datetime(2015, 12, 1),
        'email': ['airflow@example.com'],
        'email_on_failure': False,
        'email_on_retry': False,
        'retries': 1,
        'retry_delay': timedelta(minutes=5),
        'schedule_interval': '@hourly',
    }

    dag = DAG('tutorial', catchup=False, default_args=default_args)

In the example above, if the DAG is picked up by the scheduler daemon on 2016-01-02 at 6 AM, (or from the command line), a single DAG Run will be created, with an `execution_date` of 2016-01-01, and the next one will be created just after midnight on the morning of 2016-01-03 with an execution date of 2016-01-02.

If the `dag.catchup` value had been True instead, the scheduler would have created a DAG Run for each completed interval between 2015-12-01 and 2016-01-02 (but not yet one for 2016-01-02, as that interval hasn’t completed) and the scheduler will execute them sequentially. This behavior is great for atomic datasets that can easily be split into periods. Turning catchup off is great if your DAG Runs perform backfill internally.

## External Triggers

Note that `DAG Runs` can also be created manually through the CLI while running an `airflow trigger_dag` command, where you can define a specific `run_id`. The `DAG Runs` created externally to the scheduler get associated to the trigger’s timestamp, and will be displayed in the UI alongside scheduled `DAG runs`.

In addition, you can also manually trigger a `DAG Run` using the web UI (tab “DAGs” -> column “Links” -> button “Trigger Dag”).

## To Keep in Mind

- The first `DAG Run` is created based on the minimum `start_date` for the tasks in your DAG.
- Subsequent `DAG Runs` are created by the scheduler process, based on your DAG’s `schedule_interval`, sequentially.
- When clearing a set of tasks’ state in hope of getting them to re-run, it is important to keep in mind the `DAG Run`’s state too as it defines whether the scheduler should look into triggering tasks for that run.

Here are some of the ways you can unblock tasks:

- From the UI, you can clear (as in delete the status of) individual task instances from the task instances dialog, while defining whether you want to includes the past/future and the upstream/downstream dependencies. Note that a confirmation window comes next and allows you to see the set you are about to clear. You can also clear all task instances associated with the dag.
- The CLI command `airflow clear -h` has lots of options when it comes to clearing task instance states, including specifying date ranges, targeting task_ids by specifying a regular expression, flags for including upstream and downstream relatives, and targeting task instances in specific states (`failed`, or `success`)
- Clearing a task instance will no longer delete the task instance record. Instead it updates max_tries and set the current task instance state to be None.
- Marking task instances as failed can be done through the UI. This can be used to stop running task instances.
- Marking task instances as successful can be done through the UI. This is mostly to fix false negatives, or for instance when the fix has been applied outside of Airflow.
- The `airflow backfill` CLI subcommand has a flag to `--mark_success` and allows selecting subsections of the DAG as well as specifying date ranges.