Stefan Seelmann commented on AIRFLOW-2747:

Initial PR: https://github.com/apache/incubator-airflow/pull/3596

> Explicit re-schedule of sensors
> -------------------------------
>                 Key: AIRFLOW-2747
>                 URL: https://issues.apache.org/jira/browse/AIRFLOW-2747
>             Project: Apache Airflow
>          Issue Type: Improvement
>          Components: core, operators
>    Affects Versions: 1.9.0
>            Reporter: Stefan Seelmann
>            Assignee: Stefan Seelmann
>            Priority: Major
>             Fix For: 2.0.0
>         Attachments: Screenshot_2018-07-12_14-10-24.png
> By default sensors block a worker and just sleep between pokes. This is very 
> inefficient, especially when there are many long-running sensors.
> There is a hacky workaroud by setting a small timeout value and a high retry 
> number. But that has drawbacks:
>  * Errors throws by sensors are hidden and the sensor retries too often
>  * The sensor is retried in a fixed time interval (with optional exponential 
> backoff)
>  * There are many attempts and many log files are generated
>  I'd like to propose an explicit reschedule mechanism:
>  * A new "reschedule" flag for sensors, if set to True it will raise an 
> AirflowRescheduleException that causes a reschedule.
>  * AirflowRescheduleException contains the (earliest) re-schedule date.
>  * Reschedule requests are recorded in new `task_reschedule` table and 
> visualized in the Gantt view.
>  * A new TI dependency that checks if a sensor task is ready to be 
> re-scheduled.
> Advantages:
>  * This change is backward compatible. Existing sensors behave like before. 
> But it's possible to set the "reschedule" flag.
>  * The poke_interval, timeout, and soft_fail parameters are still respected 
> and used to calculate the next schedule time.
>  * Custom sensor implementations can even define the next sensible schedule 
> date by raising AirflowRescheduleException themselves.
>  * Existing TimeSensor and TimeDeltaSensor can also be changed to be 
> rescheduled when the time is reached.
>  * This mechanism can also be used by non-sensor operators (but then the new 
> ReadyToRescheduleDep has to be added to deps or BaseOperator).
> Design decisions and caveats:
>  * When handling AirflowRescheduleException the `try_number` is decremented. 
> That means that subsequent runs use the same try number and write to the same 
> log file.
>  * Sensor TI dependency check now depends on `task_reschedule` table. However 
> only the BaseSensorOperator includes the new ReadyToRescheduleDep for now.
> Open questions and TODOs:
>  * Should a dedicated state `UP_FOR_RESCHEDULE` be used instead of setting 
> the state back to `NONE`? This would require more changes in scheduler code 
> and especially in the UI, but the state of a task would be more explicit and 
> more transparent to the user.
>  * Add example/test for a non-sensor operator
>  * Document the new feature

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