title: Airflow backfills and new dag runs
body: <p>I have a DAG which has "DAG runs" since 1 Jan 2015 until today, scheduled every day. Tasks in the DAG are not "past dependent", meaning during a backfill they can be executed in any date order.</p>
  
  <p>If I need to backfill a task in the DAG, I clear all the task instances (from today to past) using UI, then all DAG runs switch to "running" state and the task start backfilling from 1 Jan 2015 to today. Tasks are time consuming, so even when executed in parallel by multiple threads / workers, the backfill can finish only in a few days.</p>
  
  <p>The problem is that new "DAG runs" for tomorrow, the day after tomorrow, etc. won't be added by scheduler until the backfill is finished so we fail to calculate new days' data on time. Is there any way to prioritize tasks for new days as they come and continue backfilling after tasks for the new day are finished?</p>
  
  <p>P.S. Backfilling can also be done using "airflow backfill" CLI but this approach has its own problems so for now I'm interested in the backfilling technique described above.</p>