title: How do I queue up backfills in airflow?
body: <p>I have DAG where <code>max_active_runs</code> is set to 2, but now I want to run backfills for 20ish runs. I actually expected airflow to sort of schedule all the backfills but only start 2 at a time, but that doesn't seem to happen. When I run the backfill command it starts two, but the command doesn't return since it didn't manage to start them all, instead, it keeps on trying until it succeeds. </p>
  
  <p>So what I expected was this:</p>
  
  <ol>
  <li>I ran the backfill command</li>
  <li>All the runs are marked as running</li>
  <li>Command returns since now everything should be scheduled</li>
  <li>Two of the runs start</li>
  </ol>
  
  <p>What I experienced:</p>
  
  <ol>
  <li>I ran the backfill command</li>
  <li>Two runs are marked as running and start</li>
  <li>Command doesn't return since it can't start the rest</li>
  </ol>
  
  <p>The experienced behavior makes it hard to just start a backfill and the shutdown your computer. So am I doing something wrong?</p>
  
  <p><strong>Update</strong>
  Using <code>trigger_dag</code> instead of backfill did what I wanted it to do. When running with backfill it seems like the command needed to be running for it to continue, feels weird. The difference with <code>trigger_dag</code> is that it trigger the dag and then it let airflow deal with it. Maybe it has something to do with how the backfill command is executed when using <code>gcloud composer environments run &lt;env&gt; --location=&lt;location&gt; backfill -- ...</code>?</p>