title: Airflow DAG B runs many times after DAG A is completed for that day
body: <p>DAG A has schedule '0 6 * * *'.</p>
  
  <p>DAG B has schedule '*/5 * * * *'.</p>
  
  <p>However, DAG B should only start running for that day once DAG A has completed for that day. </p>
  
  <p>I've played around with SubDags and ExternalTaskSensor but haven't yet found a satisfactory solution and I'm sure I'm missing something good. Recommendations? </p>
  
  <p><strong>Edit</strong>: say DAG A is my ETL. DAG B has some tasks that query my database and require that data to be up-to-date. DAG B gets run throughout the day, but only once the ETL is completed. </p>
  
  <p>I can see using ShortCircuitOperator, for example, and having the condition be "DAG A that ran today is completed." But how could I write this condition? </p>