title: Airflow + sqlalchemy short-lived connections to metadata db
body: <p>I deployed the latest airflow on a centos 7.5 vm and updated <code>sql_alchemy_conn</code> and <code>result_backend</code> to postgres databases on a postgresql instance and designated my executor as <code>CeleryExecutor</code>. Without enabling any dag at all and even with no airflow scheduler started, I see about one connection established  every 5 seconds and then disposed to run a <code>SELECT 1</code> and a <code>SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1</code>. </p>
  
  <p>The number of short-lived connections drastically increase when one starts the scheduler and turns on dags. Does anyone know the reason for this? Is this a heartbeat check or task status check? With <code>sql_alchemy_pool_enabled = True</code> in airflow.cfg should these connections not be longer lived? Is there a log that I can look to pinpoint the source of these connections with sub-second life?</p>
  
  <p>Config values used for reference</p>
  
  <pre><code>    executor = CeleryExecutor
      sql_alchemy_conn = postgres://..../db1
      sql_alchemy_pool_enabled = True
      sql_alchemy_pool_size = 5
      sql_alchemy_max_overflow = 0
      parallelism = 32
      dag_concurrency = 16
      max_active_runs_per_dag = 16
      worker_concurrency = 16
      broker_url = redis://...
      result_backend = db+postgresql+psycopg2://.../db2
      job_heartbeat_sec = 5
      scheduler_heartbeat_sec = 5
  </code></pre>