ashb commented on a change in pull request #16182:
URL: https://github.com/apache/airflow/pull/16182#discussion_r663760971



##########
File path: airflow/models/dagbag.py
##########
@@ -166,6 +166,34 @@ def dag_ids(self) -> List[str]:
         """
         return list(self.dags.keys())
 
+    @provide_session
+    def has_dag(self, dag_id: str, session: Session = None):
+        """
+        Checks the "local" cache, if it exists, and is not older than the 
configured cache time, return True
+        else check in the DB if the dag exists, return True, False otherwise
+        """
+        from airflow.models.serialized_dag import SerializedDagModel
+
+        sd_last_updated_datetime = 
SerializedDagModel.get_last_updated_datetime(
+            dag_id=dag_id,
+            session=session,
+        )
+        sd_has_dag = sd_last_updated_datetime is not None
+        if dag_id not in self.dags:
+            return sd_has_dag
+        if dag_id not in self.dags_last_fetched:
+            return sd_has_dag
+        min_serialized_dag_fetch_secs = 
timedelta(seconds=settings.MIN_SERIALIZED_DAG_FETCH_INTERVAL)
+        if timezone.utcnow() < self.dags_last_fetched[dag_id] + 
min_serialized_dag_fetch_secs:
+            return sd_has_dag
+        if sd_has_dag:
+            return True

Review comment:
       So something like this (pseudo-python):
   
   ```python
           if dag_id in self.dags and timezone.utcnow() < 
self.dags_last_fetched[dag_id] + min_serialized_dag_fetch_secs:
               return True
   
           sd_last_updated_datetime = 
SerializedDagModel.get_last_updated_datetime(
               dag_id=dag_id,
               session=session,
           )
           # etc ...
   ```




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