Airflow: Why is there a start_date for operators?

Question

I don\'t understand why do we need a \'start_date\' for the operators(task instances). Shouldn\'t the one that we pass to the DAG suffice?

Also, if the current time

Answer 1

Just to add to what is already here. A task that depends on another task(s) must have a start date >= to the start date of its dependencies.

For example:

• if task_a depends on task_b
• you cannot have
  • task_a start_date = 1/1/2019
  • task_b start_date = 1/2/2019
  • Otherwise, task_a will not be runnable for 1/1/2019 as task_b will not run for that date and you cannot mark it as complete either

Why would you want this?

• I would have liked this logic for a task, which was an external task sensor waiting for the completion of another dag. But the other dag had a start date after the current dag. Therefore, I didn't want the dependency in place for days when the other dag didn't exist