Airflow - trigger a specific task from external

Question

In Apache Airflow, let's say I want to set up a DAG that has 3 tasks.

Task A Task B Task C

When the DAG gets scheduled, I want task A to run, followed by Task B (when A completes). However, I want task C to only run when some external code triggers it (and it shouldn't poll and wait for an external condition to be satisfied; I want it to wait until it receives an external request to start execution, but only if A and B are completed). I also don't want to create another DAG for task C.

Is this possible? How to set up please? Will it require another task between B and C?

Thanks for any advice on how this can be achieved.

Answer 1

The best way to achieve this is to have two tasks leading into task C, so for example:

A >> [B, x] >> C

Where x is a new task. Then you can set x to be your other trigger condition from somewhere external.

For example if you're waiting for a certain file to be delivered for C to execute, create x as a BranchOperator, and only return C from it if the file exists.