[jira] [Created] (AIRFLOW-764) max_active_runs_per_dag not respected for DAGs triggered manually within a few seconds of one another

["Jeffrey Enns (JIRA)" <ji...@apache.org>]

Jeffrey Enns created AIRFLOW-764:
------------------------------------

             Summary: max_active_runs_per_dag not respected for DAGs triggered manually within a few seconds of one another
                 Key: AIRFLOW-764
                 URL: https://issues.apache.org/jira/browse/AIRFLOW-764
             Project: Apache Airflow
          Issue Type: Bug
          Components: core, executor
    Affects Versions: Airflow 1.7.1.3
         Environment: debian linux, mysql with localexecutor
            Reporter: Jeffrey Enns
         Attachments: test_dag.py, test_dag_screen.png, test_job.sh, trigger_two.sh

Given the following configuration:
```
[core]
executor = LocalExecutor
max_active_runs_per_dag = 1
parallelism = 20
dag_concurrency = 1
```

Even with `max_active_runs_per_dag=1`, it is possible to cause two (or more) DAG runs to run in parallel by triggering the runs manually within a few seconds/milliseconds of one another. Task Instances from the distinct DAG runs will show as active in the “Task Instances” web view at the same time.

I only looked at the scheduler code briefly, but it looked as if a race condition would be possible for manually triggered DAGs that could lead to this behaviour.

I’ve attached a test DAG and two shell scripts I used to reliably reproduce this behaviour. Put `test_dag.py` and `test_job.sh` in the DAGs folder, and then run `trigger_two.sh` to reproduce the bug. 

Also attached is a screenshot showing DAG runs (for the dag ‘race_dag’) running in parallel after following the steps described immediately above (note the execution date, start date, and end date for each TI).



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