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From "Paul Yang (JIRA)" <>
Subject [jira] [Commented] (AIRFLOW-160) Parse DAG files through child processes
Date Tue, 31 May 2016 07:28:12 GMT


Paul Yang commented on AIRFLOW-160:

In the preliminary PR, the child process handles both the parsing of the DAG definition file
and scheduling of task instances. Because scheduling needs to be done anyway, the periodic
parsing of DAG definition files remains. We could prioritize files (or force a refresh) based
on an API call in addition though.

> Parse DAG files through child processes
> ---------------------------------------
>                 Key: AIRFLOW-160
>                 URL:
>             Project: Apache Airflow
>          Issue Type: Improvement
>          Components: scheduler
>            Reporter: Paul Yang
>            Assignee: Paul Yang
> Currently, the Airflow scheduler parses all user DAG files in the same process as the
scheduler itself. We've seen issues in production where bad DAG files cause scheduler to fail.
A simple example is if the user script calls `sys.exit(1)`, the scheduler will exit as well.
We've also seen an unusual case where modules loaded by the user DAG affect operation of the
scheduler. For better uptime, the scheduler should be resistant to these problematic user
> The proposed solution is to parse and schedule user DAGs through child processes. This
way, the main scheduler process is more isolated from bad DAGs. There's a side benefit as
well - since parsing is distributed among multiple processes, it's possible to parse the DAG
files more frequently, reducing the latency between when a DAG is modified and when the changes
are picked up.
> Another issue right now is that all DAGs must be scheduled before any tasks are sent
to the executor. This means that the frequency of task scheduling is limited by the slowest
DAG to schedule. The changes needed for scheduling DAGs through child processes will also
make it easy to decouple this process and allow tasks to be scheduled and sent to the executor
in a more independent fashion. This way, overall scheduling won't be held back by a slow DAG.

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