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https://issues.apache.org/jira/browse/AIRFLOW-14?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15262337#comment-15262337
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Chris Riccomini commented on AIRFLOW-14:
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> DRJ needs to make sure to refresh from the db to check for a lock immediately
> before running it
Isn't there a race condition where two DRJs check the lock in the DB before
running, both see it's unset, and both set it to themselves? Even if they then
check to verify that they are the owners of the lock, it's not trustworthy. The
only way I can see this working is if a real read/write lock is obtained. Is
SQLAlchemy doing this?
> DagRun Refactor (Scheduler 2.0)
> -------------------------------
>
> Key: AIRFLOW-14
> URL: https://issues.apache.org/jira/browse/AIRFLOW-14
> Project: Apache Airflow
> Issue Type: Improvement
> Reporter: Jeremiah Lowin
> Assignee: Jeremiah Lowin
> Labels: backfill, dagrun, scheduler
>
> For full proposal, please see the Wiki:
> https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=62694286
> Borrowing from that page:
> *Description of New Workflow*
> DagRuns represent the state of a DAG at a certain point in time (perhaps they
> should be called DagInstances?). To run a DAG – or to manage the execution of
> a DAG – a DagRun must first be created. This can be done manually (simply by
> creating a DagRun object) or automatically, using methods like
> dag.schedule_dag(). Therefore, both scheduling new runs OR introducing ad-hoc
> runs can be done by any process at any time, simply by creating the
> appropriate object.
> Just creating a DagRun is not enough to actually run the DAG (just as
> creating a TaskInstance is not the same as actually running a task). We need
> a Job for that. The DagRunJob is fairly simple in structure. It maintains a
> set of DagRuns that it is tasked with executing, and loops over that set
> until all the DagRuns either succeed or fail. New DagRuns can be passed to
> the job explicitly via DagRunJob.submit_dagruns() or by defining its
> DagRunJob.collect_dagruns() method, which is called during each loop. When
> the DagRunJob is executing a specific DagRun, it locks it. Other DagRunJobs
> will not try to execute locked DagRuns. This way, many DagRunJobs can run
> simultaneously in either a local or distributed setting, and can even be
> pointed at the same DagRuns, without worrying about collisions or
> interference.
> The basic DagRunJob loop works like this:
> - refresh dags
> - collect new dagruns
> - process dagruns (including updating dagrun states for success/failure)
> - call executor/own heartbeat
> By tweaking the DagRunJob, we can easily recreate the behavior of the current
> SchedulerJob and BackfillJob. The Scheduler simply runs forever and picks up
> ALL active DagRuns in collect_dagruns(); Backfill generates DagRuns
> corresponding to the requested start/end dates and submits them to itself
> prior to initiating its loop.
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