ashb commented on a change in pull request #11467:
URL: https://github.com/apache/airflow/pull/11467#discussion_r512558521
##########
File path: docs/scheduler.rst
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@@ -65,3 +65,72 @@ If you want to use 'external trigger' to run future-dated
execution dates, set `
This only has effect if your DAG has no ``schedule_interval``.
If you keep default ``allow_trigger_in_future = False`` and try 'external
trigger' to run future-dated execution dates,
the scheduler won't execute it now but the scheduler will execute it in the
future once the current date rolls over to the execution date.
+
+Running more than one scheduler
+-------------------------------
+
+Airflow 2.0 is the first release that officially supports running more one
scheduler concurrently -- both for
+performance reasons and for resiliency.
+
+Overview
+""""""""
+
+The HA scheduler is written to take advantage of the existing metadata
database. This was primarily done for
+operational simplicity: every component already has to speak to this DB, and
by not using direct communication
+or consensus algorithm between schedulers (Raft, Paxos, etc.) nor another
consensus tool (Apache Zookeeper, or
+Consul for instance) we have kept the "operational surface area" to a minimum.
+
+The scheduler now uses the serialized DAG representation to make it's
scheduling decisions and the rough
+outline of the scheduling loop is:
+
+- Check for any DAGs needing a new DagRun, and create them
+- Examine a batch of DagRuns for schedulable TaskInstances or complete DagRuns
+- Select schedulable TaskInstances, and whilst respecting Pool limits and
other concurrency limits, enqueue
+ them for execution
+
+This does however place some requirements on the Database.
+
+Database Requirements
+"""""""""""""""""""""
+
+The short version is that users of PostgreSQL 9.6+ or MySQL 8+ are all ready
to go -- simple start running as
Review comment:
"simply" and "just" are bad words to use in cos `simple` -> `you can`
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