While load testing our user creation code path in production, we
identified that executing the DB statement to update the `Group#user_count` column within a
transaction is creating a bottleneck for us. This is because the
creation of a user and addition of the user to the relevant groups are
done in a transaction. When we execute the DB statement to update
`Group#user_count` for the relevant group, a row level lock is held
until the transaction completes. This row level lock acts like a global
lock when the server is creating users that will be added to the same
group in quick succession.
Instead of updating the counter cache within a transaction which the
default ActiveRecord `counter_cache` option does, we simply update the
counter cache outside of the committing transaction.
Co-authored-by: Rafael dos Santos Silva <xfalcox@gmail.com>
Co-authored-by: Rafael dos Santos Silva <xfalcox@gmail.com>
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