PERF: Update Group#user_count counter cache outside DB transaction (#19256)

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>
This commit is contained in:
Alan Guo Xiang Tan
2022-11-30 22:52:08 +08:00
committed by GitHub
parent 9f022112e3
commit 7c321d3aad
7 changed files with 52 additions and 11 deletions

View File

@ -1,6 +1,22 @@
# frozen_string_literal: true
RSpec.describe GroupUser do
fab!(:group) { Fabricate(:group) }
fab!(:user) { Fabricate(:user) }
describe 'callbacks' do
it "increments and decrements `Group#user_count` when record is created and destroyed" do
group_user = GroupUser.new(user: user, group: group)
expect do
group_user.save!
end.to change { group.reload.user_count }.from(0).to(1)
expect do
group_user.destroy!
end.to change { group.reload.user_count }.from(1).to(0)
end
end
it 'correctly sets notification level' do
moderator = Fabricate(:moderator)