With_advisory_lock test with Multiple threads fails about with_advisory_lock HOT 5 OPEN

(original author here, but realize it's been 6 years since I wrote this, and I'm no longer the maintainer).

If you're not in a transaction, (depending on the rdbms, but certainly true with MySQL), consistency guarantees are simply not present. It's never a bad idea to be in a transaction, as autocommit (at least used to be) much slower than explicit transaction boundaries.

from with_advisory_lock.

Thanks for jumping on this one (SO op here). Can you elaborate a bit on “consistency guarantees are not present”? Does that mean that locks work only some of the time, or would it mean that in the context of Rails, acquiring db lock is not always guaranteed?

@allanohorn Thanks for the tip on ‘create_or_find_by’ - I ended up implementing something very similar that relies on catching unique constraint exceptions before I knew about ’create_or_find_by’

from with_advisory_lock.

With MySQL, if you're not in a txn, read results may not reflect the committed state of the database, depending on what engine you're using and how you've configured it.

from with_advisory_lock.

I have same problem with rspec tests. When I disable with_advisory_lock, it works as expected

from with_advisory_lock.

I had some problems but I've been able to resolve them by setting rspecs use_transactional_fixtures to false (as I should have done in the first place given I use database_cleaner).

I've written two tests to be able to see that w/o locks two threads run async and with locks they get synchronized. They work as expected, both letting database_cleaner use transactions or not.

RSpec.describe :with_advisory_lock, type: :model do
  def thread_1(lock, result)
    Thread.new do
      ActiveRecord::Base.with_advisory_lock(lock) do
        result << "thread_1"
        sleep 0.01
        result << "/thread_1"
      end
    end
  end

  def thread_2(lock, result)
    Thread.new do
      ActiveRecord::Base.with_advisory_lock(lock) do
        result << "thread_2"
      end
    end
  end

  it "can run two different synchronized tasks in parallel" do
    global_result = []

    10.times do
      result = []
      [
        thread_1("lock_1", result),
        thread_2("lock_2", result)
      ].each(&:join)

      global_result << result
    end

    expect(global_result).to include(%w[thread_1 thread_2 /thread_1])
  end

  it "can synchronize tasks with the same key" do
    10.times do
      result = []
      [
        thread_1("same_lock", result),
        thread_2("same_lock", result)
      ].each(&:join)

      # check thread_1 and /thread_1 are siblings
      expect(result.index("thread_1")).to eq(result.index("/thread_1") - 1)
    end
  end
end

from with_advisory_lock.