Concurrent transactions in test

Questions / Help
1

I want to test:

  • Many processes write to database in concurrent.
  • Bad implementation (without transaction) results breaks data.
  • Collect implementation (with transaction) results update data collectly. Not break and not lost any data.

Can I use two or more DB connecion in test?

In default Phoenix test env. Two tasks is run in serialized.
I think second task wait for first task free the connection.

defmodule EctoMysqlDemo.ConcurrentTest do
  use EctoMysqlDemo.ModelCase

  def select_connection_id() do
    %{rows: [[connection_id]]} = Ecto.Adapters.SQL.query!(Repo, "select connection_id()")
    connection_id
  end

  test "two concurrent transaction is serialized in test" do
    start_time = System.monotonic_time(:milliseconds)

    task0 = Task.async(fn->
      Repo.transaction(fn->
        :timer.sleep(100)
        select_connection_id()
      end)
    end)

    task1 = Task.async(fn->
      Repo.transaction(fn->
        :timer.sleep(100)
        select_connection_id()
      end)
    end)

    cid0 = Task.await(task0)
    cid1 = Task.await(task1)

    elapsed_time = System.monotonic_time(:milliseconds) - start_time
    assert 200 < elapsed_time # not about 100ms
    assert cid0 == cid1       # used same connection
  end
end

I read Ecto.Adapters.SQL.Sandbox, I found how to use one connection in two or more processes.