To be clear update_all isn’t the thing that makes the difference here, it’s that you’re using inc to do the math in the database, and inc requires update_all.
For example this uses update_all but is still wrong
new_balance=current_balance + 5
Account |> where(id: ^account_id) |> update(set: [balance: new_balance]) |> Repo.update_all([])
The math is happening in Elixir, so it’s subject to race conditions. If you want to do the math in Elixir you need to use a lock like I showed you in option #2. That lock requires a transaction, it doesn’t matter whether that transaction is handled via Multi or via Repo.transaction with an anonymous function.
