When saving computed values in the database in your rails app, you must be aware that is possible to find unexpected errors in the result thanks to race conditions.

I have already shared an exercise to help you get more sensitivity about when an implementation can save a wrong value thanks to race conditions.

Here I want to share one tip to help you avoid save the wrong value due race conditions while trying to save a computed value.

Using the account balance as an example

To talk about something concrete I will use the “account balance” as an example, but you can use this approach for different types of calculations.

The account balance example

Imagine that you have an Account record that has many entries, and you want to update the balance each time an Entry is created. The balance is the sum of the amount of each entry.

Imagine each account will need to create many entries concurrently, maybe on different background jobs or different requests. So if you want to calculate the balance and save it just after an entry is created, you could have problems with race conditions.

Tip: Use with lock in the full operation

Instead of “just” saving the balance and maybe save a wrong value, you can wrap the full operation in a with_lock block.

By calling with_lock you will start a transaction and acquire a lock for the row in one go. The block is called from within a transaction, and the object is already locked.

class Account < ExampleRecord
  has_many :entries

  def create_entry(amount:)
    with_lock do
      # This block is called within a transaction,
      # and account is already locked
      entries.create(amount: amount)
      update_balance
    end
  end

  def update_balance
    balance = entries.balance
    update!(balance: balance)
  end
end

class Entry < ExampleRecord
  belongs_to :account, touch: true

  def self.balance
    sum(:amount)
  end
end

You can learn more of with_lock and the ActiveRecord::Locking::Pessimistic module on the rails api.

Are we avoiding race conditions?

We are not avoiding race conditions, other problems can happen, but we are at least avoiding saving the wrong value due to race conditions.

Is this a perfect fix?

No, locks can produce other type of problems, like deadlocks.

Do you know other problems with this solution?

If you have experience with other problems with this solution, please leave a comment =)

Related articles

• Saving incorrect computed values thanks to race conditions
• Pick a safe previous date to avoid race conditions saving computed values
• Touch + fragment caching to avoid race conditions saving computed values
• Use after_touch to avoid to handle race conditions saving computed values