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.
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.
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
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.
Instead of “just” saving the balance and maybe save a wrong value, you can wrap the full operation in a
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.
We are not avoiding race conditions, other problems can happen, but we are at least avoiding saving the wrong value due to race conditions.
No, locks can produce other type of problems, like deadlocks.
If you have experience with other problems with this solution, please leave a comment =)
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