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.
As far as I understand, if you are using Postgres with the default isolation level Read Committed, you can compute and save the value on an
after_touch like this:
class Account < ExampleRecord has_many :entries after_touch :update_balance def create_entry(amount:) entries.create(amount: amount) 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
Or at least, why do I think that it works…
after_touch works something like this:
PostgreSQL uses by default the Read Committed isolation level.
In this mode (as far as I understand), if within a transaction there is an
UPDATE of a row and in another concurrent transaction an
UPDATE of the same row is attempted, PostgreSQL will wait for the first transaction to be committed before continuing with the second one.
This makes the operations we perform on
after_touch to be isolated and avoids the problem of saving a wrong error due to another transaction registering another entry for the same account concurrently.
You can check the examples to explore possible race conditions when caching custom computed values in Rails.
And more specifically you can test this example that uses
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, if the calculation of the value that you are trying to save takes a lot of time and your application is really concurrent, it can queue up many transactions and cause other problems.
Also the reason why it works, can be a little obscure.
If you have experience with other problems with this solution, please leave a comment =)
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