Solving A Race Condition When Using find_or_create_by

Question

I am using

@current_device ||= Device.find_or_create_by!(udid: request.headers["udid"])

and sometimes there is a race condition, where due to network behavior, 2 requests come at the same time, which causes 2 devices to be created with the same udid, even though there is a validation on the udid column for uniqueness.

I tried to engulf this with

begin
    @current_device ||= Device.find_or_create_by!(udid: request.headers["udid"])
rescue ActiveRecord::RecordInvalid => e
            if e.message == 'Validation failed: Udid has already been taken'
                retry #to compensate/handle possible(and very happening) race condition
             else
                Rollbar.error(e)
             end
end

But it doesn't work.

Is there a better way to handle this race condition?

Lock the table first. See http://stackoverflow.com/q/17267417/398670 — Craig Ringer, Jun 18 '15 at 11:33
Never depend on ActiveRecord validations to ensure uniqueness, add a unique constraint inside the database and handle the exception in Ruby. — mu is too short, Jun 18 '15 at 16:02
You can have races if multiple connections are trying to update the same `Device` but the unique constraint inside the database will prevent duplicates. — mu is too short, Jun 18 '15 at 17:52

score 3 · Answer 1 · answered Aug 10 '15 at 04:19

3

https://rietta.com/blog/2015/05/04/validates-uniqueness-race-condition-in-ruby-on-rails/

Fix race condition at the database level

answered Aug 10 '15 at 04:19

brookz

477
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Solving A Race Condition When Using find_or_create_by

1 Answers1