Mutex in rails API among several client request

Question

I face a problem with mutual exclusion in my rails API. I have some Event object in my database which contain the number of person who will attend the event.

Consider my Event method which add a person to the event :

class Event
    def add_person
        nb = self.number_of_person + 1
        # Some stuff
        self.number_of_person = nb
        self.save
    end
end

Now, imagine that client A and B want to participate in the event E. Rails is gonna receive 2 request and will call the add_person method in parallel. At the end, it is possible that the number of person in the Event E be not incremented by 2 but by 1.

This is what I want to be able to do :

Client A                 | Client B                 | Possible in parallel
--------------------------------------------------------------------------
Event.find(1).show       | Event.find(1).show       | yes
Event.find(1).add_person | Event.find(1).add_person | no
Event.find(1).add_person | Event.find(2).add_person | yes

I only want to queue the add_person method on the same object.

Is there a way to do this ?

Answer 1

A standard ActiveRecord transaction will prevent deadlock, or other contention. You might have to use a pessimistic locking strategy. Databases are designed to prevent exactly this kind of thing.