Robot Has No Heart

Xavier Shay blogs here

A robot that does not have a heart

Acts_as_state_machine locking

consider the following!

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class Door < ActiveRecord::Base
  acts_as_state_machine :initial => :closed

  state :closed
  state :open, :enter => :say_hello

  event :open do
    transitions :from => :closed, :to => :open
  end

  def say_hello
    puts "hello"
  end
end

door = Door.create!

fork do
  transaction do
    door.open!
  end
end

door.open!

# >> hello
# >> hello

It’s broken, you can only open a door once. This is a classic double-update problem. One way to solve is with pessimistic locking. I made some codes that automatically lock any object when you call an event on it.

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class ActiveRecord::Base
  # Forces all state transition events to obtain a DB lock
  def self.obtain_lock_before_all_state_transitions
    event_table.keys.each do |transition|
      define_method("#{transition}_with_lock!") do
        self.class.transaction do
          lock!
          send("#{transition}_without_lock!")
        end
      end
      alias_method_chain "#{transition}!", :lock
    end
  end
end

class Door < ActiveRecord::Base
  # ... as before

  obtain_lock_before_all_state_transitions
end

beware! Your state transitions can now throw ActiveRecord::RecordNotFound errors (from lock!), since the object may have been deleted before you got a chance to play with it.

If you’re not using any locking in your web app, you’re probably doing it wrong. Just sayin’.

A pretty flower Another pretty flower