rails - implementing a simple lock to prevent user's from editing the same data concurrently

Question

I have an app where I need to prevent users from editing data while it is being edited by a different user. I'm trying to think of the best way to do it and wanted to ask for ideas. So far, I've created a settings model that stores application wide configuration on the db in key/value pairs. So, for the lock, I have a settings instance that's called LOCKED_TABLE_UID, and it stored the user_id of the user editing the table or null (nil) if the table is free.

>> lock = Setting.find_by_key('LOCKED_TABLE_UID')

Then, I implemented 2 methods in my application controller to acquire and release the lock:

# current_user returns the user currently logged in
def acquire_lock
  lock = Setting.find_by_key("LOCKED_TABLE_UID")
  if lock.value
    # if lock taken, see if it's the current_user or someone else
    if lock.value.to_i == current_user.id.to_i
      return true
    else
      return false
    end
  else
    # lock is free, assign it to this user
    lock.value = current_user.id
    return true if lock.save
  end
end

def release_lock
  lock = Setting.find_by_key("LOCKED_TABLE_UID")
  if lock.value
    # the lock belongs to current_user, so he can release it
    if lock.value.to_i == current_user.id.to_i
      lock.value = nil
      return true if lock.save
    else
      # not your lock, go away
      return false
    end
  else
    # lock is free, quit bugging
    return true
  end
end

What I want is to create some kind of block code that contains the locking mechanism, something like this:

def some_crud_action
  requires_locking do |lock|
    if lock
      # do some CRUD stuff here
    else
      # decline CRUD and give some error
    end
  end
end

I'd appreciate help on this - but I'm also open to other suggestions on how to accomplish all that, or some things I may have overlooked. This lock doesn't have to be atomic, but fairly basic and most important - that it works :) thanks.

Answer 1

Have you seen the ActiveRecord built-in locking feature?

• Optimistic Locking
• Pessimistic Locking

Answer 2

You're almost there. Create your require_locking? action as you see fit. Then process it with a before_filter.

 before_filter :requires_locking?, :only => [:update, :destroy]
 after_filter :release_lock, :only => [:update, :destroy]

 def requires_locking do |lock|
   unless acquire_lock
      lock = Setting.find_by_key("LOCKED_TABLE_UID")
      user_with_lock = User.find(lock.value)
      flash[:message] = "Action denied: Table locked by: #{user_with_lock.name}"
      redirect_to :back
   end
 end