concurrency issue

Hi,

On my site, if a lot of people place orders at the same time, the
available quantity left in stock for a product seems to get wrong
every so often.

Here's my execute_purchase method, which is a before_filter on the
Order model:

    def execute_purchase
      Product.transaction do
        product.lock!
        if product.quantity >= quantity # enough in stock
          if credit_card_needed?
            response = authorize_payment
            if response.success?
              product.update_attribute(:quantity, product.quantity -
self.quantity)
            else
              raise FulfillmentError, response.message
            end
          end
        else
          raise FulfillmentError, "not enough in stock"
        end
      end
    rescue FulfillmentError => e
      errors.add_to_base e.message
      return false
    end

So, I enter the transaction. Lock the product. I see if there's
enough in stock. If there's enough in stock, then I authorize the
payment -- if that's successful, then I decrement the available
quantity. Since I'm in a transaction and I've locked the record,
this method should be fine, right?

So, what happens is sometimes we sell more than we have. So, the
product quantity isn't always updated when an order happens.

I'm just trying to guess so forgive me if I'm saying something stupid

Let's think to two almost simultaneous request A and B

this is the timing

1) request A land on your page and load the product (quantity = 3) and
A wants to by 1
2) request B land on your page and load the product (quantity = 3) and
B wants to buy 1
3) request A execute the transaction setting quantity = 3 -1 => 2
4) request B execute the transaction setting quantity = 3 - 1 => 2

That's how you end up with still 2 available even if you've sold 2 items.

update_attribute doesn't run validations so in this case lock_version
doesn't protect you.

Paolo

Shouldn't the
   product.lock!
line give me a lock on that row? So, the other requests need to wait
until the transaction is over with. Then the product should get
reloaded with current data.

Right?

Yes, you lock the row, but the point is at that point it

Shouldn't the
   product.lock!
line give me a lock on that row? So, the other requests need to wait
until the transaction is over with.

The above is correct, it waits, but what at this point product is has
been already loaded and it contains quantity 3

Then the product should get
reloaded with current data.

The above is incorrect

If you don't reload explicitly the ActiveRecord object it doesn't
reload itself, it will keep forever a STALE image of the database.

Paolo

Sorry for the mess in the previous message, cut and paste error.

Basically when you instantiate an AR object it loads the data at that
moment and never updates until you call the .reload method.
An AR object is not aware of what happens to its image in the database.

Paolo

http://api.rubyonrails.org/classes/ActiveRecord/Locking/Pessimistic.html#M000646

Says that the record should be reloaded.

In fact, here's the source code for lock:

71: def lock!(lock = true)
72: reload(:lock => lock) unless new_record?
73: self
74: end

Am I not understanding this correctly?

Joe

Sorry, you're definitely right, it should reload the object so what
I've said is not valid.

Paolo

I just noticed you decrease the quantity only when credit_card_needed?

Is that right?

Paolo

Hi,

What about doing preventive stock update:

Here's my execute_purchase method, which is a before_filter on the
Order model:

    def execute_purchase
      Product.transaction do
        if product.quantity >= quantity # enough in stock
          if credit_card_needed?

UPDATE products SET quantity=quantity-#{product.quantity}

            response = authorize_payment
            if response.success?

msg Success

            else

UPDATE products SET quantity=quantity+#{product.quantity}

              raise FulfillmentError, response.message
            end

else # what if credit card is not needed ??

          end
        else
          raise FulfillmentError, "not enough in stock"
        end
      end
    rescue FulfillmentError => e
      errors.add_to_base e.message
      return false
    end

Imho no lock should be necessary in this case.
You simply risk to answer that there's no more stock whereas there is some 5 ms later.
Better than the opposite.

Jean-Christophe Michel