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

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