Concurrency issue


#1

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.


#2

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


#3

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?


#4

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


#5

I just noticed you decrease the quantity only when credit_card_needed?

Is that right?

Paolo


#6

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

On 14/05/07, removed_email_address@domain.invalid removed_email_address@domain.invalid wrote:

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


#7

Hi,

What about doing preventive stock update:

Le 14 mai 07, à 22:31, removed_email_address@domain.invalid a écrit :

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 M.

symetrie.com

Better Nested Set for rails:
http://opensource.symetrie.com/trac/better_nested_set


#8

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


#9

On 14/05/07, removed_email_address@domain.invalid removed_email_address@domain.invalid wrote:

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