Ryan,
You are completely right about the atomic operation. The below works
well with a hard coded value, but I need to insert $quantity since
that will vary based on the adjustment needed from an order or stock
being added. I'm stuck with the code change into the literal value
there.
$this->updateAll(
array('Venue.inventory' => 'Venue.inventory+$quantity'),
array('Venue.id' => $id));
Thanks,
Brian
On Mar 1, 2:06 pm, Ryan Schmidt <[email protected]> wrote:
> On Mar 1, 2011, at 12:46, Ryan Schmidt wrote:
>
>
>
> > On Mar 1, 2011, at 11:35, lirc201 wrote:
>
> >> I like the example on the weblink which is just using MySQL
> >> functionality, it is very clever. Thanks for the help and push. My
> >> code is working! I did like the security tricks I discovered for my
> >> updates below to only pass a field list.
>
> >> function updateInventory($id, $quantity) {
> >> $inv = $this->getQuantity($id); <- function is in the
> >> same model
> >> $this->data['Model']['id'] = $id;
> >> $this->data['Model']['inventory'] = $inv + $quantity;
> >> if($this->save($this->data, array(
> >> 'validate' => 'false',
> >> 'fieldList' => array('id',
> >> 'inventory')))) {
> >> ... logic for return values ...
> >> }
> >> }
>
> > This is not atomic. Between the line where you get the current quantity and
> > the line where you save the updated quantity, someone else might have
> > updated the quantity, and you'd be wiping out their change. Consider two
> > managers simultaneously adding 5 to the inventory of the same item. The
> > expected result would be that in the end the inventory is incremented by
> > 10, but because of the race condition in your code, it might only be
> > incremented by 5.
>
> The problem is more apparent if you think about the case of a customer buying
> an item on your site and decreasing the inventory.
>
> Let's say your inventory for item id 3 is 10, and you have two users about to
> buy one each of item id 3. Both users happen to click the Buy button at the
> same instant, which will cause the method updateInventory(3, -1) to be called
> for each user. If this method runs at the same time for both users, then for
> both users getQuantity() will return 10, in both cases you'll add $quantity
> to it, thereby decreasing it by one, and in both cases you'll save 9 to the
> database -- even though two users have successfully bought the item and the
> inventory should now be 8. Clearly this will be a problem as your database
> might say you have more quantity than you actually do and customers will try
> to buy product you no longer have.
>
> The solution is to use a single atomic SQL UPDATE statement, as I showed
> earlier (or rather, the CakePHP equivalent of that), or use an SQL
> transaction (and ensure your database supports transactions -- for example
> for MySQL, you'd use InnoDB tables instead of MyISAM).
--
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