Design Strategy - Helper methods that throw Exceptions

Question

I am implementing a plugin for a client that will use the 'Max Sale Quantity' value of a Magento Stock Item on a product as a means of setting an upper-bound on total customer purchases.

Because I need to check this condition in multiple events, controller actions, I cannot contain it to merely an Observer where I could create a local protected function that checks this condition from my Resource Model.

In my requirements I will be limiting the cart additions. But I find myself in a dilemma: from the controller addAction I need to rely on throw - however from sales_quote_merge_before it seems that I could effectively unset the offending products and post a message to the customer/session, though, I could throw and catch, respectively as with the controller action instead.

My questions are:

• Is this bad practice to effectively rely on throw from the module helper?
• Is this, rather, a model method that I need to invoke?
• What is the best design strategy that allows me to keep my module DRY?

Edit:

As of EE 1.12 I see only 4 instances of Helper/Data.php files that throw - 3 of them are Api2/Oauth. Mage/Core does it once as well, but this seems to be the only other instance. Various 3rd parties look like they take different approaches here.

Any feedback is welcome.

Answer 1

Saying to the customer, "we are sorry, you can't purchase so much" is a exception. The expected case is, there is enough and you put the products in the cart. And one more word: There are try and catch blocks inside of magento, for example in \Mage_Checkout_CartController::addAction:

// [...]
} catch (Mage_Core_Exception $e) {
        if ($this->_getSession()->getUseNotice(true)) {
            $this->_getSession()->addNotice(Mage::helper('core')->escapeHtml($e->getMessage()));
// [...]

Let me say it this way: They are there to be used :-)