Tuesday, November 18, 2008

Let's not get physical

In my last post, I promised to reveal how class diagrams can be your secret weapon in the war against imprecise requirements. How can this be, you wonder? Sure, class diagrams capture entities, attributes, and associations, but they're completely incapable of expressing the hopes, fears, aspirations and unwarranted hostility of my end users. Surely we need all the subtlety and nuance of natural language to fully capture the essence of what we're being required to build? Surely we need to capture requirements using the flowing prose of use cases?

As we've discussed, precision is certainly something to be aspired to in requirements specifications, and yet, the legal profession provides compelling testimony to the horrible things that happen to beautiful prose when we impose precision upon it. Let's begin to consider some other things we can express clearly, unambiguously, and some might say beautifully, via everybody's favorite UML diagram.

Suppose we've been asked to build an online storefront offering stolen military hardware at low-low prices. The company, Arms4Less, was founded by several retired mercenaries now holed up somewhere in Honduras. Our primary contact is this crusty old guy we know only as "Sarge". The first thing he says the system needs to do is "store orders for stuff."

"You know", he says, "we sell stuff. Hand grenades, bazookas, napalm cannisters, that kinda thing. We'll need the usual shopping cart deal, letting customers tell us how much of each product they want." We sketch out the diagram below.
As you would expect, an Order consists of 1 or more (1..*) items, and each OrderItem specifies the quantity of some product being ordered. Each Product has a price. Now, hopefully you're regarding this model and saying, "Yeah? So? Why am I wasting time reading this stupid guy's blog when I could be learning Spring?" In a moment we'll make this model a lot more interesting. The diamond, by the way, signifies a "composite" relationship between an Order and its items, which essentially means that if we delete an Order, all of its items will get deleted too. (You would of course know what the diamond means if you'd been paying attention instead of doing email in that UML course your boss sent you to.) If the other symbols being used here are mysterious to you, go hang out with Ambler and come back when you're done.

A tremendously useful, but largely underused (Ambler's overview doesn't even mention them), facility of class diagrams is the specification of "derived" attributes. Derived attribute values are - uh - derived. That is, their values don't need to be stored since they can be completely determined from other data values. For example, Sarge explains that we need to total up each order and make the customer's got enough dough to cover it. "We do this for the customer's own protection", Sarge explains. "You don't wanna know what's gotta happen if the guy can't pay us." And indeed, we really don't wanna know until a later iteration. So, we add 2 derived attributes to the model to deal with the order total.

We've added a total attribute to both the Order and the OrderItem entities. Note that each has a leading slash, which means "derived" in UML. The OrderLine total is, not surprisingly, derived by multiplying the quantity by the price of the associated product (quantity*product.price). The Order total is the sum of the totals of its items (sum(items.total)). We mentioned earlier that derived attributes don't need to be stored, and indeed you can see how we can simply calculate these attributes' values on the fly. Certainly, however, there are situations in which we store derived attributes if, for example, they're too expensive to calculate dynamically. Such a decision however is purely an implementation choice. Logically, these attributes are derived.† This is all about what the system must do, not how it will do it. So let's keep our functional requirements logical. As Sarge might say, "You don't wanna get physical with me unless you're good and ready, computer boy."

Now, you're probably still a little underwhelmed by all this. But as we'll see in our next episode, we've only just scratched the surface of the expressive power of the friendly, little class diagram.

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