Chapter 9 Domain Models 1CS6359 Fall 2012 John Cole.

Chapter 9

Domain Models

1CS6359 Fall 2012 John Cole

Why do this?

• If you don’t understand the domain, you can’t program for it effectively

• Lower the representational gap between mental model and software model

CS6359 Fall 2012 John Cole 2

Objectives

• Identify conceptual classes related to the current iteration

• Create an initial domain model• Model appropriate attributes and associations


What is a Domain Model?

• Illustrates noteworthy concepts in a domain. That is, defines what the system is about

• Models the things in your system and the way they relate to each other

• A domain model is conceptual, not a software artifact

• A visual dictionary


What’s the Difference?


Conceptual Class:

SaleSale

amtamt

itemitem

Software Artifacts:

SalesDatabaseSalesDatabase

SaleSale

Double amt;Double amt;

Item item;Item item;

void print()void print()

Business Object Model

• Class diagrams with no methods, just fields• May show domain objects • Associations between classes• Attributes of conceptual classes


Domain Model, visually


Conceptual Classes

• An idea, thing, or object• Symbol—Words or images representing a

conceptual class• Intension—Definition of a conceptual class• Extension—The set of examples to which the

class applies• It is not a data model


How to Create a Domain Model

• Find the conceptual classes• Draw them as classes in a UML class diagram• Add associations necessary to record

relationships• Add attributes necessary for information to be

preserved• Use existing names, the vocabulary of the

domain


How do I find Conceptual Classes?

• Reuse or modify existing models. If there are published models, prior art, or books, use them

• Use a category list (Table 9.1)• Identify noun phrases (linguistic analysis)


Noun Phrase Identification


• Consider the following problem description, analyzed for Subjects, Verbs, Objects:

The ATM verifies whether the customer's card number and PIN are correct. S V O O OIf it is, then the customer can check the account balance, deposit cash, and withdraw cash. S V O V O V O Checking the balance simply displays the account balance. S O V ODepositing asks the customer to enter the amount, then updates the account balance. S V O V O V O Withdraw cash asks the customer for the amount to withdraw; if the account has enough cash, S O V O O V S V Othe account balance is updated. The ATM prints the customer’s account balance on a receipt. O V S V O O

Noun Phrases

Analyze each subject and object as follows:• Does it represent a person performing an action?

Then it’s an actor, ‘R’.• Is it also a verb (such as ‘deposit’)? Then it may be a

method, ‘M’.• Is it a simple value, such as ‘color’ (string) or ‘money’

(number)?Then it is probably an attribute, ‘A’.

• Which NPs are unmarked? Make it ‘C’ for class. Verbs can also be classes, for example:• Deposit is a class if it retains state information


Where are the Terms?

• Some are in the use case• Some come from domain experts• Natural language is imprecise and ambiguous,

so you need to use judgment


POS example


You can create a list, or you can use a set of class diagrams, per the table below

StoreRegister SaleItem

Payment

SalesLineItem

Cashier Customer Manager

ProductCatalog

ProductSpecification

Monopoly Game Domain Model


Maintaining the Model

• Usually the model is a guideline to thinking, not an end in itself. Thus it will change as you learn more

• Who will use the updated model?


Report Objects

• Including reports in a domain model usually isn’t useful because the information is derived from other objects.

• There are special cases, such as receipts, that should be in the model.


Mapmaker (domain terms) Strategy

• Use existing names so far as possible. Learn the terms your users use.

• Exclude out-of-scope features. If a feature is not in the current iteration, don’t use it.

• Model the “unreal” world (i. e. Telecom) by listening carefully to the vocabulary of experts. “Port” means two different things in telecom and shipping.


Attributes vs. Classes

• Common mistake: representing something as an attribute when it should have been a class.

• “If we do not think of some conceptual class X as a number or text in the real world, it should probably be a class, not an attribute.”

• Should “store” be an attribute of Sale or should it be a class?


Description Classes

• A description class contains information that describes something else

• E. g. Product Description records the price, picture, text description (and what else?) of an item


Why Use Them?

• If we keep all the information in, say, a sales line item, once all of that item are sold, there is no record of what the item was

• How does this relate to database design?


Associations

• Relationship between (instances of) classes that indicates some interesting and meaningful connection

• What objects need memory of a relationship?• In Monopoly, where each piece is and which

player owns which token• No need to remember numbers on the dice


Associations (cont’d)

• Avoid having too many; this adds communication paths and complexity

• Will they be implemented in software?• UML: Line between classes with the name of

the association• Name with VerbPhrase-ClassName format. E.

g. Sale Paid By CashPayment• Each end of an association is called a Role


Associations (cont’d)

• Multiplicity: how many instances of class A can be associated with an instance of class B.

• E. g. an instance of Store can be associated with many (zero or more) Items

• This is at a particular moment, not over time• Discussion of 0..1 or 1• Multiple associations: Flight FliesTo and FliesFrom an

airport• Common Associations List on p. 155


Attributes• An Attribute is a logical data value of an object• You can also have derived attributes denoted

by / before the name• Full syntax: visibility Name : type multiplicity =

default {property string}• +pi : Real=3.14159 {read-only}


Sale

datestartTime : Time

attributes

Attribute Types

• Usually primitive data types, as well as things like Color, DateTime, Zip code, etc. The latter are specializations of primitive types

• It should not normally be a class, such as Sale or Airport

• Relate conceptual classes with an association, not an attribute. No foreign keys

• Attributes in code are okay


New Data Type Classes

• Things like ItemID or ClaimNumber are not always simple data types, even though they look like them

• If the attribute contains separable pieces, it can be its own class. For example, a Claim Number has the state, year, and day encoded in it

• If it has operations associated with it


New Data Type Classes (cont’d)

• If it has other attributes, such as a sale price• If it has units, such as currency• If it is an abstraction of one or more types

with these properties


Quantities with Units

• Represent them with distinct classes with an associated unit


Payment

amount : Number

Payment Quantity

amount : Number

Unit

...

Payment

amount : Quantity

Has-amount1*

Is-in1*

not useful

quantities are pure datavalues, so suitable to showin attribute section better

Payment

amount : Money

variation: Money is aspecialized Quantity whoseunit is a currency

Is the Domain Model Correct?

• No, but it’s a good approximation• It gets better with each iteration; don’t try to

get it all at once• Domain model usually done in the elaboration

phase


Chapter 9 Domain Models 1CS6359 Fall 2012 John Cole.

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