part iii elaboration iteration 1 basicswkchen/courses/ooad/ooad101_2/... · 2018. 3. 2. · – do...

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PART IIIPART IIIElaboration Iteration 1Elaboration Iteration 1

BasicsBasics

inc. elaboration construction transition

iteration phase

development cycle

Chapter 8Chapter 8Iteration 1Iteration 1

Basics Basics

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Iteration 1Iteration 1• Iteration 1 of the elaboration phase

– Requirements and Emphasis: Core OOA/D Skills

– Book iteration 1 is NOT architecture-centricand risk-driven.

• In Iterative Development– Don't Implement All the Requirements at

Once – Incremental Development for the Same Use

Case Across Iterations

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Iteration 1Iteration 1• Use case implementation may be spread across iterations

1A use case or feature is often too complex to complete in one short iteration.

Therefore, different parts or scenarios must be allocated to different iterations.

Use CaseProcess Sale

2 3 . . .



Use CaseProcess Rentals

Feature: Logging

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POS Iteration 1POS Iteration 1• Requirements for iteration 1 of the POS application

– Implement a basic, key scenario of the Process Sale use case: entering items and receiving a cash payment.

– Implement a Start Up use case as necessary to support the initialization needs of the iteration.

– Nothing fancy or complex is handled, just a simple happy pathscenario, and the design and implementation to support it.

– There is no collaboration with external services, such as a tax calculator or product database.

– No complex pricing rules are applied.– The design and implementation of the supporting UI, database,

and so forth, would also be done

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Elaboration Elaboration 11• Elaboration: Build the core architecture, resolve the

high-risk elements, define most requirements, and estimate the overall schedule and resources.

• Elaboration is the initial series of iterations during project– the core, risky software architecture is programmed and tested– the majority of requirements are discovered and stabilized– the major risks are mitigated or retired

• Elaboration often consists of two or more iterations; – each iteration is recommended to be 2~6 weeks

• Elaboration is not a design phase or a phase when the models are fully developed in preparation for implementation – that would be waterfall.

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Elaboration Elaboration 22• Elaboration

– Not throw-away prototypes; the code and design areproduction quality (a production subset of the final system).

– More commonly called executable architecture or Architectural baseline

• Some key ideas and best practices will manifest in elaboration:– do short time boxed risk-driven iterations– start programming early– adaptively design, implement, and test the core and risky parts

of the architecture– test early, often, realistically– adapt based on feedback from tests, users, developers– write most of the use cases and other requirements in detail,

through a series of workshops, once per elaboration iteration

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Elaboration Elaboration 33

A description of the user interface, paths of navigation, usability models, and so forth.

Use-Case Storyboards, UI Prototypes

This includes the database schemas, and the mapping strategies between object and non-object representations.

Data Model

A learning aid that summarizes the key architectural issues and their resolution in the design. It is a summary of the outstanding design ideas and their motivation in the system.

Software Architecture Document

This is the set of diagrams that describes the logical design. This includes software class diagrams, object interaction diagrams, package diagrams, and so forth.

Design Model

This is a visualization of the domain concepts; it is similar to a static information model of the domain entities.

Domain Model

CommentArtifact

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Planning the Next Iteration Planning the Next Iteration • Organize requirements and iterations by risk,

coverage, and criticality.– Risk includes both technical complexity and other

factors, such as uncertainty of effort or usability.– Coverage implies that all major parts of the system

are at least touched on in early iterations perhaps a "wide and shallow" implementation across many components.

– Criticality refers to functions the client considers of high business value.

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POS Risk ListPOS Risk List

……Low

Affects security subdomain.…

Maintain Users…

Medium

Scores high on all rankings.Pervasive. Hard to add late.…

Process SaleLogging…

High

CommentRequirement (Use Case or Feature)Rank

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Chapter 9Chapter 9Domain Models Domain Models

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IntroductionIntroduction• Domain model

– The most important and classic model in OO analysis. – A visual representation of conceptual classes or real situation

objects in a domain.– Also called conceptual models, domain object models, and

analysis object models.– “focusing on explaining 'things' and products important to a

business domain”, such as POS related things.

• Guideline– Avoid a waterfall-mindset big-modeling effort to make a

thorough or "correct" domain model– such over-modeling efforts lead to analysis paralysis, with little

or no return on the investment.

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Sample UP artifact influence Sample UP artifact influence

Process Sale

1. Customer arrives ...2. ...3. Cashier enters item identifier.4....

Use Case Text

Operation: enterItem(…)

Post-conditions:- . . .

Operation Contracts

Sale

date. . .

SalesLineItem

quantity

1..*1 . . .

. . .

the domain objects, attributes, and associations that undergo state changes

Domain Model

Use-Case Model

Design Model

: Register

enterItem(itemID, quantity)

: ProductCatalog

spec = getProductSpec( itemID )

addLineItem( spec, quantity )

: Sale

. . .

conceptual classes in the domain inspire the names of some software classes in the design

conceptual classes –terms, concepts attributes, associations

Cashier: …Item ID: …...

Glossary

elaboration of some terms in the domain model

Require-ments

Business Modeling

Design

Sample UP Artifact Relationships

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POS Domain ModelPOS Domain Model

Register

Item

Store

addressname

Sale

date time

Payment

amount

SalesLineItem

quantity

Stocked-in

*

Houses

1..*

Contained-in

1..*

Records-sale-of

0..1

Paid-by

1

1

1

1

1

1

0..1

1

Captured-on

conceptor domain object

association

attributes

Partial domain model drawn withUML class diagram

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Domain Model Domain Model 11• Domain model is illustrated with UML class

diagrams (no operations). It provides a conceptual perspective. – domain objects or conceptual classes– associations between conceptual classes– attributes of conceptual classes

• Elements NOT suitable in a domain model– Software artifacts, such as a window or a database,

unless the domain being modeled is of software concepts, such as a model of graphical user interfaces.

– Responsibilities or methods.

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Domain Model Domain Model 22• A domain model shows real-situation conceptual

classes, not software classes

Sale

dateTime

visualization of a real-world concept in the domain of interest

it is a not a picture of a software class

SalesDatabase software artifact; not part of domain modelavo

id

software class; not part of domain model

Sale

datetime

print()

avoid

• A domain model does not show software artifacts or classes

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Domain Model Domain Model 44

Lower representational gap with OO modeling.

Payment

amount

Sale

datetime

Pays-for

Payment

amount: Money

getBalance(): Money

Sale

date: DatestartTime: Time

getTotal(): Money. . .

Pays-for

UP Domain ModelStakeholder's view of the noteworthy concepts in the domain.

UP Design ModelThe object-oriented developer has taken inspiration from the real world domain in creating software classes.

Therefore, the representational gap between how stakeholders conceive the domain, and its representation in software, has been lowered.

1 1

1 1

A Payment in the Domain Model is a concept, but a Payment in the Design Model is a software class. They are not the same thing, but the former inspired the naming and definition of the latter.

This reduces the representational gap.

This is one of the big ideas in object technology.

inspires objects

and names in

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Lower representational gap Lower representational gap

Use case Domain model Design

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Guideline: Create a Domain Guideline: Create a Domain ModelModel

• Guideline: Create a Domain Model– Bounded by the current iteration

requirements under design• Find the conceptual classes (next slide)• Draw them as classes in a UML class diagram.• Add associations

– Record relationships for which there is a need to preserve some memory.

• Add attributes– Necessary to fulfill the information requirements.

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Guideline: Find Conceptual Guideline: Find Conceptual Classes Classes 11

• Three strategies to find conceptual classes1. Reuse or modify existing models. 2. Use a category list.3. Identify noun phrases

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• Use a Category List

Register, LedgerFlightManifest

where is the transaction recorded?Guideline: Important.

ItemFlight, Seat, Meal

product or service related to a transaction or transaction line itemGuideline: Transactions are for something (a product or service). Consider these next.

SalesLineItemtransaction line itemsGuideline: Transactions often come with related line items, so consider these next.

Sale, PaymentReservation

business transactionsGuideline: These are critical (they involve money), so start with transactions.

ExamplesConceptual Class Category

POS, Monopoly, airline reservation system

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Store, Bin Board Airplane

containers of things (physical or information)

Product CatalogFlight Catalog

catalogsGuideline: Descriptions are often in a catalog.

Product DescriptionFlight Description

descriptions of thingsGuideline: See p. 147 for discussion.

Item, Register Board, Piece, Die Airplane

physical objectsGuideline: This is especially relevant when creating device-control software, or simulations.

Sale, Payment, Flightnoteworthy events, often with a time or place we need to remember

StoreAirport, Plane, Seat

place of transaction; place of service


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DailyPriceChangeListRepairSchedule

schedules, manuals, documents that are regularly referred to in order to perform work

Cash, Check, LineOfCreditTicketCredit

financial instruments

Receipt, LedgerMaintenanceLog

records of finance, work, contracts, legal matters

Credit Authorization SystemAir Traffic Control

other collaborating systems

Item Square (in a Board) Passenger

things in a container


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• Identify noun phrases.– Identify the nouns and noun phrases in textual

descriptions of a domain, and consider them as candidate conceptual classes or attributes

– Some of these noun phrases may refer to conceptual classes that are ignored in this iteration (e.g., "Accounting" and "commissions"), and some may be simply attributes of conceptual classes.

– A weakness of this approach is the imprecision of natural language; different noun phrases may represent the same conceptual class or attribute, among other ambiguities.

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Find Conceptual Classes: Find Conceptual Classes: POS POS 11

• Main Success Scenario (or Basic Flow):1.Customer arrives at a POS checkout with goods and/or

services to purchase.2.Cashier starts a new sale.3.Cashier enters item identifier.4.System records sale line item and presents item description,

price, and running total. Price calculated from a set of price rules.

Cashier repeats steps 2-3 until indicates done.5.System presents total with taxes calculated.6.Cashier tells Customer the total, and asks for payment.7.Customer pays and System handles payment.8.System logs the completed sale and sends sale and payment

information to the external Accounting (for accounting and commissions) and Inventory systems (to update inventory).

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9.System presents receipt.10.Customer leaves with receipt and goods (if any).

Extensions (or Alternative Flows):. . .7a. Paying by cash:

1.Cashier enters the cash amount tendered.2.System presents the balance due, and releases the cash drawer.3.Cashier deposits cash tendered and returns balance in cash to

Customer.4.System records the cash payment.

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• For iteration-1, the basic cash-only scenario of Process Sale. – Sale, Cashier, Cash, Payment, – Customer, Sales Line Item, – Store, Item, Product Description, – Register, Product Catalog, Ledger.

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Guideline:Guideline: Agile Modeling Agile Modeling Maintain the Model in a ToolMaintain the Model in a Tool

• Perfection is not the goal of Agile, and agile models are usually discarded shortly after creation. – From this viewpoint, there is no motivation

to maintain or update the model.

• If someone wants the model maintained and updated with new discoveries– do the drawing with a UML tool.

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Guideline: Report Objects Guideline: Report Objects Include 'Receipt' in the ModelInclude 'Receipt' in the Model

• Receipt is a noteworthy term in the POS domain. Butperhaps it's only a report of a sale and payment, and duplicate information. Two factors to consider– Exclude it: Showing a report of other information in a

domain model is not useful since all its information is derived or duplicated from other sources.

– Include it: it has a special role in terms of the business rules: It confers the right to the bearer of the receipt to return bought items.

• Receipt will be excluded in this iteration. – Until the iteration that tackles the Handle Returns use case

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GuidelinesGuidelines• Guideline: Think Like a Mapmaker; Use Domain

Terms (next page)– Make a domain model in the spirit of how a

cartographer or mapmaker works:• Use the existing names in the territory. For example, if

developing a model for a library, name the customer a "Borrower" the terms used by the library staff.

• Exclude irrelevant or out-of-scope features. • Do not add things that are not there.

• Guideline: How to Model the Unreal World?– It requires a high degree of abstraction, and

listening carefully to the core vocabulary and concepts that domain experts use.

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Think Like a Mapmaker; Use Think Like a Mapmaker; Use Domain TermsDomain Terms

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Guideline:Guideline: A Common Mistake A Common Mistake with Attributes vs. Classeswith Attributes vs. Classes

• If we do not think of some conceptual class X as a number or text in the real world, X is probably a conceptual class, not an attribute.– In the real world, a store is not considered a number

or text, the term suggests a legal entity, an organization, and something that occupies space. Therefore, Store should be a conceptual class.

Sale StorephoneNumber

SaleStore

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Guideline:Guideline: When to Model with When to Model with 'Description' Classes'Description' Classes

• A description class contains information that describes something else. For example, a ProductDescription that records the price, picture, and text description of an Item.– Problems: if implemented in software similar to the domain model,

it has duplicate data (space-inefficient, and error-prone). Because the description, price, and itemID are duplicated for every Iteminstance of the same product

– A particular Item may have a serial number; it represents a physical instance. A ProductDescription wouldn't have a serial number

ItemserialNumber

ItemdescriptionpriceserialNumberitemID

productDescriptiondescriptionpriceitemID

1 *

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Association Association 11• Association

– a relationship between classes (instances of those classes) that indicates some meaningful and interesting connection.

• Guideline: When to Show an Association?– Associations imply knowledge of a relationship that needs to be

preserved for some duration. • Guideline: Avoid Adding Many Associations

– Many lines on the diagram will obscure it with "visual noise."

• Perspectives: Will the Associations Be Implemented In Software?– It is meaningful in a purely conceptual perspective in the real

domain.– Many associations will be implemented in software as paths or

navigation and visibility.

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Association Association 22• Guideline

– Name an association based on a ClassName-VerbPhrase-ClassName format where the verb phrase creates a sequence that is readable and meaningful.

– Association names should start with a capital letter, since an association represents a classifier of links between instances;

– e.g. Good: Sale Paid-by CashPayment– e.g. Bad: Sale Uses CashPayment– e.g. Good: Player Is-on Square– E.g. Bad: Player Has Square

• Applying UML: Roles– Each end of an association is called a role. Roles may optionally

have multiplicity expression, name, navigability.

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Association Association 33• Applying UML: Multiplicity

– Multiplicity defines how many instances of a class A can be associated with one instance of a class B

ItemStore Stocks

*

multiplicity of the role

1

zero or more; "many"

one or more

one to 40

exactly 5

T

T

T

T

*

1..*

1..40

5

T3, 5, 8

exactly 3, 5, or 8

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Association Association 44• Applying UML: Multiple Associations Between Two

Classes

Flight Airport

F lies-to

Flies-from

*

* 1

1

• Guideline: Find Associations with a Common Associations List (for business information systems)

CustomerPaymentPassengerTicket

A is a role related to a transaction B

ItemSalesLineItem (or Sale), FlightReservation

A is a product or service for a transaction (or line item) B

SalesLineItemSaleA is a line item of a transaction B

CashPaymentSaleCancellationReservation

A is a transaction related to another transaction B

ExamplesCategory

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Association Association 55

SalesLineItemSalesLineItem, SquareSquare, CityCity

A is next to B

CashierRegister, PlayerPiece, PilotAirplaneA uses or manages or owns B

DepartmentStore, MaintenanceAirlineA is an organizational subunit of B

CashierStore, PlayerMonopolyGame, PilotAirline

A is a member of B

SaleRegister, PieceSquare, ReservationFlightManifest

A is known/logged/recorded/ reported/captured in B

ProductDescriptionItem, FlightDescriptionFlight

A is a description for B

RegisterStore, ItemShelf, SquareBoard, PassengerAirplane

A is physically or logically containedin/on B

DrawerRegister, SquareBoard, SeatAirplaneA is a physical or logical part of B

ExamplesCategory

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POS Partial Domain ModelPOS Partial Domain Model

Reading direction

Reading direction arrow

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Monopoly Partial Domain Monopoly Partial Domain ModelModel

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Attributes Attributes 11• An attribute is a logical data value of an object.• Guideline: When to Show Attributes?

– Include attributes that the requirements (e.g., use cases) suggest or imply a need to remember information.

– e.g., a receipt (which reports the information of a sale) in theProcess Sale use case normally includes a date and time, the store name and address, and the cashier ID,

Sale

dateTime/ total : Money

attributes

derived attribute

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Attributes Attributes 22• Guideline: Where to Record Attribute

Requirements?– E.g., middleName: [0..1]– Place all such attribute requirements in the Glossary.– Use a tool that integrates UML models with a data dictionary.

• Derived Attributes– The total attribute in the Sale can be calculated or derived

from the information in the SalesLineItems.

Sale

- dateTime : Date- / total : Money

Private visibility attributes

Math

+ pi : Real = 3.14 {readOnly}

Public visibility readonly attribute with initialization

Person

firstNamemiddleName : [0..1]lastName

Optional value

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Attributes Attributes 33• Guideline: Focus on Data Type Attributes in the

Domain Model.– most attribute types should be primitive data types, such as

numbers and Booleans.

Cashier

namecurrentRegister

Cashier

name

Register

numberUses

Worse

Better

not a "data type" attribute

1 1

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Attributes Attributes 44• Guideline: Don't show complex concepts as

attributes; use associations.

Flight

Flight

destinationW orse

BetterFlies-to Airport1 1

destination is a complex concept

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Attributes Attributes 55• Guideline: Represent what may initially be considered

a number or string as a new data type class in the domain model if:– It is composed of separate sections: phone number, name of person– There are operations associated with it, such as parsing or validation.

• social security number

– It has other attributes.• promotional price could have a start (effective) date and end date

– It is a quantity with a unit.• payment amount has a unit of currency

– It is an abstraction of one or more types with some of these qualities.• item identifier in the sales domain is a generalization of types such as

Universal Product Code (UPC) and European Article Number (EAN)

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Attributes Attributes 66• Applying UML: Two ways to indicate a data type

property of an object.

OK

OK

ProductDescription

ProductDescription

itemId : ItemID

1Store

Store

address : Address

11 1ItemID

idmanufacturerCodecountryCode

Address

street1street2cityName...

Data type

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Attributes Attributes 77• Guideline: No Attributes Representing Foreign

Keys.

Cashier

namecurrentRegisterNumber

Cashier

name

Register

numberWorks-on

Worse

Better

a "simple" attribute, but being used as a foreign key to relate to another object

1 1

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Attributes Attributes 88• Guideline: Modeling Quantities and Units.

– Most numeric quantities should not be represented as plain numbers. – To represent Quantity as a distinct class, with an associated Unit. – Money is a kind of quantity whose units are currencies. Weight is a

quantity with units such as kilograms or pounds.

Payment

amount : Number

Payment Quantity

amount : Number

Unit

...

Payment

amount : Quantity

Has-amount1*

Is-in1*

not useful

quantities are pure data values, so are suitable to show in attribute section better

Payment

amount : Money

variation: Money is a specialized Quantity whose unit is a currency

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Register

id

ItemStore

nameaddress

Sale

dateTime/ total

CashPayment

amountTendered

SalesLineItem

quantity

Cashier

id

Customer

ProductCatalog

ProductDescription

itemIDdescriptionprice

Stocks

*

Houses1..*

Used-by

*

Contains1..*

Describes

*

Captured-on

Contained-in

1..*

Records-sale-of

0..1

Paid-by Is-for

Logs-completed

*

Works-on

11

1

1 1..*

1

1

1

1

1

1

1

0..1 1

1

Ledger

Records-accounts-

for

1

1

50

Monopoly Partial Domain Monopoly Partial Domain ModelModel

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Iterative and Evolutionary Iterative and Evolutionary Domain Modeling Domain Modeling

• Domain model is usually both started and completed in elaboration phase

rsData Model

sSW Architecture Document

rsDesign ModelDesign

rsGlossary

rsSupplementary Specification

rsVision

rsUse-Case Model (SSDs)Requirements

sDomain ModelBusiness Modeling

T1..T2C1..CnE1..EnI1Iteration

Trans.Const.Elab.Incep.ArtifactDiscipline

Chapter 10Chapter 10System Sequence System Sequence Diagrams (Diagrams (SSDSSD))

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UP artifact influenceUP artifact influence

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POS SSD: a Process Sale POS SSD: a Process Sale ScenarioScenario


:System: Cashier

endSale

makePayment(amount)

a UML loop interaction frame, with a boolean guard expression

external actor to system

Process Sale Scenario

system as black box

the name could be "NextGenPOS" but "System" keeps it simple

the ":" and underline imply an instance, and are explained in a later chapter on sequence diagram notation in the UML

a message with parameters

it is an abstraction representing the system event of entering the payment data by some mechanism

description, total

return value(s) associated with the previous message

an abstraction that ignores presentation and medium

the return line is optional if nothing is returned

total with taxes

change due, receipt

makeNewSale

[ more items ]loop

Guard (bool)

return line

instance

message

TimeSSD name

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System Sequence Diagram System Sequence Diagram 11• System sequence diagram (SSD)

– a picture that shows, for one particular scenario of a use case, the events that external actors generate, their order, and inter-system events.

– The system is treated as a black box; the emphasis of the diagram is events that cross the system boundary from actorsto systems.

– UML sequence diagrams is used to draw SSD.

• Guideline: Draw an SSD for a main success scenario of each use case, and frequent or complex alternative scenarios.

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System Sequence Diagram System Sequence Diagram 22• Motivation: Why draw an SSD?

– A software system must reacts to:• External events from actors• Timer events• Faults or exceptions

– Therefore it is useful to identify external input events (system events).

• Is an important part of analyzing system behavior.• System behavior is description of what the system does.• SSD describes system behavior.• Use cases and system operation contracts also describe

system behavior.

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System Sequence Diagram System Sequence Diagram 33• SSDs are derived from use cases; they show one

scenario.

: Cashier :System

Simple cash-only Process Sale scenario:

1. Customer arrives at a POS checkout with goods and/or services to purchase.2. Cashier starts a new sale.3. Cashier enters item identifier.4. System records sale line item and presents item description, price, and running total. Cashier repeats steps 3-4 until indicates done.5. System presents total with taxes calculated.6. Cashier tells Customer the total, and asks for payment.7. Customer pays and System handles payment....


endSale

makePayment(amount)

description, total

total with taxes

change due, receipt

makeNewSale

[ more items ]loop


SSD name

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System Sequence Diagram System Sequence Diagram 44• System events should be expressed at the abstract level of

intention rather than in terms of the physical input device.– "enterItem" is better than "scan" (laser scan) because it captures the

abstract intent of the operation.– Non committal with respect to design choices about what interface is

used to capture the system event (laser scanner, keyboard, voiceinput ..)


scan(itemID, quantity)

: Cashier

worse name

better name

:System

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System Sequence Diagram System Sequence Diagram 55• Guideline: Show details of SSD in the glossary.

– The elements shown in SSDs (operation name, parameters, return data) are terse.

– Put detail of these elements in glossary.

• Iterative and evolutionary SSDs– Create SSD only for the next iteration– Inception phase: SSDs are not usually motivated in inception.– Elaboration phase: most SSDs are created during elaboration, when it

is useful to identify the details of the system events to clarify what major operations, write system operation contracts, and possibly to support estimation.

inc. elaboration construction transition

iteration phase

development cycle

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Monopoly SSDMonopoly SSD

1.Observer requests new game initialization, enters number of players.2.Observer starts play.3.System displays game trace for next player move Repeat step 3 until a winner or Observer cancels.

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Chapter 11Chapter 11Operation Operation Contracts Contracts

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Operation Contracts: Operation Contracts: IntroductionIntroduction

• Operation contracts– Part of the UP Use-Case Model– They provide more analysis detail on the

effect of the system operations implied in the use cases

• Operation contracts use a pre- and post-condition form to describe detailed changes to objects in a domain model, as the result of a system operation.

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Sample UP Artifact Influence Sample UP Artifact Influence

Operation: enterItem(…)

Post-conditions:- . . .

Operation Contracts

Sale

date. . .

SalesLineItem

quantity

1..*1 . . .

. . .

Domain Model

Use-Case Model

Design Model: Register


: ProductCatalog

spec = getProductSpec( itemID )

addLineItem( spec, quantity )

: Sale

Require-ments

Business Modeling

Design

Sample UP Artifact Relationships

: System

enterItem(id, quantity)

Use Case Text

System Sequence Diagrams

makeNewSale()

system events

Cashier

Process Sale

: Cashier

use case

names

system operations

Use Case Diagram

Vision

SupplementarySpecification

Glossary

starting events to design for, and more detailed requirements that must be satisfied by the software

Process Sale

1. Customer arrives ...2. ...3. Cashier enters item identifier.

the domain objects, attributes, and associations that undergo changes

requirements that must be satisfied by the software

ideas for the post-conditions

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POS Operation Contract POS Operation Contract • Contract CO2: enterItem

– Operation: enterItem(itemID: ItemID, quantity: integer)

– Cross References: Use Cases: Process Sale– Preconditions: There is a sale underway.– Postconditions:

• A SalesLineItem instance sli was created (instance creation).• sli was associated with the current Sale (association

formed).• sli.quantity became quantity (attribute modification).• sli was associated with a ProductDescription, based on

itemID match (association formed).

Words in black are learning aid, not part of the contract

See domain model (next page)

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65


Register

id

ItemStore

nameaddress

Sale

dateTime/ total

CashPayment

amountTendered

SalesLineItem

quantity

Cashier

id

Customer

ProductCatalog

ProductDescription


Stocks

*

Houses

1..*

Used-by

*

Contains1..*

Describes

*

Captured-on

Contained-in

1..*

Records-sale-of

0..1

Paid-by Is-for

Logs-completed

*

Works-on

11

1

1 1..*

1

1

1

1

1

1

1

0..1 1

1

Ledger

Records-accounts-

for

1

1

enterItem

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Sections of a Contract Sections of a Contract • Operation:

– Name of operation, and parameters

• Cross References:– Use cases this operation can occur within

• Preconditions:– Noteworthy assumptions about the state of the

system or objects in the Domain Model before execution of the operation. (non-trivial assumptions)

• Postconditions:– The state of objects in the Domain Model after

completion of the operation.

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67

System Operations System Operations 11• System operations

– The public interface of the system (as a black boxcomponent).

• The entire set of system operations, across all use cases, defines the public system interface, viewing the system as a single component or class.

– SSDs show system events• Input system events imply the system has system operations

to handle the events

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System Operations System Operations 22

: Cashier


endSale()

makePayment(amount)

description, total

total with taxes

change due, receipt

makeNewSale()

these input system events invoke system operations

the system event enterItem invokes a system operationcalled enterItem and so forth

this is the same as in object-oriented programming when we say the message foo invokes the method (handling operation) foo

[ more items ]loop

:System


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Postconditions Postconditions 11• Postconditions definition

– Describe changes in the state of objects in the domain model• Including instances created, associations formed or broken, and

attributes changed.

• Postconditions are not actions to be performed during the operation– They are observations about the domain model objects that are

true when the operation has finished• Postconditions fall into these categories:

– Instance creation and deletion.– Attribute change of value.– Associations (UML links) formed and broken.

• How are postconditions related to the Domain Model?– What instances can be created; What associations can be formed

in the Domain Model.

70

Postconditions Postconditions 22• Motivation: Why Postconditions?

– First, contracts aren't always necessary unless more detail and precision is useful

– Postconditions support fine-grained detail and precision in declaring what the outcome of the operation must be.

– A contract is an excellent tool of requirements analysis that describes in great detail the changes required by a system operation without having to describe how they are to be achieved

• Guideline: How To Write a Postcondition?– Express postconditions in the past tense to emphasize state

changes that arose from an operation, not an action to happen.

• (better) A SalesLineItem was created.• (worse) Create a SalesLineItem, or, A SalesLineItem is created.

36

71

Postconditions Postconditions 33• Why write postconditions in the past tense?

– Think about them using The Stage and Curtain analogy (next page)

• The system and its objects are presented on a theatre stage.– 1.Before the operation, take a picture of the stage.– 2.Close the curtains on the stage, and apply the system operation– 3.Open the curtains and take a second picture.– 4.Compare the before and after pictures, and express as

postconditions the changes in the state of the stage

• Guideline: How Complete Should Postconditions Be? Agile vs. Heavy Analysis– Generating a complete and detailed set of postconditions

for all system operations is not likely or necessary.– Treat the creation of postconditions as an initial best guess,

knowing that they are not complete and perfect.

72

PostconditionsPostconditions 44

Before After

37

73

Example: enterItem Example: enterItem PostconditionsPostconditions

• Postconditions of the enterItem system operation.– Instance Creation and Deletion

• After an enterItem (itemID and quantity) operation, what new objectshould have been created? A SalesLineItem. Thus:

– A SalesLineItem instance sli was created (instance creation).

– Attribute Modification• After an enterItem (itemID and quantity) operation, what attributes of

new or existing objects should have been modified? The quantity of the SalesLineItem should have become equal to the quantity parameter. Thus:

– sli.quantity became quantity (attribute modification).

– Associations Formed and Broken• After an enterItem (itemID and quantity) operation, what associations

between new or existing objects should have been formed or broken? The new SalesLineItem should have been related to its Sale, and related to its ProductDescription. Thus:

– sli was associated with the current Sale (association formed).– sli was associated with a ProductDescription, based on itemID match

(association formed).

74

Guideline: Should We Update Guideline: Should We Update the Domain Modelthe Domain Model

• Guideline: Should We Update the Domain Model?– It's common during the creation of the contracts to

discover the need to record new conceptual classes, attributes, or associations in the domain model.

– Enhance it as you make new discoveries while thinking through operation contracts.

– In iterative and evolutionary methods (and reflecting the reality of software projects), all analysis and design artifacts are considered partial and imperfect, and evolve in response to new discoveries

38

75

Guideline: When Are Guideline: When Are Contracts UsefulContracts Useful

• There are situations where the details and complexity of required state changes are awkward or too detailedto capture in use cases– Consider an airline reservation system

• The system operation addnewreservation: the complexity is very high regarding all the domain objects that must be changed, created, and associated.

• These fine-grained details can be written up in the use case, but it will make it extremely detailed (e.g., Noting each attribute in all the objects that must change).

– The postcondition offers and encourages a very precise, analytical language that supports detailed thoroughness

• If developers can comfortably understand what to do without them, then avoid writing contracts.

76

Guideline: How to Create and Guideline: How to Create and Write Contracts Write Contracts 11

• How to Create and write contracts? Identify system operations from the SSDs. For system operations that are complex and

perhaps subtle in their results, or which are not clear in the use case, construct a contract.

To describe the postconditions, use the following categories:• Instance creation and deletion• Attribute modification• Associations formed and broken

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77

Guideline: How to Create and Guideline: How to Create and Write Contracts Write Contracts 22

• Writing Contracts– Write the postconditions in a declarative, passive past form

(was ..) to emphasize the observation of a change rather than how it is going to be achieved.

• (better) A SalesLineItem was created.• (worse) Create a SalesLineItem.

– To establish an association between existing objects or those newly created.

• After the enterItem operation is complete, the newly created instance was associated with Sale; thus:

– The SalesLineItem was associated with the Sale.

• The most common problem– Forget to include the forming of associations. Particularly when

new instances are created.

78

Example: NextGen POS Example: NextGen POS Contracts Contracts 11

System Operations of the Process Sale Use Case• Contract CO1: makeNewSale

– Operation: makeNewSale()– Cross References: Use Cases: Process Sale– Preconditions: none– Postconditions:

• A Sale instance s was created (instance creation).• s was associated with a Register (association formed).• Attributes of s were initialized.

• Keep it as light as possible, and avoid all artifacts unless they really add value.

• Contract CO2: enterItem– Operation: enterItem(itemID: ItemID, quantity: integer)– Cross References: Use Cases: Process Sale– Preconditions: There is a sale underway.– Postconditions:

• A SalesLineItem instance sli was created (instance creation).• sli was associated with the current Sale (association formed).• sli.quantity became quantity (attribute modification).• sli was associated with a ProductDescription, based on itemID match (association

formed).

There is no sale underway

See domain model (next next page)

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79

Example: NextGen POS Example: NextGen POS Contracts Contracts 22

• Contract CO3: endSale– Operation: endSale()– Cross References: Use Cases: Process Sale– Preconditions: There is a sale underway.– Postconditions:

• Sale.isComplete became true (attribute modification).

• Contract CO4: makePayment– Operation: makePayment( amount: Money )– Cross References: Use Cases: Process Sale– Preconditions: There is a sale underway.– Postconditions:

• A Payment instance p was created (instance creation).• p.amountTendered became amount (attribute modification).• p was associated with the current Sale (association formed).• The current Sale was associated with the Store (association formed); (to

add it to the historical log of completed sales)

See domain model (next page)

Add isCompleteattribute

80


Register

id

ItemStore

nameaddress

Sale

dateTime/ total

CashPayment

amountTendered

SalesLineItem

quantity

Cashier

id

Customer

ProductCatalog

ProductDescription


Stocks

*

Houses

1..*

Used-by

*

Contains1..*

Describes

*

Captured-on

Contained-in

1..*

Records-sale-of

0..1

Paid-by Is-for

Logs-completed

*

Works-on

11

1

1 1..*

1

1

1

1

1

1

1

0..1 1

1

Ledger

Records-accounts-

for

1

1

makeNewSale

makePayment

41

81

Applying UML: Operations, Applying UML: Operations, Contracts, and the OCL Contracts, and the OCL

• The UML formally defines operations:– An operation is a specification of a transformation or query that an

object may be called to execute– Operation is an abstraction, not an implementation.

• By contrast, a method is an implementation of an operation• In UML, an operation has a signature (name and parameters) and

– is associated with a set of UML Constraint objects classified as preconditions and postconditions that specify the semantics of the operation

• Object Constraint Language (OCL) – A rigorous language that can be used to express constraints of UML

operartions.– Example:

System::makeNewSale( ) pre : post : …

• Guideline: Keep things simple; use natural language instead of OCL.

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Process: Process: Operation Contracts Operation Contracts Within the UPWithin the UP

• In the UML, operations exists at many levels, from System down to fine-grained classes, such as Sale.– Operation contracts for the System level are part of the Use-Case

Model.

• Inception phase– Contracts are not motivated during inception, they are too

detailed.

• Elaboration phase– If used at all, most contracts will be written during elaboration,

when most use cases are written. Only write contracts for the most complex and subtle system operations.

inc . e labora tion construction transition

ite ration phase

developm ent cycle

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