Requirements To Design--Iteratively

Chapter 12Applying UML and Patterns

Craig Larman

Introduction A transition from primarily a requirements focus

to primarily a design and implementation focus will occur in each iteration.

Actual modeling that has been explored so far is realistically done in just a few days.

Other activities such as proof-of-concept programming, planning, setting up the environment could take weeks

Objectives During Transition Motivate the transition to design activities Shift emphasis to designing a solution for

iteration as collaborating software objects Contrast the importance of object design skill

versus UML notation knowledge

On to Object Design During object design, a logical solution based on

the object-oriented paradigm is developed. Interaction diagrams illustrate how objects

collaborate to fulfill the requirements. In practice, after or in parallel with drawing

interaction diagrams, (design) class diagram can be synergistically drawn

Object Design vs UML Notation Drawing UML interaction diagrams is the result

of making decisions about the object design. The object design skills are what really matter

Fundamental object design requires knowing: Principles of responsibility assignment Design patterns

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Logical Architecture and UML Package Diagrams

Chapter 13Applying UML and Patterns

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Architectural Dimension and ViewsThe common system descriptions are: logical architecture--the system as a

conceptual organization in layers, packages, classes, interfaces and subsystems.

deployment architecture--the system as an allocation of processes to processing unit and network configurations

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Layers “A coarse-grained grouping of classes

packages or subsystems that has cohesive responsibility for a major aspect of the system.”

Higher layers call upon services of lower layers Typical layers in OO systems:

User Interface Application Logic and Domain Objects Technical Services (e.g. database interface

support or error logging objects) – typically independent and reuseable

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Architectural Patterns and Pattern Categories Architectural patterns: Relates to large-

scale design and typically applied during the early iterations

Design patterns: Relates to small and medium-scale design of objects and frameworks

Idioms: Relates to language or implementation-oriented low-level design solutions.

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Inter-Layer and Inter-Package Coupling It is informative to include a diagram in the

logical view that shows the coupling between the layers and packages.

Emphasizes the dynamics of how objects across the layers connect and communicate.

The interaction diagram focuses on the logical view and on the collaborations between the layers and package boundaries.

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Partial coupling between Packages

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Package Diagrams UML Package Diagrams are often used to

show the contents of components, which are often packages in the Java sense.

Packages, as components, can be nested inside other packages.

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Package Diagram

UI Domain

Swing Web Sales

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Designing Application Logic with Objects Possible Alternative: create one class and put all

logic in it Violates the spirit of object orientation

Better Alternative: create software objects with names drawn from the real world Assign application logic responsibilities to them.

Skill and experience required to do good job of choosing objects and assigning responsibilities.

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Domain Layer and Domain Model These are not the same thing Domain model shows the real world Domain layer shows the software architecture Domain model inspires the Domain layer, and

is the source of many of the concepts, especially class names.

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Information Systems Layers A three-tier (layer) architecture has interface,

Application logic and storage layers Separates the application logic into distinct

logical middle tier of software. The interface tier is relatively free of

application processing The middle tier communicates with the back-

end storage layer.

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Example:

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Alternative Two-tier Design In this design, the application logic is placed

within window definitions, which read and writes directly to database.

There is no middle tier that separates out the application logic.

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The Model-View Separation Principle Do not connect or couple non-UI objects

directly to UI objects Model (domain) objects should not have direct

knowledge of view (UI) objects Do not put application logic in the UI object

methods Only model (domain) classes should

encapsulate the behavior of application logic UI objects should only receive UI events,

delegate application logic requests, and initialize UI elements

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Model-View Separation Advantages Supports cohesive model definitions that

focuses on the domain process, rather than on interfaces

Allows separate development of the model and user interface layers.

Minimizes impact of requirements changes in the interface upon the domain layer

Allow new views to be easily connected to an existing domain layer, without affecting the domain layer

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Model-View Separation Advantages Allow multiple simultaneous views on the

same model object Allow execution of the model layer

independent of the user interface layer Allow easy porting of the model layer to

another user interface framework