Module 3: Logical Design and Behavioral Design Patterns

Relevant Behavior Patterns The purpose of this section is to present three behavioral design patterns that might be useful in the logical design of an application: State, Authentication,

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Instructor Notes

This module provides students with knowledge about how to use behavioral design patterns in the transition from conceptual to logical design Students will also learn how to apply this knowledge to a sample application: an Automated Teller Machine (ATM)

After completing this module, students will be able to:

! Describe behavioral design patterns

! Identify the appropriate architecture template for a given set of use cases

! Describe how an ATM application can take advantage of behavioral design patterns

Materials and Preparation

This section provides the materials and preparation tasks that you need to teach this module

Required Materials

To teach this module, you need the following materials:

! Microsoft® PowerPoint® file 1910A_03.ppt

! Module 3: Logical Design and Behavioral Design Patterns

! Lab 3: Logical Design and Behavioral Design Patterns

Preparation Tasks

To prepare for this module, you should:

! Read all of the materials for this module

! Complete the lab

Presentation:

60 Minutes

Lab:

30 Minutes

Module Strategy

Use the following strategy to present this module:

! Introduction to Behavioral Design Patterns The purpose of this section is to introduce students to behavioral design patterns and their use in the logical design It is important to emphasize which design patterns are discovered in order to simplify the task of creating

a logical design and to provide consistency Patterns are not limited to those that have been published in the design pattern literature New patterns are being added all the time For examples, refer to William Stamatakis’ book

Microsoft Visual Basic Design Patterns, Microsoft Press®, 2000

In the topic “Interaction with Users or Other Systems,” focus on the step of discovering design patterns that are associated with the facade, the business logic, and the data access layer (DAL) Emphasize that although the discovery of design patterns is an art, there is a need to formalize the patterns discovered to allow for their correct use

Note that this course is not recommending a methodology or process for creating logical and physical designs For ease of learning, the layers are presented from the front of a system to the back (user services to data services) This is not necessarily the order in which you would actually perform design work

Also, you probably would not derive the entire logical design and then the physical design, as implied by the labs in this course For learning purposes, that is the order of the labs In reality, you will probably perform iterations over the logical and physical designs

! ATM Scenario The purpose of this section is to introduce students to the logical design process by using a simple example of an ATM The purpose of this example

is to illustrate how common behavioral design patterns are identified The section allows you to present a statement of the ATM problem, present the ATM use cases, identify the architecture template for an ATM application, and review the sequence diagram of an ATM object model

! Relevant Behavior Patterns The purpose of this section is to present three behavioral design patterns that might be useful in the logical design of an application: State,

Authentication, and Queue Refer students to Appendix B, “Additional Design Patterns Used in Market Purchasing,” for a formal description of Authentication and Queue

! Best Practices Discovering common behavioral patterns in an application is an important logical design validation step It confirms that the design is based on recurrent themes

Lab Strategy

! Lab 3: Logical Design and Behavioral Design Patterns The purpose of Lab 3 is for students to learn how to apply behavioral design patterns Students will probably not derive the exact same answers as the lab answers This is acceptable as long as the student answers are justified and reflect the principles discussed in the module Discuss with students their answers to Lab 3

! Describe behavioral design patterns

! Identify the appropriate architecture template for a given set of use cases

! Describe how an ATM application can take advantage of behavioral design patterns

In this module, you will learn

about behavioral design

patterns

# Introduction to Behavioral Design Patterns

! The Problem

! Interaction with Users or Other Systems

! Maintaining Business Rules

! Data Store

Behavioral design patterns represent behavior that can be translated into common algorithms and assignments of responsibilities between objects They describe not only common activities that objects perform, but also common patterns of communication between objects

In this section, the following topics will be covered:

! The problem

! Interaction with users or other systems

! Maintaining business rules

In this section, you will learn

about behavioral design

patterns

Enterprise applications, while they might differ in their purpose, are architecturally similar, as was shown in Module 2, “Architecture and Structural Design Patterns.”

Similarly, enterprise objects that represent different entities within an enterprise have many things in common In particular, they share similar behavior The purpose of behavioral pattern design is to identify common patterns for interaction with users and other systems, for maintaining business rules, and for storing data

Discovering useful patterns and using them in the logical design of objects is an art The approach being prescribed for the logical design in object-oriented analysis and design is to first identify the architecture template, to then identify the enterprise objects that belong to each of the architecture template layers, and

to finally identify the behavioral design pattern for each of the enterprise objects

The goal for this approach is to use in a design as many common elements as possible

An excellent logical design presents the correct mix of an architectural template, objects, and object behavioral patterns

Because it is impossible to identify all of the pieces (the architecture, objects, and object behavioral patterns) simultaneously, the problem is partitioned into first identifying the architecture template, then identifying the objects, and finally identifying the behavioral design patterns applicable for each object

In this slide, you will learn

about the problem facing

application designers that

necessitates behavioral

design patterns

Interaction with Users or Other Systems

! Interaction with Users

! Interaction with Other Systems

If you think about the kind of behavior that is involved in user interaction or interaction with other systems, you will realize that there are some common behavioral design patterns

Interaction with Users

For interaction with users, you could consider the following behavioral design patterns:

! Command Command is a behavioral design pattern that represents requests as objects, thereby allowing objects to be separate from the commands they execute For example, a toolkit may provide user interface elements such as buttons and menu items However, the toolkit cannot possibly know the actions with

which a user will connect those elements A Button class will likely

perform many different functions in the same application How will it call the correct function when pressed? The Command pattern allows an appropriate concrete command class to be associated with user interface

elements at run time When the button is pressed, it calls the Command

object with which it was initialized

! Memento Memento is a behavioral design pattern that captures and externalizes an object’s internal state An example of using a Memento occurs when a user logs on, and the state of the application at the time they logged on

previously is restored This state is then exposed to all of the other objects in the application

Topic Objective

To provide a background

about interaction with users

and other systems

Lead-in

In this topic, you will learn

how to identify common

behavioral design patterns

in interaction with users and

other systems

Interaction with Other Systems

For interaction with other systems, you might consider using a Queue behavioral design pattern

Queue is a behavioral design pattern that defines a many-to-many dependency between objects so that one or more objects post messages to a message queue and all of their dependents can retrieve the messages from the message queue You might not find the Queue behavioral design pattern in the design patterns literature The Queue behavioral pattern was discovered during the

development of this course and the accompanying Market Purchasing application The Queue behavioral design pattern is described in detail in Appendix B: “Additional Design Patterns used in Market Purchasing.”

Design patterns are not limited to the patterns that are formalized in the design patterns literature You should discover and add your own patterns based on your observation of common behavior or interactions between objects in your enterprise applications Looking for common behavior is an important task when deriving design patterns Many patterns seem obvious once you discover them because similar characteristics that define a pattern recur on many systems

Maintaining Business Rules

! Maintaining Integrity

! Maintaining Consistency

The second objective of an enterprise application is to maintain the business rules that drive the enterprise Maintaining business rules includes maintaining their integrity and consistency by having uniform behavior There are some design patterns that can be used to reflect common behavior in business rules These behavioral design patterns include the following:

! Observer Observer is a behavioral design pattern that defines a one-to-many dependency between objects so that when a publisher object changes state, all of its subscriber objects are notified For example, consider a component that needs to monitor stock prices A stock ticker component could publish all stock prices as they occur Subscriber components could listen to the stock ticker The subscribers would be called when a stock price changes In

a more sophisticated example, filters could be used so that subscribers only get called when certain stock prices change or when they change above or below a certain price level

! Visitor Visitor is a behavioral design pattern that represents an operation that needs

to be performed on elements of an object structure Visitor allows new operations be defined without changing existing classes in the object structure An example of using a Visitor occurs in an order pipeline In an order pipeline, it is necessary to calculate taxes The objects that calculate taxes are Visitors to the order pipeline object structure that extend the order processing without changing its existing classes or structure

You will explore some of these patterns in more detail later in this course

Topic Objective

To provide background

about interaction with users

and other systems

Lead-in

In this topic, you will learn

how to identify common

behavioral design patterns

in interaction with users and

other systems

! Iterator Iterator is a behavioral design pattern that allows access to the elements of

an aggregate object sequentially without exposing its underlying representation An example is a requisition that consists of multiple line item details To calculate the total amount for the requisition, it is necessary

to iterate through all of the line item details

! State State is a behavioral design pattern that allows an object to alter its behavior when its internal state changes

You will explore some of these patterns in more detail later in the course

In the three preceding slides, you have seen examples of using behavioral design patterns in the three main features of an enterprise application The decision of which behavioral pattern belongs to any given feature is arbitrary In reality, you will find that any behavioral pattern can be used with any feature

Topic Objective

To provide background

about data persistence

Lead-in

In this topic, you will learn

how to identify common

behavioral design patterns

in data persistence

# ATM Scenario

! Problem Statement

! ATM Use Cases

! ATM Architecture Template

! ATM Sequence Diagram

In this section, you will be introduced to a simple example of an ATM With this example, you will learn how to identify common behavioral design patterns

In this section, the following topics will be covered:

! Problem statement

! ATM use cases

! ATM architecture template

! ATM sequence diagram

In this section, you will learn

about the ATM system, its

use cases, its architectural

template, and its sequence

diagram

Furthermore, the system must limit its liability by limiting the amount that can

be withdrawn from an account per day

Finally, the ATM system must have high availability, so it must operate whether the main bank processing system is running or not In other words, an ATM must be capable of operating independently

In this topic, you will learn

what constitutes an ATM

application