Session 3:
Design Patterns
Analysis and Specification of Information Systems
Winter 2008
Eran Toch
http://www.technion.ac.il/~erant
Specification and Analysis of Information Systems Spring 2005 1

Agenda
• What are Design Patterns?
• Façade Pattern
• 3-Tier Pattern
• Composite Pattern
• Go Back to a Safe Place Pattern
• Summary
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 2

Patterns in Architecture
• Does this room makes
you feel happy?
• Why?
– Light (direction)
– Proportions
– Symmetry
– And more...
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 3

What is a Design Pattern?
A description of a recurrent problem
and of the core of possible solutions
In short, a
reference
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 4

Why do we need Patterns?
• Reusing design knowledge
– Problems are not always unique. Reusing existing
experience might be useful.
– Patterns give us hints to “where to look for problems”.
• Establish common terminology
– Easier to say, "We need a Façade here“.
• Provide a higher level of abstraction
– Frees us from dealing with the details too early
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 5

History of Design Patterns
Christopher Alexander
The Timeless Way of Building Architecture 1970’
A Pattern Language: Towns, Buildings, Construction
Gang of Four (GoF) Object Oriented
Design Patterns: Elements of 1995’
Reusable Object-Oriented Software Software Design
Other Areas:
Many Authors HCI, Organizational Behavior, 2007’
Education, Concurent Programming…
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 6

Structure of a design pattern*
• Pattern Name and Classification
• Intent
– a Short statement about what the pattern does
• Motivation
– A scenario that illustrates where the pattern would be useful
• Applicability
– Situations where the pattern can be used
*According to GoF
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 7

Structure – cont’d
• Structure
– A graphical representation of the pattern
• Participants
– The classes and objects participating in the pattern
• Collaborations
– How to do the participants interact to carry out their
responsibilities?
• Consequences
– What are the pros and cons of using the pattern?
• Implementation
– Hints and techniques for implementing the pattern
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 8

Classification of GoF Patterns
Taken from Vince Huston’s site about Design Patterns
http://home.earthlink.net/~huston2/dp/patterns.html
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 9

Agenda
• What are Design Patterns?
• Façade Pattern
• 3-Tier Pattern
• Composite Pattern
• Go Back to a Safe Place Pattern
• Summary
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 10

Façade – an Example
• A robot has four classes:
– Camera (that can identify objects)
– Arm (that can move)
– Pillars (that can grab)
– Operator – managing other classes
Subsystem: mechanics
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 11

Problems
1. How much does the operator needs to know regarding the
mechanics?
2. For instance, we want to identify an object and move it to
some predefined location. The code would look like:
oldLocation = Camera.identify(object);
Arm.move(oldLocation);
Pliers.close();
Arm.move(newLocation);
Pliers.open();
• Problem: No encapsulation
– Operator needs to know a lot: structure + behavior
– Preconditions
– Agility
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 12

The Façade Pattern
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 13

Façade - Definition
• Intent
– Provide a unified interface to a set of interfaces in a
subsystem.
– Façade defines a higher-level interface that makes the
subsystem easier to use.
• Applicability
– Use the Façade pattern
• to provide a simple, default view of a complex subsystem.
• to layer your subsystems. Use a façade as the entry point to
each layer.
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 14

Façade Example
Operators interacts
only with the
Façade class
The Façade Class
“knows” and
controls the
subsystem objects
Example
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 15

Façade – the System Level
• Encapsulation at higher
levels:
– General patterns for
constructing subsystems.
– Each subsystem is
represented by a façade
interface
– Inner details are
encapsulated
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 16

Façade - Consequences
• Good:
– Shields clients from subsystem components
– It promotes weak coupling between the subsystem and its
clients.
– Façades help layer a system
• Bad:
– Duplicated methods
– Lower visibility of subsystem functionality
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 17

More Structural Patterns
• Adapter - Match interfaces of different classes
• Bridge - Separates an object’s interface from its
implementation
• Decorator - Add responsibilities to objects
dynamically
• Flyweight - A fine-grained instance used for efficient
sharing
• Proxy - An object representing another object
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 18

Agenda
• What are Design Patterns?
• Façade Pattern
• 3-Tier Pattern
• Composite Pattern
• Go Back to a Safe Place Pattern
• Summary
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 19

Buying a Computer: Multiple Channels
An e-commerce system:
–Products
–Customers
–Orders
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 20

Managing Shopping Systems
• Actions are
specified twice
• Adding new actions
is difficult
• Manageability
– Different teams
working on
store/web etc...
Example
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 21

3-Tier Pattern
• Intent
– Separating application view, business logic and data
• Structural Pattern
– Can be expressed using specialized sterotypes in class
diagram
• Origin
– By Ivar Jacobson (not GoF)
– Based on the MVC (Model-View-Controller) pattern
See: Rumbaugh et al, Object Oriented Modeling and Design, Prentice Hall, 1991.
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 22

3-Tier Structure
Represents the system’s
interface to the outside
world
Represents application
processes
Represents consistent
data
This is a regular class diagram!
Illustration
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 23

Step 1 – Boundary Tier
• In the simplest form, each actor gets an interface:
• However, it can be more complicated:
– Dividing complex interfaces
– Combining interfaces
– Generalizing interfaces
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 24

Boundary Tier Member Distribution
• Boundary tier objects are created when the session starts and are
destroyed when the session ends.
• Contains:
– Methods operated by the user.
– Attributes of the interface.
Interface
configuration Pull Push
properties messages messages
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 25

Step 2 – Control Tier
We add a control tier.
Basically, every use-case
gets a control class that
handles it’s process
Controls can be
decomposed into sub-
processes. Again,
inheritance can be used
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 26

Control Tier Member Distribution
• Control objects are created when the action (use-case) starts and are
destroyed when it ends
• Contain mainly methods, some of them double the ones in the
boundary tier objects
• May contain attributes that are used for the process’s flow management
Process state Flow management of
variables other control objects
Calling interface
objects
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 27

Step 3 – Entity Tier
We add the Entity classes.
These classes compose
most of the classes in the
regular class-diagram
mode
Entity classes have mainly
associations, rather than
dependencies, because
they are consistent
through the processes
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 28

Entity Tier Member Distribution
• Entity tier objects represent the consistent data in the system.
They are created and destroyed according to the processes
that control them
• Contain mainly attributes and some data-handling methods
Example
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 29

3 Tier Usage patterns
“l” Structure “V” Structure “Y” Structure
Used for date Used for communication Used for communication
update / retrieve + update / retrieve
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 30

3-Tier Consequences
• Good:
– Re-use of Model
components
– Flexible processes
– Easier support for new
types of views
Transaction Info Loan
executing viewing Manage • Bad:
– Increased design
complexity.
Date
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 31

Behavioral Patterns
• Chain of Responsibilities - A way of passing a
request between a chain of objects
• Command - Encapsulate a command request as an
object
• Interpreter - A way to include language elements in
a program
• Iterator - Sequentially access the elements of a
collection
• Mediator - Defines simplified communication
between classes
• Memento - Capture and restore an object's internal
state
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 32

Behavioral Patterns – cont’d
• Observer - A way of notifying change to a number of
classes
• State - Alter an object's behavior when its state
changes
• Strategy - Encapsulates an algorithm inside a class
• Template Method - Defer the exact steps of an
algorithm to a subclass
• Visitor - Defines a new operation to a class without
change
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 33

Agenda
• What are Design Patterns?
• Façade Pattern
• 3-Tier Pattern
• Composite Pattern
• Go Back to a Safe Place Pattern
• Summary
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 34

An Example
Client
Files
Folders
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 35

A Solution?
• Folder:
– For each action
(delete, display,
copy etc), there is
special treatment
for files and folders.
• Explorer:
– Each type of object
is manipulated
separately
• Scalability:
– Handling larger
Example
number of elements
(disks, CD, USB…)
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 36

The Composite Solution
Example
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 37

Composite: Structure
Illustration
• An abstract base class (Component) specifies the uniform
behavior.
• Primitive and Composite classes are subclassed.
• Composite Manages components uniformly, using add and
remove.
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 38

Composite: Consequences
• Good
– It makes it easy to add new kinds of components.
– It makes clients simpler.
• Bad
– It makes it harder to restrict the type of components of a
composite.
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 39

Composite: Known Uses
Graphical User Interface Arithmetic Expressions
A * (B + (C * D))
*
A ()
+
B *
C D
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 40

Agenda
• What are Design Patterns?
• Façade Pattern
• 3-Tier Pattern
• Composite Pattern
• Go Back to a Safe Place Pattern
• Summary
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 41

Human / Computer Interaction Patterns
• Human / Computer Interaction is about how users
interact with computers
• Some interactions are recurrent and can be
expressed by patterns:
– Form
– Control Panel
– WYSIWYG Editor
– Hierarchical Set
– Map of Navigable Spaces
– Go Back to a Safe Place
– More…
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 42

Computers can be S C A R Y
Sometimes an innocent user gets into a state
she don’t want to be in…
And then, the terror!!!
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 43

Solution
• Provide a way to go back to a checkpoint of the
user's choice.
The "Home" button
The “undo" feature
and the “Back”
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 44

Agenda
• What are Design Patterns?
• Composite Pattern
• Façade Pattern
• 3-Tier Pattern
• Go Back to a Safe Place Pattern
• Summary
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 45

Summary
 Patterns learned:
– Façade, Composite, 3-Tier, Go Back to a Safe Place.
 Advantages of Design Patterns:
– They capture expertise and make it accessible to non-
experts.
– They form a vocabulary that helps developers communicate
better.
– They help people understand a systems more quickly when it
is documented with the patterns it uses.
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 46

Relevancy to the Course
• Most of the design patterns are relevant to the design and
coding phases
• However, some of them are very relevant as a tool for the
analysis phase
Chain of resp. Visitor Flyweight
3-Tier Façade Composite Singleton Prototype
Analysis Design
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 47

References
• Design Patterns: Elements of Reusable Object-Oriented Software,
Gamma E. et el., 1995, Addison-Wesley.
• The original patterns for architecture by C. Alexander
http://www.enumerable.com/dev/leveltwo/patternsframe.htm?/dev/leveltwo
• A course from Bob Tarr from UMBC University
http://www.research.umbc.edu/~tarr/dp/fall00/cs491.html
• The Design Patterns Java Companion, James W. Cooper (an online
version of the book)
http://www.patterndepot.com/put/8/JavaPatterns.htm
• A Site dedicated to Design Patterns by Vince Huston
http://home.earthlink.net/~huston2/dp/patterns.html
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 48

References – con’d
• Seven Habits of Successful Pattern Writers, John Vlissides
http://hillside.net/patterns/papers/7habits.html
• COMMON GROUND: A Pattern Language for Human-
Computer Interface Design, Jenifer Tidwell,
http://www.mit.edu/~jtidwell/common_ground_onefile.html
• Design patterns for .NET:
http://www.dofactory.com/Patterns/Patterns.aspx
• Patterns for Concurrent, Parallel, and Distributed Systems
http://www.cs.wustl.edu/~schmidt/patterns-ace.html
• J2EE Patterns Catalog
http://java.sun.com/blueprints/patterns/catalog.html
Intro | Façade | 3-Tier | Composite | Safe Place | Summary 49

Thanks!
Specification and Analysis of Information Systems Spring 2005 50