Chapter 8 Structural Patterns: Proxy and Faade

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Chapter 8 -Structural Patterns Proxy and
Façade
CIS 476/566 Software Architecture and Design
Patterns

• Dr. Brahim Medjahed
• brahim_at_umich.edu

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Agenda

• Proxy Pattern
• Façade Pattern

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Design Patterns in GoF The Big Picture
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Proxy Pattern
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Proxy Pattern - Intent

• One of the most used patterns
• Provide a surrogate or placeholder for another
  object to control access to it
• Reduces the cost of accessing objects
• Uses another object (the proxy) that acts as a
  stand-in for the real object
• The proxy creates the real object only if the
  user asks for it

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Motivation

• A proxy is
• A person authorized to act for another person
• An agent or substitute
• Authority to act for another
• There are situations in which a client does not
  or can not reference an object directly, but
  wants to still interact with the object
• A proxy object can act as the intermediary
  between the client and the target object

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Client Proxy Real Object

• The proxy object has the same interface as the
  target object
• The proxy holds a reference to the target object
  and can forward requests to the target as
  required (delegation)
• In effect, the proxy object has the authority the
  act on behalf of the client to interact with the
  target object

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Scenario 1

• Document editor that embed graphical objects in a
  document
• Creating graphical objects expensive
• But, opening a document should be fast!
• Solution
• Creating expensive objects on demand (e.g., when
  an object becomes visible)
• But, we dont want to complicate the editors
  implementation
• What do we put in the document in place of the
  image?

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Scenario 1 (contd)

• Solution (contd)
• Images are stored and loaded separately from text
• If a RealImage is not loaded a ProxyImage
  displays a grey rectangle in place of the image
• ProxyImage also stores its extent, that is its
  width and height
• The client cannot tell that it is dealing with a
  ProxyImage instead of a RealImage

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Scenario 1 UML Class Diagram
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Scenario 2

• An Internet Service Provider (ISP) notices that
  many of its clients are frequently accessing the
  same web pages,
• Multiple copies of the web documents being
  transmitted through its server.
• Solution ISP's server can cache recently
  accessed pages
• When a client request arrives, the server can
  check to see if the document is already in the
  cache and then return the cached copy.
• The ISP's server accesses the target web server
  only if the requested document is not in the
  cache or is out of date.
• The cache acts as a proxy

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Proxy Pattern - Applicability

• Proxies are useful wherever there is a need for a
  more sophisticated reference to a object than a
  simple pointer or simple reference can provide

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Proxy Pattern Applicability (contd)

• Remote Proxy - Provides a reference to an object
  located in a different address space
• Proxy local representative of the real subject
  in a different address space (e.g., Stubs in RPC)
• Virtual Proxy - Allows the creation of a memory
  intensive object on demand (e.g., scenario 1).
• Protection Proxy - Provides different clients
  with different levels of access to a target
  object (see Exercise 2)

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Proxy Pattern Applicability (contd)

• Smart Reference Proxy - Provides additional
  actions whenever a target object is referenced
• Counting the number of references to the object
• Lock real subject to prevent that other client
  simultaneously change it.
• Cache Proxy - Provides temporary storage of the
  results of expensive target operations so that
  multiple clients can share the results (e.g.,
  scenario 2)

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Proxy Pattern - Structure
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Proxy Pattern Structure At Run-Time
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Participants

• Proxy
• Maintains a reference that lets the proxy access
  the real subject.
• Provides an interface identical to Subject's so
  that a proxy can by substituted for the real
  subject.
• controls access to the real subject and may be
  responsible for creating and deleting it.
• Subject
• Defines the common interface for RealSubject and
  Proxy so that a Proxy can be used anywhere a
  RealSubject is expected.
• RealSubject
• Defines the real object that the proxy
  represents.

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Example

• Code for UML class diagram in slide 15
• do-It() displays the message called
  realSubjet.do-It()

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Example - Client

•   // Client test application   class
  Client      static void Main()          //
  Create proxy and request a service       Proxy p
  new Proxy()      p.do-It()       

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Example Subject and RealSubject

•   // "Subject"  abstract class Subject
        public abstract void do-It()        
  // "RealSubject"   class RealSubject
  Subject      public override void
  do-It()          Console.WriteLine("Called
  RealSubject.do-It()")      

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Example - Proxy

• // "Proxy"   class Proxy Subject      RealSu
  bject rs    public override void
  do-It()          if (rs null)            
    rs new RealSubject()            rs.do-It(
  )        

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Exercise 1

• Implement UML Class Diagram in Slide 10

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Exercise 1 - Implementation

• To be done in class

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Exercise 2

• The RealClient class stores an account number.
  Only users who know a valid password can access
  this account number.
• The RealClient is protected by a ProtectionProxy
  which knows the password.
• If a user wants to get an account number, first
  the proxy asks the user to authenticate only if
  the user entered a correct password does the
  proxy invoke the RealClient to get the account
  number for the user.
• Questions
• UML Class Diagram
• Implementation

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Exercise 2 UML Class Diagram

• To be done in class

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Exercise 2 Implementation

• To be done in class

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Façade Pattern
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Façade Pattern - Intent

• Provides a unified interface to a set of objects
  in a subsystem.
• A facade defines a higher-level interface that
  makes the subsystem easier to use.

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Motivation

• Context
• Often, an application contains several complex
  packages.
• A programmer working with such packages has to
  manipulate many different classes
• Problem
• Programmers need to use only a subset of a
  complex system
• Or, they need to interact with the system in a
  particular way
• Example record all calls to specific methods
• Solution
• Façade presents a new interface for the clients
  of the existing system

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Interaction Through Façade
Client Classes
Subsystem Classes
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Scenario
Compiler
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Façade Pattern - Structure
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Participants

• Façade   
• Knows which subsystem classes are responsible for
  a request.
• Delegates client requests to appropriate
  subsystem objects.
• Subsystem classes
• Implement subsystem functionality.
• Handle work assigned by the Façade object.
• Have no knowledge of the façade and keep no
  reference to it.

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Façade - Applicability

• You want to provide a simple interface to a
  complex subsystem
• There are many dependencies between clients and
  the implementation classes of an abstraction
• You want to layer your system

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Façade - Collaborations

• Client communicate with the subsystem by sending
  requests to Façade, which forwards them to the
  appropriate subsystem objects
• Clients that use the façade dont have to access
  its subsystem objects directly

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Example
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Exercise

• Use the Facade pattern as a MortgageApplication
  object which provides a simplified interface to a
  large subsystem of classes measuring the
  creditworthyness of an applicant.
• The façade has one method IsEligible(Customer
  cust, int amount)
• The subsystem has 4 classes
• Customer constructor
• Bank hasSufficientSavings()
• Loan hasNoBadLoans()
• Credit hasGoodCredit()
• Questions
• UML class diagram
• Implementation

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Exercise UML Class Diagram

• To be done in class

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Exercise Implementation

• To be done in class