Design Pattern: PROXY

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Design Pattern PROXY

• Wei-Tek Tsai
• Department of Computer Science and Engineering
• Arizona State University
• Tempe, AZ

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Definition

• A Proxy is a surrogate or a placeholder for
  another object to control access to the real
  objects.
• Sometimes the Proxy pattern is known as an
  Ambassador pattern when dealing with distributed
  objects.
• Applicability The Proxy pattern is applicable
  when there is a need for sophistication in a
  reference to an object. There is added function
  between a client and the intended subject object.

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Proxy Structure
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Proxy Pattern More Structure

• The Subject defines the common interface for the
  RealSubject and Proxy.
• The Proxy can be used any place a RealSubject is
  expected.
• The RealSubject defines the real object that the
  Proxy represents.

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Proxy Document Editor Example
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Proxy Pattern Properties

• Maintains the ability to access the real
  object, while screening requests for direct
  manipulation of the object.
• Provides an identical interface to the Subject
  so that it can be substituted for the Subject in
  any instance.
• Control access to the Subject, even managing
  it.
• Has other responsibilities defined by the type
  of Proxy
• Remote Proxies
• Virtual Proxies
• Protection Proxies
• Smart Reference Proxies

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Remote Proxies

• Are responsible for encoding requests (with its
  accompanying arguments) to forward to its
  corresponding real object, which is found in
  another address space (Remote or Distributed
  Object).
• This Proxy is an Ambassador to a distributed
  object.
• This Proxy hides the fact from the user that the
  object is distributed.

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Virtual Proxies

• Cache information about the real object, so that
  they only access the object when it is absolutely
  necessary.
• This Proxy is used to access expensive objects,
  such as objects high in storage, access time, or
  another limit resource, created on demand.
• Example ImageProxy

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Protection Proxies

• Protect access the the original object.
• This means that screen users to access the real
  object or only allow access to a limited public
  interface.
• While the real object may have a private
  interface is then a subset of the real interface.
• Example KernelProxies in the Choices Operating
  System that provide a protected access to
  operating system objects.

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Smart Reference Proxy

• A Smart Reference Proxy is a replacement for a
  bare pointer
• A Smart Reference Proxy performs additional
  actions when an object is accessed
• Count the number of references to the real object
  so that it can be automatically freed when there
  are no more references.
• Create a transient representation for a
  persistent object that is loading the object into
  memory.
• Copy on write, that is normally just increment
  the reference count when asked to copy the
  Subject, only doing the actual copy when the
  client requests an operation that modifies the
  Subject.
• Check for locking before forwarding an access
  request.

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Proxy Pattern Participants Collaborations

• Participants
• Proxy
• Subject
• Defines the common interface for ConcreteSubject
  and Proxy so that Proxy can be used anywhere a
  ConcreteSubject is expected
• ConcreteSubject
• Defines the real object that the Proxy represents
  
• Collaborations
• The Proxy forwards all requests to the
  ConcreteSubject. But before doing so, the Proxy
  performs additional actions depending on the kind
  of Proxy.

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Proxy Pattern Consequences

• A Proxy pattern introduces a level of indirection
  when accessing an object.
• A Remote Proxy can hide the fact that an object
  resides in a different address space.
• A Virtual Proxy can perform optimizations like
  creating an object on demand.
• Both Protected Proxies and Smart References allow
  additional housekeeping tasks when an object is
  accessed.

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Proxy Pattern Distinguishing Aspects

• The Proxy is a stand in for a Subject when it
  is inconvenient or undesirable to access the
  Subject directly, as it may be on a remote
  machine, has restricted access, or is persistent.
• The Proxy maintains a reference to the Subject
  object and forwards all requests to the Concrete
  or RealSubject, but will perform additional
  actions before forwarding the request.
• The Proxy provides the same interface, or a
  subset, as the original object.
• The Proxy pattern models a one to one
  relationship and this relationship is a static
  relationship.