Proxy

Proxy

Provide a substitute or placeholder for another object to control access to it.

Problem

• The access to an object should be controlled.
  • create an object only when it is needed.
  • check if clients have the needed access rights.
• Additional functionality should be provided when accessing an object.
  • marshal / unmarshal data

Solution

Define a separate Proxy object that

• can be used as substitute for another object and
• implements the same interface (Subject) as an original service object with additional functionality to control the access to this subject.

Common Structure

• Subject
  • defines the common interface for RealSubject and Proxy
• RealSubject
  • defines the real object that the proxy represents
• Proxy
  • maintains a reference to the real subject in order to access its operations.
  • provides an interface identical to Subject's so that a proxy can be substituted for the real subject.
  • controls access to the real subject and may be responsible for creating and deleting it.

Collaboration

• Proxy forwards requests to RealSubject when appropriate, depending on the kind of proxy (see the sequence diagram above).

Proxy variants

• remote proxy
  • is responsible for encoding a request and for sending the encoded request to the real subject in a different space.
• virtual proxy
  • may cache additional information about the real subject so that it can postpone creating an expensive object and accessing it (e.g. ImageProxy caches a thumbnail of a huge image)
• protection proxy
  • checks if the caller has the required permissions to perform a request.

Benefits

• Decoupling clients from the location of server components
  • Clients are not affected by migration of servers or changes in the networking infrastructure
  • Client code becomes more stable and reusable
• Enhanced efficiency and lower cost
  • load-on-demand strategy: avoid unnecessary loads from disk
  • Works even when the RealSubject is not ready
• Separation of housekeeping code from functionality
  • Manages the lifecycle of a RealSubject object (creation, deletion)

Drawbacks

• Less efficiency due to indirection: All proxies introduce an additional layer of indirection.
• Overkill via sophisticated strategies: Be careful with intricate strategies for caching or loading on demand - they do not always pay.

Example

Definition

Usage

Comparison with other patterns

• Adapter provides a different interface to the object it adapts. Proxy provides the same interface as its subject.

• Facade provides a simplified interface to a larger complex system with more subsystems. Proxy provides a interface to a single object.

• Decorator can have similar implementations as proxies, but decorators have a different purpose. A decorator adds one or more responsibilities to an object at runtime, whereas the behavior of a proxy cannot be changed at runtime.