AdapterVersusProxyVersusFacadePatternComparison

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One of the more frequent questions I get in class is "what's the difference betweem Adapter, Proxy, and Facade? They really seem the same to me".

This is mostly because the runtime relationships are awfully similar:

AdapterProxyFacade.jpg

This is a good example of the critical notion that patterns are not diagrams, or code snippets.

Proxy vs. Adapter

  • The Proxy changes the behavior of the Service, but preserves its interface.
  • The Adapter changes the interface of the Service, but preserves it behavior.
  • A Client can use the Proxy or the Service Entity in the same way.
  • A Client designed to use the Adapter would not be able to use the Service Entity without it.
  • The Proxy can be cast to the interface of the Service. The Adapter can be cast to the interface the Client expects.

Adapter vs. Facade

  • The Adapter is used to preserve existing polymorphism. In other words, the interface you are adapting to probably already exists, and is probably already committed to.
  • The Facade provides a more idealized interface, but it can be developed incrementally as new needs of the subsystem are discovered.
  • Adapters are usually small, and therefore do not raise performance concerns, typically.
  • Facades tend to be large, and therefore it may be useful to make sure they are re-entrant (so as to avoid the need for multiple instances).
  • The "Adaptee" is probably fine as it is, you're using the Adapter because you've committed to a different interface.
  • The "Facadee" is probably not fine as it is, and you are communicating this to other developers by using the term "Facade".

Proxy vs. Facade

  • Proxies are optional, Facades typically are not.
  • The purpose of the Proxy is to add behavior. The purpose of the Facade is to simplify, which may actually involve removing behavior