Facade

fa·çade also fa·cade   (f-säd)
n.
  1. The face of a building, especially the principal face.
  2. An artificial or deceptive front: ideological slogans that were a façade for geopolitical power struggles.

[French, from Italian facciata, from faccia, face, from Vulgar Latin *facia, from Latin facis. See dh- in Indo-European Roots.]

Purpose
  • Provide a simple interface to a complex system. By implementing the simpler interface in terms of the backend interface[s], the facade adds value.
  • Decouple backend API from the client API. This is useful when backend API is not sufficiently stable to use as a client API.
  • Realize a layer's API. Consolidate a collection of class's APIs, hiding the class structure from the client.

An articulation point allows 1 endpoint to move (i.e., interface to change) without forcing another endpoint to move. For example, your elbow is an articulation point; it allows you to move your hand without moving your upper arm.

A facade is an adaptor or bridge when:

  • A client is controlling an object B
  • B's interface may change
  • we want to hide these changes from B's clients.
The solution is to interpose an articulation point A: a class that presents a stable interface to clients, but which translates a client's requests to control B into B's [possibly unstable] interface.

Proxy: we may want to decouple a service's client interface from the service's actual API.
Structure Facade structure
Consequences The articulation point is an extra class. If B's interface is stable & simple, then A is extra baggage.
Implementation
  • From the client's standpoint, the backend API is private. Declarations enforce this:
    • The facade and backend classes are in a separate package from the client.
    • The backend API is package-private
    • The facade API is public.
Sample Code
Related Patterns Bridge, Adapter