Façade

• Want to reduce complexities of a system.
• Decouple subsystems , reduce its dependency, and improve portability.
• Make an entry point to your subsystems.
• Minimize the communication and dependency between subsystems.
• Security and performance consideration.
• Shield clients from subsystem components.
• Simplify generosity to specification.
• Related patterns include
  • Abstract Factory, which is often used to create an interface for a subsystem in an independent way, and can be used as an alternative way to a facade.
  • Singleton, which is often used with a facade.
  • Mediator, which is similar to facade, but a facade doesn't define new functionality to the subsystem.

Example

JDBC design is a good example of Façade pattern. A database design is complicated. JDBC is used to connect the database and manipulate data without exposing details to the clients.

Security of a system may be designed with Façade pattern. Clients' authorization to access information may be classified. General users may be allowed to access general information; special guests may be allowed to access more information; administrators and executives may be allowed to access the most important information. These subsystems may be generalized by one interface. The identified users may be directed to the related subsystems.

interface General {
    public void accessGeneral();
}
interface Special extends General {
    public void accessSpecial();
}
interface Private extends General {
    public void accessPrivate();
}

class GeneralInfo implements General {
    public void accessGeneral() {
	//...
    }
    //...
}
class SpecialInfo implements Special{
    public void accessSpecial() {
        //...
    }
    public void accessGeneral() {}
    //...
}
class PrivateInfo implements Private, Special {
    public void accessPrivate() {
	// ...
    }
    public void accessSpecial() {
	//...
    }
    public void accessGeneral() {
	// ...
    }
    //...
}

class Connection {
   //...
    
   if (user is unauthorized) throw new Exception();
   if (user is general) return new GeneralInfo();
   if (user is special) return new SpecialInfo();
   if (user is executive) return new PrivateInfo();
   //...
}

The above code example illustrates that the whole system is not exposed to the clients. It depends on the user classification.

interface General {
    public void accessGeneral();
}

interface Special extends General {

    public void accessSpecial();

}

interface Private extends General {

    public void accessPrivate();

}

class GeneralInfo implements General {

    public void accessGeneral() {

	//...
    }

    //...
}

class SpecialInfo extends GeneralInfo implements Special{

    public void accessSpecial() {

        //...
    }

}

class PrivateInfo extends SpecialInfo implements Private {

    public void accessPrivate() {

	// ...

    }
   
    //...
}

To avoid repeated code, SpecialInfo become subclass of GeneralInfo and PrivateInfo becomes subclass of SpecialInfo. When a person is exposed to special information, that person is allowed to access general information also. When a person is exposed to private information, that person is allowed to access general information and special information also.

When you design a mortgage process system, you may consider the process of checking client's bank, credit and other loan information. Facade design may be a choice.

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