Façade vs. Mediator

...most sites point out that the mediator “adds functionality”...

The facade only exposes the existing functionality from a different perspective.

The mediator "adds" functionality because it combines different existing functionality to create a new one.

Take the following example:

You have a logging system. From that logging system, you can either log to a file, to a socket, or to a database.

Using the facade design pattern you would "hide" all the relationships from existing functionality behind a single "interface" the one that the facade exposes.

Client code:

 Logger logger = new Logger();
 logger.initLogger("someLogger");
 logger.debug("message");

The implementation may involve the interaction of many objects. But at the end, the functionality already exists. Probably the "debug" method is implemented as follows:

Implementation:

 class Logger { 

      private LoggerImpl internalLogger;
      private LoggerManager manager;

      public void initLogger( String loggerName ) {
          this.internalLogger = manager.getLogger( loggerName ); 
      }

      public void debug( String message ) { 
          this.internalLogger.debug( message );
      }     
 }

The functionality already exists. The facade only hides it. In this hypothetical case, the LoggerManager handles the creation of the correct logger, and the LoggerImpl is a package-private object that has the "debug" method. This way the Facade is not adding functionality it is just delegating to some existing objects.

On the other hand the mediator adds new functionality by combining different objects.

Same Client code:

 Logger logger = new Logger();
 logger.initLogger("someLogger");
 logger.debug("message");

Implementation:

 class Logger { 

      private java.io.PrintStream out;
      private java.net.Socket client;
      private java.sql.Connection dbConnection;
      private String loggerName;


      public void initLogger( String loggerName ) {
               this.loggerName = loggerName;
               if ( loggerName == "someLogger" ) { 
                    out = new PrintStream( new File("app.log"));
               } else if ( loggerName == "serverLog" ) { 
                    client = new Socket("127.0.0.1", 1234 );
               } else if( loggerName == "dblog") { 
                    dbConnection = Class.forName()... .
               }

      }

      public void debug( String message ) { 

               if ( loggerName == "someLogger" ) { 
                    out.println( message );
               } else if ( loggerName == "serverLog" ) { 
                    ObjectOutputStrewam oos = 
                           new ObjectOutputStrewam( client.getOutputStream());
                    oos.writeObject( message );
               } else if( loggerName == "dblog") { 
                    Pstmt pstmt = dbConnection.prepareStatment( LOG_SQL );
                    pstmt.setParameter(1, message );
                    pstmt.executeUpdate();
                    dbConnection.commit();
               }
      }
 }

In this code, the mediator is the one that contains the business logic to create an appropriate "channel" to log and also to make the log into that channel. The mediator is "creating" the functionality.

Of course, there are better ways to implement this using polymorphism, but the point here is to show how the mediator "adds" new functionality by combining existing functionality ( in my sample didn't show very much sorry ) but imagine the mediator, read from the database the remote host where to log, then creates a client and finally write to that client print stream the log message. This way the mediator would be "mediating" between the different objects.

Finally, the facade is a structural pattern, that is it describes the composition of the objects, while the mediator is a behavioral, that is, it describes the way the objects interact.

I hope this helps.

I'm using mediator to add log file functionality.

It works like this:

• Obj A tells the mediator it needs something done.
• The mediator sends the message to various client objects.
• Obj B does the thing Obj A needs, and sends an appropriate message back via the mediator.
• Meanwhile, Obj C is also sent both messages by the mediator, and logs the results. That way, we can get user statistics from the log files.
• Obj D could be an error checker as well, so that if Obj B responds that Obj A's request is impossible, Obj D could be the thing that reports that to the user. Errors can now be logged in a different file than regular activity, and could use some other means to behave (beeping, whatever) that Obj A shouldn't really concern itself with.

under related patterns, gof says: Facade (185) differs from Mediator in that it abstracts a subsystem of objects to provide a more convenient interface. Its protocol is unidirectional; that is, Facade objects make requests of the subsystem classes but not vice versa. In contrast, Mediator enables cooperative behavior that colleague objects don't or can't provide, and the protocol is multidirectional.