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Software architecture design pattern
In software engineering, the mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioral pattern due to the way it can alter the program's running behavior. In object oriented programming, programs often consist of many classes. Business logic and computation are distributed among these classes. However, as more classes are added to a program, especially during maintenance and/or refactoring, the problem of communication between these classes may become more complex. This makes the program harder to read and maintain. Furthermore, it can become difficult to change the program, since any change may affect code in several other classes. With the mediator pattern, communication between objects is encapsulated within a mediator object. Objects no longer communicate directly with each other, but instead communicate through the mediator. This reduces the dependencies between communicating objects, thereby reducing coupling.

The mediator design pattern is one of the twenty three well known design patterns that describe how to solve recurring design problems to design flexible and reusable object oriented software, that is, objects that are easier to implement, change, test, and reuse.

Tight coupling between a set of interacting objects should be avoided. It should be possible to change the interaction between a set of objects independently. Defining a set of interacting objects by accessing and updating each other directly is inflexible because it tightly couples the objects to each other and makes it impossible to change the interaction independently from (without having to change) the objects. And it stops the objects from being reusable and makes them hard to test. Tightly coupled objects are hard to implement, change, test, and reuse because they refer to and know about many different objects.

Define a separate (mediator) object that encapsulates the interaction between a set of objects. Objects delegate their interaction to a mediator object instead of interacting with each other directly. The objects interact with each other indirectly through a mediator object that controls and coordinates the interaction. This makes the objects loosely coupled. They only refer to and know about their mediator object and have no explicit knowledge of each other. See also the UML class and sequence diagram below.

The essence of the mediator pattern is to "define an object that encapsulates how a set of objects interact". It promotes loose coupling by keeping objects from referring to each other explicitly, and it allows their interaction to be varied independently. Client classes can use the mediator to send messages to other clients, and can receive messages from other clients via an event on the mediator class.