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Broker Design Broker Design Patterns: Façade and Patterns: Façade and MediatorMediator

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ObjectivesObjectives

To present the structure, behavior, and characteristics of broker patterns

To present the Façade and Mediator design patterns

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TopicsTopics

Broker patterns The Façade pattern The Mediator pattern

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Broker Pattern Broker Pattern StructureStructure

SupplierBrokerClient

The Client must access the Broker and the Broker must access the Supplier

Most Broker patterns elaborate this basic structure

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Broker Pattern Broker Pattern BehaviorBehavior

:Broker:Client :Supplier

requestService()obtainService()

sd BrokerBehavior

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Broker Pattern Broker Pattern AdvantagesAdvantages

Simplify the Supplier—A Broker can augment the Supplier’s services.

Decompose the Supplier—A complex Supplier can offload some of its responsibilities to a Broker.

Facilitate Client/Supplier Interaction—A Broker may present a different interface, handle interaction details, etc.

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Broker Example: Broker Example: Iterator FormIterator Form

«supplier»ConcreteCollection

«broker»ConcreteIterator

* 1

iterator() : Iterator

«interface»Collection

«interface»Iterator

Client reset()isDone() : booleangetCurrent() : Objectnext()

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Broker Example: Iterator Broker Example: Iterator BehaviorBehavior

i:ConcreteIterator:Client :ConcreteCollection

reset()

e=getCurrent()

next()

done=isDone()

done=isDone()

checkStatus()

fetchValue()

checkStatus()

loop [!done]

sd IteratorAsBroker

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The Façade PatternThe Façade Pattern

The Façade pattern eases interaction between a client and a sub-system of suppliers by providing a simpler interface to the sub-system.

The broker class is a façade that provides simplified sub-system services to clients.

Analogy: a travel agent

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Façade Pattern Façade Pattern StructureStructure

Façade

Complex Subsystem

Client

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Façade ExamplesFaçade Examples

Interface to a compiler Interface to a user interface

(from the application side) Interface to a memory

management system

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When to Use a FaçadeWhen to Use a Façade Use the Façade pattern when there is

a need to provide a simplified interface to a complex sub-system.

Façades can also help decouple systems.• If the façade mediates all interaction with

a client, then the sub-system can be changed without affecting the client.

A façade may work like an adapter by providing a new interface to a sub-system (adapters are discussed later).

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The Mediator PatternThe Mediator Pattern

The Mediator pattern reduces coupling and simplifies code when several objects must negotiate a complex interaction.

Classes interact only with a mediator class rather than with each other.

Classes are coupled only to the mediator where interaction control code resides.

Mediator is like a multi-way Façade pattern.

Analogy: a meeting scheduler

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Using a MediatorUsing a Mediator

collaboratorA

mediator

collaboratorD

collaboratorC

collaboratorB1: op1() 2.1: op2()

1.3: op3()

3: op4()

2: op2()

collaboratorA

collaboratorD

collaboratorC

collaboratorB

1.2: op2()1.5: op2()1.1: op1()

1.4: op4()

1: op()

UnmediatedCollaboration

MediatedCollaboration

2.2: op3()

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Mediator Pattern Mediator Pattern StructureStructure

Mediator Collaborator

ColleagueB

ColleagueA

ColleagueC

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Mediator as a BrokerMediator as a Broker

«broker»Mediator

Collaborator

«supplier»ColleagueB

«client»ColleagueA

«supplier»ColleagueC

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Mediator BehaviorMediator Behavior

self:Mediator :ColleagueA

consult()

:ColleagueB :ColleagueC

consult()

consult()

notify()

sd requestService()

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When to Use a When to Use a MediatorMediator

Use the Mediator pattern when a complex interaction between collaborators must be encapsulated to

• Decouple collaborators,• Centralize control of an interaction, and• Simplify the collaborators.

Using a mediator may compromise performance.

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Mediators, Façades, and Mediators, Façades, and Control StylesControl Styles

The Façade and Mediator patterns provide means to make control more centralized.

The Façade and Mediator patterns should be used to move from a dispersed to a delegated style, but not from a delegated to a centralized style.

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SummarySummary Broker patterns use a Broker class to

facilitate the interaction between a Client and a Supplier.

The Façade pattern uses a broker (the façade) to provide a simplified interface to a complex sub-system.

The Mediator pattern uses a broker to encapsulate and control a complex interaction among several suppliers.