ICTA Technology Meetup 06ICTA Technology Meetup 06

Enterprise ApplicationDesign Patterns

(Part 01)

By Crishantha Nanayakkara

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AgendaEnterprise Design Patterns – An Introduction

Enterprise Design Pattern Catalogs

Gang Of Four (GOF) Patterns

What Next? ­ Enterprise Application Architecture Design Patterns

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Design PatternsDesign PatternsAn IntroductionAn Introduction

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● “The gap between the best software engineering practice and the average practice is very wide – perhaps wider than the other engineering ”

– Fred Brooks (A famous author, ACM award winner)

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What is a Pattern?

This means, a This means, a Design PatternDesign Pattern gives you a solution to a gives you a solution to a common common recurringrecurring design problem design problem

However,However,

The Design Patterns are not meant to be laws or rules; The Design Patterns are not meant to be laws or rules; they are just guidelines that you can even alter to fit to they are just guidelines that you can even alter to fit to your needsyour needs

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How to create your own design pattern?

● Firstly, get the complete design pattern catalog and get familiar with them

● If you find a new pattern, then apply,

The Rule of ThreePattern can be called a pattern if it has been applied in real

world solution by at least three times

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OO vs Design Patterns

● Knowing the OO basics (abstraction, encapsulation, inheritance, polymorphism) will not make you a good designer!!!

Oh!!!What should 

I do then?

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OO vs Design Patterns

● Patterns show you how to build systems with good OO design qualities

● Patterns do not give you code. They give general solutions to design problems

● Patterns are not invented they are discovered

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Are patterns always great?● NO

– Patterns are a tool, which only be used when it is needed

– Heavy use of patterns will lead to complexity

– Design patterns can be used as a shared vocabulary for communicating the design to others irrespective of the programming language

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What are anti-patterns?● If a design pattern gives you a general

solution to a recurring problem in a particular context then what does an anti-pattern give u?– An anti-pattern tells you how to go from a

problem to a BAD solution

● Why worry about finding anti-patterns?– To prevent other developers from making the

same mistake

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(Reference: as3dp.com)

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Enterprise Design Enterprise Design Pattern CatalogsPattern Catalogs

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GOF Design Patterns

Enterprise Application ArchitectureDesign Patterns 

Enterprise Integration Design Patterns 

SOA Patterns

Cloud Patterns

Security Patterns

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What are Pattern Catalogs?The much formal way of specifying pattern descriptionsThe much formal way of specifying pattern descriptions

The first and the The first and the most definite most definite Fundamental Design Pattern Design Pattern Catalog is Catalog is GOFGOF

(There are 23 (There are 23 fundamental fundamental patterns in it!!)patterns in it!!)

Design Patterns: Elements of Reusable Object-Oriented Software (Addison-Wesley, 1995)

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Gang of Four (GOF) Design Patterns Catalog

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Enterprise Application Architecture Patterns Catalog

(http://martinfowler.com/eaaCatalog)

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Enterprise Intergration Patterns Catalog

http://www.eaipatterns.com/

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GOF Design PatternsGOF Design PatternsAn IntroductionAn Introduction

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Behavioural PatternsBehavioural Patterns

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GOF Design Pattern Organization

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GOF Design Pattern Relationships

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Strategy Pattern

● Design Principle 1

Identify the aspects of your application that vary and separate them from what stays the same

● Design Principle 2

Program to an interface not to an implementation

● Design Principle 3

Favor composition over inheritance

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Strategy Pattern

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Strategy Pattern

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Strategy Pattern

Reference: Head First Design Patterns

28Reference: Head First Design Patterns

29Reference: Head First Design Patterns

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The Observer Pattern

Subject

Observer 1

Observer 2

Observer 3

Unsubscribed

Subscribers

Publisher

●When the in Subject changes, subscribers are notified●The observers are subscribed to the Subject to receive updates when the Subject data changes

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ConcreteCommand

Objects

Invoker Receiver

Command Pattern

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Creational PatternsCreational Patterns

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Singleton Pattern

However, the synchronization is bit expensive

This overcomes the multiple threading issue.

This way is good if the performance is not an issue

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The Simple Factory

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The Simple Factory

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Simple Factory

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The Factory MethodThe factory method pattern encapsulates the object

creation by letting subclasses to decide what objects to create

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The Factory Method

Factory objects are createdthrough INHERITANCE

This is the “Factory Method”

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Dependency Inversion Principle

● Before Factory Method ● After Factory Method

This is all about reducing dependencies to concrete classes in our code

Abstract Class

Concrete Classes

Though this principle is quite similar to “Program to an interface” principle, this makes a bigger statement

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Structural PatternsStructural Patterns

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Decorator Pattern

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Decorator Pattern

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Cannot Connect

Can Connect

Decorator Pattern

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Decorator Pattern

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“Adapter is used when you have an abstract interface, and you want to map that interface to another object, which has similar functional role, but a different interface”

Basically it is used to Convert the interface of an object into something else

Adapter vs Decorator Pattern

● Adapter Pattern

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“Decorator is used to extend the functionality of an object while maintaining its own interface”

● Decorator Pattern

Since both “Decorator” and “Adapter” patterns are used to “wrap” objects, they are basically known as “Wrapper Patterns”

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Facade Pattern

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Facade Pattern● Facades don’t encapsulate the sub system

classes

● They provide a simplified interface to their functionality

● The sub system classes still remain for direct access to outside clients

● One sub system can have any number of facades linked to it

● Decouples the client from a sub system – If you change any of the sub systems, your client code does not need to change.

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Facade Pattern

● Design Principle

The principle of least knowledge

Talk only to your immediate friends

This prevents us from creating a large number of classes coupled together. Having too many dependencies creates a fragile design

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● There are different kinds of proxy types

● The above is called the “Remote Proxy”

● The “Remote Proxy” acts as a local representative for an remote object. A method call on an “Remote Proxy” results the call being transferred on the wire and the result is returned back to the proxy and to the client

Proxy Pattern Remote Proxy

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● The “client object” acts like its making remote calls. ● The “proxy object” pretends to be like the “remote object”● Actually what happens is, the “client object” calls methods in the “proxy object” (in its local heap), which handles all low level details of communication

Proxyobject

Clientobject

RemoteobjectRemote call

Local heap Remote heap

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Composites

Leaf Nodes

Composite Pattern

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MenuComponent

MenuItem

Menu

MenuItems

Composite Pattern

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All components must implement the MenuComponent interface;

Why? In order for the composite to work transparently to the client, you must implement the same interface for all objects in the composite

Sometimes, the best possible way is to throw Run-time Exceptions to all unrelated methods

Composite Pattern

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The MenuComponent

Both Menu andMenuItem components should implement this interface

Having default implementation will be difficult for each method

Hence, exceptions are thrown if any invalid method is called through a component

MenuItem

Menu

Com

mon

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