Design Patterns Zsolt Tóth - University of Miskolctothzs/edu/design...Principles & Laws SOLID SOLID...

PrinciplesDesign Patterns

Zsolt Tóth

University of Miskolc

2017

Zsolt Toth (UM) Principles 2017 1 / 31

Introduction

Principlesemphasize the key concepts.can answer the question "Why ...?"always have to be born in mind.affect on everything.

Architecturesallow the modeling of software systems.facilitate the design of big systems.

Architectural Patternsgive a general solution for frequent problems.


Principles & Laws

Outline

1 Principles & LawsOOPSOLIDDesign Laws

2 Software ArchitecturesCentralized Systems

Client - Server Modeln-Tier Architecture

Distributed SystemsService Oriented ArchitectureMicro services

3 Architectural Patterns


Principles & Laws OOP

Basics of Object Oriented Programming

ClassObjectInheritancePolymorphismEncapsulationInformation HidingMessage

Static Type vs Dynamic TypeLate BindingAbstract class vs InterfaceSpecialization vsEncapsulationScopesInstances


Principles & Laws SOLID

SOLID

Basic Principles of ObjectOriented DesignApplied TogetherAffect on Design PatternsLanguage IndependentRobert C. MartinEarly 2000

Single fesponsibility PrincipleOpen-Close PrincipleLiskov Substitution PrincipleInterface Segregation PrincipleDependency Inversion Principle



Single Responsibility Principle

"A class should have only one reason to change."

Consequences+ Simple classes and methods+ Easy-to-understand+ Improve Code Quality+ Facilitate testing- More classes- Difficult to stick to it.

While a Button has state, text, iconand callback function,

it is rendered by the currentGUI Engine.the event is forwarded to itscallback function, when theButton is clicked.



Open-Close Principle

"Software entities (classes, modules, functions, etc.) shouldbe open for extension, but closed for modification."

ConsequencesDo not override!Polymorphism

+ Abstraction+ Stable behavior+ Better interface design- No overriding

abstract class Template{

abstract void method1();abstract void method2();

final voidtemplateMethod(){

method1();method2();}

}



Liskov Substitution Principle

"A variable v should be substituted by any object whosedynamic type is a subtype of the static type of v withoutchanging its desired properties."

Static types areas abstract as possible.as concrete as necessary.

+ Re-usability+ Abstractness- Well-designed class hierarchy

required

CollectionsListSetMapQueueStack

Differences?



Interface Segregation Principle

"Provide as small and specific interfaces to the client as theyare needed."

+ Abstractness+ Loose coupling+ Smaller interfaces- More interfaces

Service definitionComponent based designFacade



Dependency Inversion Principle

"Abstractions should not depend on details. Details shoulddepend on abstractions."

Use interfacesAsk dependencies, do notinstantiate them.

+ Facilitate testing+ Decoupling+ Re-usability- More interfaces to maintain- Inverts the thinking

Constructor definitionpublicprotected

MockingInterface and implementationshould be separated.Database Connectivity


Principles & Laws Design Laws

Divide and Conquer

Whole is greater than the sum of its parts.

Politics, Warfare, ...Engineering

Interior DesignHeating systemIKEA Furniture

Mechanical PartsCogs, Pulley

CircuitsAD/DA ConverterAmplifiers

Software ComponentsSubsystemsDBMSs

AlgorithmsParallelism

More functionsCategorize - PartitioningSubsystems

More complexAnalyze - Define StepsMore simple, easierStill could be complex

RecursionReplaceable parts

MaintenanceAssembly

Additional CostAdministrationQuality Assurance



Pareto Principle

For many events, roughly 80% of the effects come from 20%of the causes.

80/20 ruleLaw of the vital fewEconomy & BusinessSoftware Engineering

Bug fixingLoad testingOptimization

Software ProjectsEasy to start

Let’s rewrite!Hard to finish

Testing, Documentation, ...

Usually unfinished"It satisfies industrial needs."

Full of bugs.Unprofessional work.Poor design.But someone pays for it.



KISS

Keep It Simple, Stupid!

U. S. Navy1960sSimplicityOccam’s Razor

In Software EngineeringLimit of class / method sizeDesign specific toolsUnix commandsCombine them



Don’t Repeat Yourself

Eliminate redundancyRe-usabilityAbstractionRefactoringctrl+c, ctrl+v

ExceptionsPerformance Issues

Database ReplicationsSecurity Issues

RAID



Gartner Hype Cycle - Amara’s Law



Brook’s Law

Adding manpower to a late software project makes it later"

Project ManagementTakes time to becomeproductiveCommunication overheadO(n2)

The mythical man-month

AgileTest Driven DevelopmentScrum

Sprint 2-4 weeksTeam ≈8 member


Software Architectures

Outline









Abstract structure of SoftwareSystemsIndependent of

PlatformProgramming Language

Depend onProject Goal

Purpose of the SystemScalability

Target Users1, 100, 10.000, ...Expected usage?Expected load?


Software Architectures Centralized Systems

Monolithic Architecture

Only one processInstalled on a single computer

MainframesDesktop Applications

Unix commandsls, ln, ps, mkdir,grep, chmod, chown

Computer Games, WordprocessorsOff-line workInstallationComplex computationsOur first programs

+ Simple, easy-to-understandthe Architecture

+ Independent from otherapplications

+ Self-contained- Unmaintainable Application- No Modularity



Component Based Design

Independent developmentunitA part of the systemProvide service via itsinterfaceUse other components

TestsUnitComponentIntegration

DependenciesBuild processDeployment



Client - Server Model

Simple ModelClients request servicesServer waits for and servesrequests

Widely usedFTP, SSHWWW, SMTP

Web ApplicationsInformation SystemsSearch EnginesSocial MediaWeb Shopse–Governmente–BankingMonitoring Systems



n-Tier ArchitectureDetailed than Client–ServerModelTiers are not LayersTiers have specific functions,purposeTypical tiers

PresentationClient–sideWeb sitesMobile / Desktopapplications

BusinessServer–sideBusiness logic

DatabaseServer–sideNot available directly.


Software Architectures Distributed Systems

Service Oriented Architecture

Collection of ServicesService - Servicecommunication

Simple data passingCoordinating some activity

SolutionsCORBADCOMWeb Services

A servicerepresents an activity.is self-contained.is black box.may use other services.


Software Architectures Distributed Systems

Micro services

Micro service is a process.Implementation of SOA.Popular since 2014.Characteristics

Fine-grainedCloud applicationsContinuous delivery

Reusability of services.

EMailServiceEmail notifications arerequired in many businessactivities.Each email has the samestructure.

sender, receiver addressessubjectcontent (generated by otherservices)


Architectural Patterns

Outline








Model View Controller

Separation of value andappearanceDesktop applicationsWeb applicationsVariants

Model View PresenterModel View ViewModel

ModelDomain objectsState

ViewGraphical appearanceVisualization

ControllerConnection between modeland viewRefresh the ViewUser interactionsEvent handlers



Data Access Object

Abstract interface for persistence storageSeparation of persistence and businesslogicStorage IndependentFacilitates usage of different DBMSsDefines expected behaviorMultiple specific implementations

Typical methodsCreateReadUpdateDelete



Data Transfer Object

Messaging between systems.Separates

internal domain objectsexternal environment

Hides sensitive dataReduce network trafficRequest - Response ObjectsMarshalling

{"coordinate": {"x": 0, "y": 0, "z": 0},

5 "zone": {"id":

"00000000-0000-0000","name": "Unknown"},"uuid":

"2ee1f927-50af-45c9"10 }



Service Locator

Dependency InjectionEliminate dependency onimplementationCentral Registry of Services

CentralizedSingle "vague" dependencyEasy-to-understandDifficult testing

Loosely coupled ServicesRun-time linkerSingle Service LocatorRequest ServicesSimple, Reusable Services.



Interceptor

Modify a service requestTransparentUsage

ConversionFiltering

ExamplesWeb Server CGIJava EE, FiltersSpring Security



Publish Subscribe

Messaging PatternMessage Queue

Sendercreates messages.puts them to the queue.

Receiverchecks the queue.consumes messages.

Queuesharedordered

IPC

Publish SubscribePublisher

creates message.publish it in certaintopics.

Subscribersubscribes to topics.checks the topics.

Topicsknows both publishers &subscribersforwards messages

Alerts


Design Patterns Zsolt Tóth - University of Miskolctothzs/edu/design...Principles & Laws SOLID SOLID...

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