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Architectural Models

Overview System architectures Software layers Architectural models Design requirements

Example : Paperless Office Input and storage of scanned documents Viewing/printing of documents on demand Networking for resource sharing and communication Accounting and data analysis

Required properties No loss/corruption/unauthorized acces of data Fast response PostalservicesCustomer servicesLondonBirminghamScannerDataBaseServerMainframePrinterService Should grow as the business expand

3January,20021

Client Server Systems

One Tier Architecture

NetworkcomputerorPCswithterminalemulationPresentation(toclients)+processing(transactions,applications)+data(management&access)

workstations

Two Tier Architecture

Client-serverfatclientorfatserver

Presentation+processingData(remotedataaccess)orpresentationDataprocessing(remoteprocedurecall)

sharedapplicationservices

shareddataservices

client

presentation

processing

data

remotedataaccess,procedurecall

SoftwareLayers

Open (distributed)servicesMiddlewareOperating systemPlatformComlocalputerand network hardwarelanguageandrun-timesupportforprograminteractionextendedservicesavailable tothoseofthe distributedsystemapplicationsconventionalanddistributedapplicationsresponsibleforbasicresource management(memory allocation/protection,process creation andscheduling,localinterprocesscommunicationandperipheraldevices handling)

Softwarelayers Servicelayers Higher-levelaccessservicesatlowerlayers Services can be located on differentcomputers Processtypes: Serverprocesses Clientprocesses peerprocesses

Platform

Importantlayers

lowest-levelhardware+software commonprogramminginterface,yet differentimplementationsofoperatingsystemfacilitiesforco-ordination&communication Middleware programmingsupportfordistributedcomputing

Middlewareprovides... supportfordistributedprocesses/objects: suitableforapplicationsprogramming communicationvia remote method invocation(Java RMI), or remote procedure call(Sun RPC) servicesinfrastructureforapplicationprograms naming,security,transactions,eventnotification,... products:CORBA,DCOMThelayeredview...

though appropriate forsimpletypesofresourcedatasharing: e.g.databasesofnames/addresses/examgrades toorestrictivefor more complex functions? reliability,security,fault-tolerance,etc,needaccesstoapplicationsdata seeend-to-endargument[Saltzer,Reed&Clarke]

Architecturalmodels Define softwarecomponents(processes,objects) waysinwhichcomponentsinteract mappingofcomponentsontotheunderlyingnetwork Whyneeded? tohandlevaryingenvironmentsandusage toguaranteeperformanceMain types of models Client-server first andmost commonly used Multipleservers to improve performanceand reliability e.g. search engines (1000s of computers) Proxyservers toreduce loadon network, provide access throughfirewall Peerprocesses when faster interactive response needed

Client server

Key:Process:Computer:ClientinvocationinvocationServer2resultServer1resultClient

Server1actsasclientforServer2

MultipleserversService

ServerClientServerClientServer

Serversmayinteract

Proxyservers

ClientWebserverProxyserverClientWebserver

intranetfirewalloutsideworld

PeerprocessesApplicationApplicationCo-ordinationcodeCo-ordinationcodeApplicationCo-ordinationcodeWhite-board(event notification)

Clientserverandmobility Mobilecode downloadedfromserver,runsonlocally e.g.webapplets Mobileagent(code + data) travelsfromcomputertoanother collectsinformation,returningtooriginBeware!Securityrisks

Webapplets

Client requestsresults, applet code isdownloaded:

ClientAppletcodeWebserver

Clientinteractswith the applet:

ClientAppletWeb serverDesignRequirementsforDSsJudging how good the architecture is... Performance howfastwillitrespond? Quality of Service arevideoframesandsoundsynchronised? Dependability doesitworkcorrectly?Performance Responsiveness fastinteractiveresponsedelayedbyremoterequests useofcaching,replication Throughput dependentonspeedofserveranddatatransfer Loadbalancing useofapplets,multipleservers Quality of Service (QoS)

Non-functionalpropertiesexperiencedbyusers: Deadlineproperties harddeadlines (must be met within Ttime units) softdeadlines (`there is a 90% chance that the videoframe will be delivered within T time units) multimediatraffic,video/soundsynchronisation dependon availabilityofsufficientresources Adaptability ability to adapt to changing system configuration

Dependability Correctness correct behaviour wrt specification e.g. use of verification Fault-tolerance ability to tolerate/recover from faults e.g. use of redundancy Security ability to withstand malicious attack e.g. use of encryption, etc

Summary Choosebetweenonetier,twotier, simple versus complex transaction processing Client-serverarchitecturemostcommon used for WWW, email, ftp, Internet services, etc butcanleadtobottlenecks multiple servers for fast response (e.g. Google searchengine based on 6,000 Linux PCs) proxy servers used to limit load (e.g. through firewall) ExpectedtomeetrequirementsofPerformance,QoSandDependability