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2

中山火厚Review

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SSocketAPI

UDP-datagramsockets

TCP-streamsockets

SClient-ServerModel

protocol/servicesession

interprocesscommunicationsandeventsynchronization

protocoldatarepresentation

Client-serverusingConnectionlessDatagramSocket

Client-serverusingconnection-orientedDatagramSocket

IterativeserversVs.Concurrentservers

Statefulserverversusstatelessserver

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1.DistributedObjects

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1.DistributedObjects

1.1MessagePassingvs.DistributedObjects

1.2TheDistributedObjectsParadigm

1.3FromRPCtoRemoteMethodInvocation

1.4JavaRemoteMethodInvocation

中山大擘AConcurrent,Connection-orientedServer

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中山火孽MessagePassingversusDistributedObjects

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BThemessage-passingparadigmisanaturalmodelfor

distributedcomputing,inthesensethatitmimicsinterhuman

communications.Itisanappropriateparadigmfornetwork

serviceswhereprocessesinteractwitheachotherthroughthe

exchangesofmessages.

QHowever,theabstractionprovidedbythisparadigmdoesnot

meettheneedsofthecomplexityofsophisticatednetwork

applications.

MessagePassingversusDistributedObjects-2

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BMessagepassingrequirestheparticipatingprocessestobe

tightly-coupled:throughouttheirinteraction,theprocesses

mustbeindirectcommunicationwitheachother.If

communicationislostbetweentheprocesses(duetofailures

inthecommunicationlink,inthesystems,orinoneofthe

processes),thecollaborationfails.

aThemessage-passingparadigmisdata-oriented.Each

messagecontainsdatamarshalledinamutuallyagreedupon

format,andisinterpretedasarequestorresponseaccording

totheprotocol.Thereceivingofeachmessagetriggersan

actioninthereceivingprocess.

E3Itisinadequateforcomplexapplicationsinvolvingalargemix

ofrequestsandresponses.Insuchanapplication,thetaskof

interpretingthemessagescanbecomeoverwhelming.

8

Thedistributedobjectparadigm

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HThedistributedobjectparadigmisaparadigmthatprovides

abstractionsbeyondthoseofthemessage-passingmodel.Asits

nameimplies,theparadigmisbasedonobjectsthatexistina

distributedsystem.

SInobject-orientedprogramming,supportedbyanobject-

orientedprogramminglanguagesuchasJava,objectsareused

torepresentanentitysignificanttoanapplication.Eachobject

encapsulates:

thestateordataoftheentity:inJava,suchdataiscontainedin

theinstancevariablesofeachobject;

theoperationsoftheentity,throughwhichthestateoftheentity

canbeaccessedorupdated.

object-orientedprogramming

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Toillustrate,considerobjectsoftheDatagramMessageclass.

Eachobjectinstantiatedfromthisclasscontainsthreestate

dataitems:amessage,thesender'saddress,andthesender's

portnumber.Inaddition,eachobjectcontainsthree

operations:

■amethodputVal,whichallowsthevaluesofthesedata

itemstobemodified,

■agetMessagemethod,whichallowsthecurrentvalueofthe

messagetoberetrieved,and

■agetAddressmethod,whichallowsthesender'saddressto

beretrieved.

LocalObjectsvs.DistributedObjects

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E2Localobjectsarethosewhosemethodscanonlybeinvoked

byalocalprocess,aprocessthatrunsonthesame

computeronwhichtheobjectexists.

HAdistributedobjectisonewhosemethodscanbeinvoked

byaremoteprocess,aprocessrunningonacomputer

connectedviaanetworktothecomputeronwhichthe

objectexists.

中山火孽TheDistributedObjectParadigm

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Inadistributedobjectparadigm,networkresourcesarerepresentedbydistributedobjects.To

requestservicefromanetworkresource,aprocessinvokesoneofitsoperationsormethods,

passingdataasparameterstothemethod.Themethodisexecutedontheremotehost,and

theresponseissentbacktotherequestingprocessasareturnvalue.

HostAHostB

objectstatedataitem

i:objectoperation

oadistributedobject

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MessagePassingversusDistributedObjects-3

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Comparedtothemessage-passingparadigm,whichisdata-

oriented,thedistributedobjectsparadigmisaction-oriented:the

focusisontheinvocationoftheoperations,whilethedata

passedtakesonasecondaryrole.Althoughlessintuitiveto

human-beings,thedistributed-objectparadigmismorenatural

toobject-orientedsoftwaredevelopment.

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1.DistributedObjects

1.1MessagePassingvs.DistributedObjects

1.2TheDistributedObjectsParadigm

1.3FromRPCtoRemoteMethodInvocation

1.4JavaRemoteMethodInvocation

AnArchetypalDistributedObjectsSystem

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*physicaldatapath

logicaldatapath

DistributedObjectSystem

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HAdistributedobjectisprovided,orexported,byaprocess,here

calledtheobjectserver.Afacility,herecalledanobjectregistry,

mustbepresentinthesystemarchitectureforthedistributed

objecttoberegistered.

HToaccessadistributedobject,aprocess-anobjectclient-looks

uptheobjectregistryforareferencetotheobjectThis

referenceisusedbytheobjectclienttomakecallstothe

methods.

Areferenceisa“handle“foranobject;itisarepresentationthrough

whichanobjectcanbelocatedinthecomputerwheretheobjectresides.

DistributedObjectSystem-2

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Logically,theobjectclientmakesacalldirectlytoaremotemethod.

Inreality,thecallishandledbyasoftwarecomponent,calledaclient

proxy,whichinteractswhichthesoftwareontheclienthostthatprovides

theruntimesupportforthedistributedobjectsystem.

Theruntimesupportisresponsiblefortheinterprocesscommunication

neededtotransmitthecalltotheremotehost,includingthemarshalling

oftheargumentdatathatneedstobetransmittedtotheremoteobject.

DistributedObjectSystem-3

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QAsimilararchitectureisrequiredontheserverside,wheretheruntime

supportforthedistributedobjectsystemhandlesthereceivingofmessages

andtheunmarshallingofdata,andforwardsthecalltoasoftwarecomponent

calledtheserverproxy.

E3Theserverproxyinterfaceswiththedistributedobjecttoinvokethemethod

calllocally,passingintheunmarshalleddataforthearguments.

HThemethodcallresultsintheperformanceofsometasksontheserverhost.

Theoutcomeoftheexecutionofthemethod,includingthemarshalleddata

forthereturnvalue,isforwardedbytheserverproxytotheclientproxy,via

theruntimesupportandnetworksupportonbothsides.

中山火孽DistributedObjectSystems/Protocols

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Thedistributedobjectparadigmhasbeenwidelyadoptedin

distributedapplications,forwhichalargenumberof

mechanismsbasedontheparadigmareavailable.Amongthe

mostwellknownofsuchmechanismsare:

〜JavaRemoteMethodInvocation(RMI),

〜theCommonObjectRequestBrokerArchitecture(CORBA)

systems,

〜theDistributedComponentObjectModel(DCOM),

〜mechanismsthatsupporttheSimpleObjectAccessProtocol

(SOAP).

Ofthese,themoststraightforwardistheJavaRMI

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1.DistributedObjects

1.1MessagePassingvs.DistributedObjects

1.2TheDistributedObjectsParadigm

1.3FromRPCtoRemoteMethodInvocation

1.4JavaRemoteMethodInvocation

20

中山火孽RemoteProcedureCalls(RPC)

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QRemoteMethodInvocationhasitsorigininaparadigmcalledRemoteProcedureCall

QIntheremoteprocedurecallmodel,aprocedurecallismadebyoneprocesstoanother,

withdatapassedasarguments.Uponreceivingacall,theactionsencodedinthe

procedureareexecuted,thecallerisnotifiedofthecompletionofthecall,andareturn

value,ifany,istransmittedfromthecalleetothecaller.

ProcessA

»executionflow

LocalProcedureCallandRemoteProcedureCall

中山火孽

hostA0

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Alocalprocedurecall

hostB

hostA

1.procionhostAmakesacall4.TheproxyonhostB

toproc2onhostB.unmarshallsthedata

2.Iheruntimesupportmapsreceivedandissuesa

thecalltoacalltotheproxycalltoproc2.

onhostA.5.Thecodeinproc2is

3.Theproxymarshallsthedataexecutedandreturns

andmakesanIPCcalltoatotheproxyonhostB.

proxyonhostB.6.Theproxymarshalls

thereturnvalueand

7.TheproxyreceivedthereturnmakesanIPCcallto

value,unmarshallsthedata,theproxyonhostA.

andforwardsthereturnvalue

toprod,whichresumesits

executionflow.

Aremoteprocedurecall

(thereturnexecutionpathisnotshown)

■22

RemoteProcedureCalls(RPC)-2

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E3Sinceitsintroductionintheearly1980s,theRemoteProcedureCallmodel

hasbeenwidelyinuseinnetworkapplications.

E3TherearetwoprevalentAPIsforthisparadigm.

theOpenNetworkComputingRemoteProcedureCall,evolved

fromtheRPCAPIoriginatedfromSunMicrosystemsintheearly

1980s.

Theotherwell-knownAPIistheOpenGroupDistributed

ComputingEnvironment(DCE)RPC.

E3BothAPIsprovideatool,rpcgen,fortransformingremoteprocedurecalls

tolocalprocedurecallstothestub.

中山火孽0

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1.DistributedObjects

1.1MessagePassingvs.DistributedObjects

1.2TheDistributedObjectsParadigm

1.3FromRPCtoRemoteMethodInvocation

1.4JavaRemoteMethodInvocation

24

中山火孽RemoteMethodInvocation

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HRemoteMethodInvocation(RMI)isanobject-oriented

implementationoftheRemoteProcedureCallmodel.It

isanAPIforJavaprogramsonly.

BUsingRMI,anobjectserverexportsaremoteobjectand

registersitwithadirectoryservice.Theobjectprovides

remotemethods,whichcanbeinvokedinclient

programs.

SSyntactically:

Aremoteobjectisdeclaredwitharemoteinterface,an

extensionoftheJavainterface.

Theremoteinterfaceisimplementedbytheobjectserver.

Anobjectclientaccessestheobjectbyinvokingtheremote

methodsassociatedwiththeobjectsusingsyntaxprovided

forremotemethodinvocations.

25

中山火孽TheJavaRMIArchitecture

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>logicaldatapath

>physicaldatapath

26

ObjectRegistry

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HTheRMIAPIallowsanumberofdirectoryservicestobe

usedforregisteringadistributedobject.

HWewilluseasimpledirectoryservicecalledtheRMI

registry,rmiregistry9whichisprovidedwiththeJava

SoftwareDevelopmentKit(SDK?.TheRMIRegistryis

aservicewhoseserver,whenactive,runsontheobject

server5shostmachine,byconventionandbydefaultonthe

TCPport1099.

OnesuchserviceistheJavaNamingandDirectoryInterface(JNDI),which

ismoregeneralthantheRMIregistry,inthesensethatitcanbeusedby

applicationsthatdonotusetheRMIAPI.

TheJavaSDKiswhatyoudownloadtoyourmachinetoobtaintheuseof

theJavaclasslibrariesandtoolssuchasthejavacompilerjavac.

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Theinteractionbetweenthestubandtheskeleton

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中山火厚TheAPIfortheJavaRMI

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HTheRemoteInterface

ElTheServer-sideSoftware

TheRemoteInterfaceImplementation

StubandSkeletonGenerations

TheObjectServer

HTheClient-sideSoftware

TheRemoteInterface

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HAJavainterfaceisaclassthatservesasatemplatefor

otherclasses:itcontainsdeclarationsorsignaturesof

methodswhoseimplementationsaretobesuppliedby

classesthatimplementstheinterface.

HAjavaremoteinterfaceisaninterfacethatinheritsfrom

theJavaRemoteclass,whichallowstheinterfacetobe

implementedusingRMIsyntax.OtherthantheRemote

extensionandtheRemoteexceptionthatmustbespecified

witheachmethodsignature,aremoteinterfacehasthe

samesyntaxasaregularorlocalJavainterface.

中山火厚Asampleremoteinterface

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//file:Somelnterface.java

//tobeimplementedbyaJavaRMIserverclass,

importjava.rmi.*

publicinterfaceSomelnterfaceextendsRemote{

//signatureoffirstremotemethod

publicStringsomeMethodl()

throwsjava.rmi.RemoteException;

//signatureofsecondremotemethod

publicintsomeMethod2(float)throws

java.rmi.RemoteException;

//signatureofotherremotemethodsmayfollow

Asampleremoteinterface-2

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HThejava.rmi.RemoteExceptionmustbelistedinthethrow

clauseofeachmethodsignature.

E3Thisexceptionisraisedwhenerrorsoccurduringthe

processingofaremotemethodcall,andtheexceptionis

requiredtobecaughtinthemethodcaller'sprogram.

E3Causesofsuchexceptionsincludeexceptionsthatmay

occurduringinterprocesscommunications,suchasaccess

failuresandconnectionfailures,aswellasproblems

uniquetoremotemethodinvocations,includingerrors

resultingfromtheobject,thestub,ortheskeletonnot

beingfound.

StubandSkeletonGenerations

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InRMI,eachdistributedobjectrequiresaproxyeachfortheobject

serverandtheobjectclient,knownsastheobjecfsskeletonandstub

respectively.Theseproxiesaregeneratedfromtheimplementationof

aremoteinterfaceusingatoolprovidedwiththeJavaSDK:theRMI

compilerrmic.

rmic<classnameoftheremoteinterfaceimplementation>

Forexample:

rmicSomelmpl

Asaresultofthecompilation,twoproxyfileswillbegenerated,each

prefixedwiththeimplementationclassname:

Somelmpl_skel.class

Somelmpl_stub.class.

Thestubfilefortheobject

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E2Thestubfilefortheobject,aswellastheremoteinterface

file,mustbesharedwitheachobjectclient-thesefileare

requiredfortheclientprogramtocompile.

E2Acopyofeachfilemaybeprovidedtotheobjectclientby

hand.Inaddition,theJavaRMIhasafeaturecalled6istub

downloading^whichallowsastubfiletobeobtainedbya

clientdynamically.

中山火孽TheObjectServer

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Theobjectserverclassisaclasswhosecodeinstantiatesand

exportsanobjectoftheremoteinterfaceimplementation.Figure10

showsatemplatefortheobjectserverclass.

importjava.rmi.*;

publicclassSomeServer{

publicstaticvoidmain(Stringargs[]){

try(

//codeforportnumbervaluetobesupplied

SomelmplexportedObj=newSomelmpl();

(RMIPortNum);

//registertheobjectunderthename''some"

registryURL="rmi://localhost:"+portNtim+"/some";

Naming.rebind(registryURL,exportedObj);

System.out.printin("SomeServerready.");

}//endtry

}//endmain

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中山火孽TheObjectServer-2

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//ThismethodstartsaRMIregistryonthelocalhost,ifit

//doesnotalreadyexistsatthespecifiedportnumber.

privatestaticvoidintRMIPortNum)

throwsRemoteException{

try(

Registryregistry=LocateRegistry.getRegistry(RMIPortNum);

registry.list();

//Theabovecallwillthrowanexception

//iftheregistrydoesnotalreadyexist

}

catch(RemoteExceptionex){

//Novalidregistryatthatport.

System.out.printin(

"RMIregistrycannotbelocatedatport"+RMIPortNum);

Registryregistry=LocateRegistry.createRegistry(RMIPortNum);

System.out.printin(

"RMIregistrycreatedatport"+RMIPortNum);

)

}//endstartRegistry

中山火孽TheObjectServer-3

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Inourobjectservertemplate,thecodeforexportinganobjectisas

follows:

//registertheobjectunderthename''some"

registryURL="rmi://localhost:"+portNum+

"/some";

Naming.rebind(registryURL,exportedObj);

TheNamingclassprovidesmethodsforstoringandobtaining

referencesfromtheregistry.Inparticular,therebindmethodallowan

objectreferencetobestoredintheregistrywithaURLintheformof

rmi://<hostname>:<portnumber>/<referencename>

Therebindmethodwilloverwriteanyreferenceintheregistrybound

withthegivenreferencename.Iftheoverwritingisnotdesirable,

thereisalsoaZ?//7(ymethod.

Thehostnameshouldbethenameoftheserver,orsimply“localhost”.

Thereferencenameisanameofyourchoice,andshouldbeuniquein

theregistry.

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中山火孽TheRMIRegistry

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HAserverexportsanobjectbyregisteringitbyasymbolicnamewithaserver

knownastheRMIregistry.

//CreateanobjectoftheInterface

Somelnterfacelobj=newSomelnterfaceC'Server1,5);

//Registertheobject;rebindwilloverwirteexisting//

registrationbysamename-bind()willnot.

Naming.rebind(t4Server1,,,obj);

QAserver,calledtheRMIRegistry,isrequiredtorunonthehostoftheserverwhich

exportsremoteobjects.

HTheRMIRegistryisaserverlocatedatport1099bydefault

QItcanbeinvokeddynamicallyintheserverclass:

importjava.rmi.registry.LocateRegistry;

LocateRegistry.createRegistry(1099);

TheRMIRegistry-2

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TAlternatively,anRMIregistrycanbeactivatedby

handusingthermiregistryutilitywhichcomes

withtheJavaSoftwareDevelopmentKit(SDK),

asfollows:

rmiregistry<portnumber>

wheretheportnumberisaTCPportnumber.If

noportnumberisspecified,portnumber1099is

assumed.

STheregistrywillruncontinuouslyuntilitisshut

down(viaCTRL-C,forexample)

39

TheObjectServer-5

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HWhenanobjectserverisexecuted,theexportingofthe

distributedobjectcausestheserverprocesstobeginto

listenandwaitforclientstoconnectandrequestthe

serviceoftheobject.

HAnRMIobjectserverisaconcurrentserver:each

requestfromanobjectclientisservicedusinga

separatethreadoftheserver.Notethatifaclient

processinvokesmultipleremotemethodcalls,these

callsw川beexecutedconcurrentlyunlessprovisionsare

madeintheclientprocesstosynchronizethecalls.

40

中山火孽TheClient-sideSoftware

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Theprogramfortheclientclassislikeanyother

Javaclass.

ThesyntaxneededforRMIinvolves

locatingtheRMIRegistryintheserverhost,

and

lookinguptheremotereferencefortheserver

object;thereferencecanthenbecasttothe

remoteinterfaceclassandtheremotemethods

invoked.

41

中山火孽TheClient-sideSoftware-2

SUNYAT-SENUNIVERSITY

importjava.rmi.*;

publicclassSomeClient{

publicstaticvoidmain(Stringargs[]){

try(

StringregistryURL=

"rmi://localhost:"+portNum+"/some";

Somelnterfaceh=

(Somelnterface)Naming.lookup(registryURL);

//invoketheremotemethod(s)

Stringmessage=h.methodi();

System.out.printin(message);

//method2canbeinvokedsimilarly

}//endtry

catch(Exceptione){

System.out.printIn("ExceptioninSomeClient:"+e);

}

}//endmain

//Definitionforothermethodsoftheclass,ifany.

}//endclass

Lookinguptheremoteobject

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ThelookupmethodoftheNamingclassisused

toretrievetheobjectreference,ifany,previously

storedintheregistrybytheobjectserver.Note

thattheretrievedreferencemustbecasttothe

remoteinterface(notitsimplementation)class.

StringregistryURL=

"rmi://localhost:n+portNum+"/some";

Somelnterfaceh=

(Somelnterface)Naming.lookup(registryURL);

43

InvokingtheRemoteMethod

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Theremoteinterfacereferencecanbeusedtoinvokeany

ofthemethodsintheremoteinterface,asinthe

example:

Stringmessage=h.methodi();

System.out.printin(message);

HNotethatthesyntaxfortheinvocationoftheremote

methodsisthesameasforlocalmethods.

HItisacommonmistaketocasttheobjectretrievedfrom

theregistrytotheinterfaceimplementationclassorthe

serverobjectclass.Insteaditshouldbecastasthe

interfaceclass.

中山火厚

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StepsforbuildinganRMIapplication

Algorithmfordevelopingtheserver-sidesoftware

中山火孽