NET Remoting with an easy example.docx

NET Remoting with an easy example.docx

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NETRemotingwithaneasyexample

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Introduction

.NETRemotingisamechanismforcommunicatingbetweenobjectswhicharenotinthesameprocess.Itisagenericsystemfordifferentapplicationstocommunicatewithoneanother..NETobjectsareexposedtoremoteprocesses,thusallowinginterprocesscommunication.Theapplicationscanbelocatedonthesamecomputer,differentcomputersonthesamenetwork,oroncomputersacrossseparatenetworks.

Microsoft.NETRemotingprovidesaframeworkthatallowsobjectstointeractwitheachotheracrossapplicationdomains.Remotingwasdesignedinsuchawaythatithidesthemostdifficultaspectslikemanagingconnections,marshalingdata,andreadingandwritingXMLandSOAP.Theframeworkprovidesanumberofservices,includingobjectactivationandobjectlifetimesupport,aswellascommunicationchannelswhichareresponsiblefortransportingmessagestoandfromremoteapplications.

RemoteObjects

Whatareremoteobjects?

Anyobjectoutsidetheapplicationdomainofthecallerapplicationshouldbeconsideredremote,wheretheobjectwillbereconstructed.Localobjectsthatcannotbeserializedcannotbepassedtoadifferentapplicationdomain,andarethereforenonremotable.

AnyobjectcanbechangedintoaremoteobjectbyderivingitfromMarshalByRefObject,orbymakingitserializableeitherbyaddingthe[Serializable]tagorbyimplementingtheISerializableinterface.Whenaclientactivatesaremoteobject,itreceivesaproxytotheremoteobject.AlloperationsonthisproxyareappropriatelyindirectedtoenabletheRemotinginfrastructuretointerceptandforwardthecallsappropriately.Incaseswheretheproxyandremoteobjectsareindifferentapplicationdomains,allmethodcallparametersonthestackareconvertedintomessagesandtransportedtotheremoteapplicationdomain,wherethemessagesareturnedbackintoastackframeandthemethodcallisinvoked.Thesameprocedureisusedforreturningresultsfromthemethodcall.

Typesof.NETRemotableObjects

Therearethreetypesofobjectsthatcanbeconfiguredtoserveas.NETremoteobjects.Youcanchoosethetypeofobjectdependingontherequirementofyourapplication.

SingleCall

SingleCallobjectsserviceoneandonlyonerequestcomingin.SingleCallobjectsareusefulinscenarioswheretheobjectsarerequiredtodoafiniteamountofwork.SingleCallobjectsareusuallynotrequiredtostorestateinformation,andtheycannotholdstateinformationbetweenmethodcalls.

SingletonObjects

Singletonobjectsarethoseobjectsthatservicemultipleclients,andhencesharedatabystoringstateinformationbetweenclientinvocations.Theyareusefulincasesinwhichdataneedstobesharedexplicitlybetweenclients,andalsoinwhichtheoverheadofcreatingandmaintainingobjectsissubstantial.

Client-ActivatedObjects（CAO）

Client-activatedobjects（CAO）areserver-sideobjectsthatareactivateduponrequestfromtheclient.Whentheclientsubmitsarequestforaserverobjectusinga"new"operator,anactivationrequestmessageissenttotheremoteapplication.Theserverthencreatesaninstanceoftherequestedclass,andreturnsanObjRefbacktotheclientapplicationthatinvokedit.AproxyisthencreatedontheclientsideusingtheObjRef.Theclient'smethodcallswillbeexecutedontheproxy.Client-activatedobjectscanstorestateinformationbetweenmethodcallsforitsspecificclient,andnotacrossdifferentclientobjects.Eachinvocationof"new"returnsaproxytoanindependentinstanceoftheservertype.

Domains

In.NET,whenanapplicationisloadedinmemory,aprocessiscreated,andwithinthisprocess,anapplicationdomainiscreated.Theapplicationisactuallyloadedintheapplicationdomain.Ifthisapplicationcommunicateswithanotherapplication,ithastouseRemotingbecausetheotherapplicationwillhaveitsowndomain,andacrossdomains,objectcannotcommunicatedirectly.Differentapplicationdomainsmayexistinsameprocess,ortheymayexistindifferentprocesses.

Contexts

The.NETruntimefurtherdividestheapplicationdomainintocontexts.Acontextguaranteesthatacommonsetofconstraintsandusagesemanticsgovernallaccesstotheobjectswithinit.Allapplicationshaveadefaultcontextinwhichobjectsareconstructed,unlessotherwiseinstructed.Acontext,likeanapplicationdomain,formsa.NETRemotingboundary.Accessrequestsmustbemarshaledacrosscontexts.

Proxies

WhenacallismadebetweenobjectsinthesameApplicationDomain,onlyanormallocalcallisrequired;however,acallacrossApplicationDomainsrequiresaremotecall.Inordertofacilitatearemotecall,aproxyisintroducedbythe.NETframeworkattheclientside.ThisproxyisaninstanceoftheTransparentProxyclass,directlyavailabletotheclienttocommunicatewiththeremoteobject.Generally,aproxyobjectisanobjectthatactsinplaceofsomeotherobject.Theproxyobjectensuresthatallcallsmadeontheproxyareforwardedtothecorrectremoteobjectinstance.In.NETRemoting,theproxymanagesthemarshalingprocessandtheothertasksrequiredtomakecross-boundarycalls.The.NETRemotinginfrastructureautomaticallyhandlesthecreationandmanagementofproxies.

RealProxyandTransparentProxy

The.NETRemotingFrameworkusestwoproxyobjectstoaccomplishitsworkofmakingaremotecallfromaclientobjecttoaremoteserverobject:

aRealProxyobjectandaTransparentProxyobject.TheRealProxyobjectdoestheworkofactuallysendingmessagestotheremoteobjectandreceivingresponsemessagesfromtheremoteobject.TheTransparentProxyinteractswiththeclient,anddoestheworkofinterceptingtheremotemethodcallmadebytheclient.

Marshaling

ObjectMarshallingspecifieshowaremoteobjectisexposedtotheclientapplication.Itistheprocessofpackaginganobjectaccessrequestinoneapplicationdomainandpassingthatrequesttoanotherdomain.The.NETRemotinginfrastructuremanagestheentiremarshalingprocess.Therearetwomethodsbywhicharemoteobjectcanbemadeavailabletoalocalclientobject:

Marshalbyvalue,andMarshalbyreference.

Marshallingobjectsbyvalue

Marshalingbyvalueisanalogoustohavingacopyoftheserverobjectattheclient.Objectsthataremarshaledbyvaluearecreatedontheremoteserver,serializedintoastream,andtransmittedtotheclientwhereanexactcopyisreconstructed.Oncecopiedtothecaller'sapplicationdomain（bythemarshalingprocess）,allmethodcallsandpropertyaccessesareexecutedentirelywithinthatdomain.

Marshallbyvaluehasseveralimplications;first,theentireremoteobjectistransmittedonthenetwork.Second,someortheentireremoteobjectmayhavenorelevanceoutsideofitslocalcontext.Forexample,theremoteobjectmayhaveaconnectiontoadatabase,orahandletoawindow,orafilehandleetc.Third,partsoftheremoteobjectmaynotbeserialiazable.Inaddition,whentheclientinvokesamethodonanMBVobject,thelocalmachinedoestheexecution,whichmeansthatthecompiledcode（remoteclass）hastobeavailabletotheclient.Becausetheobjectexistsentirelyinthecaller'sapplicationdomain,nostatechangestotheobjectarecommunicatedtotheoriginatingapplicationdomain,orfromtheoriginatorbacktothecaller.

MBVobjectsare,however,veryefficientiftheyaresmall,andprovidearepeatedfunctionthatdoesnotconsumebandwidth.Theentireobjectexistsinthecaller'sdomain,sothereisnoneedtomarshalaccessesacrossdomainboundaries.Usingmarshal-by-valueobjectscanincreaseperformanceandreducenetworktraffic,whenusedforsmallobjectsorobjectstowhichyouwillbemakingmanyaccesses.

Marshalbyvalueclassesmusteitherbemarkedwiththe[Serilaizable]attributeinordertousethedefaultserialization,ormustimplementtheISerializableinterface.

Marshallingobjectsbyreference

Marshalingbyreferenceisanalogoustohavingapointertotheobject.Marshalbyreferencepassesareferencetotheremoteobjectbacktotheclient.ThisreferenceisanObjRefclassthatcontainsalltheinformationrequiredtogeneratetheproxyobjectthatdoesthecommunicationwiththeactualremoteobject.Onthenetwork,onlyparametersandreturnvaluesarepassed.Aremotemethodinvocationrequirestheremoteobjecttocallitsmethodontheremotehost（server）.

MarshalbyreferenceclassesmustinheritfromSystem.MarshalByRefObject.

Marshallingdonebythe.NETFramework

Marshallingisdonebythe.NETFrameworkitself.Wedon�thavetowriteanycodeforit.

Channels

The.NETRemotinginfrastructureprovidesamechanismbywhichastreamofbytesissentfromonepointtotheother（clienttoserveretc.）.Thisisachievedviaachannel.Strictlyspeaking,itisaclassthatimplementstheIChannelinterface.Therearetwopre-defined.NETRemotingchannelsexistinginSystem.Runtime.Remoting.Channels,theTcpChannelandtheHttpChannel.TousetheTcpChannel,theservermustinstantiateandregistertheTcpServerChannelclass,andtheclient,theTcpClientChannelclass.

Channelselectionissubjecttothefollowingrules:

∙Atleastonechannelmustberegisteredwiththeremotingframeworkbeforearemoteobjectcanbecalled.Channelsmustberegisteredbeforeobjectsareregistered.

∙Channelsareregisteredperapplicationdomain.Therecanbemultipleapplicationdomainsinasingleprocess.Whenaprocessdies,allchannelsthatitregistersareautomaticallydestroyed.

∙Itisillegaltoregisterthesamechannelthatlistensonthesameportmorethanonce.Eventhoughchannelsareregisteredperapplicatio