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InterprocessCommunicationsMei-LingL.Liu1DistributedComputing,M.L.LiuInterprocessCommunicationsOperatingsystemsprovidefacilitiesforinterprocesscommunications(IPC),suchasmessagequeues,semaphores,andsharedmemory.DistributedcomputingsystemsmakeuseofthesefacilitiestoprovideapplicationprogramminginterfacewhichallowsIPCtobeprogrammedatahigherlevelofabstraction.Distributedcomputingrequiresinformationtobeexchangedamongindependentprocesses.2DistributedComputing,M.L.LiuIPC–unicastandmulticastIndistributedcomputing,twoormoreprocessesengageinIPCinaprotocolagreeduponbytheprocesses.Aprocessmaybeasenderatsomepointsduringaprotocol,areceiveratotherpoints.Whencommunicationisfromoneprocesstoasingleotherprocess,theIPCissaidtobea

unicast.Whencommunicationisfromoneprocesstoagroupofprocesses,theIPCissaidtobeamulticast,atopicthatwewillexploreinalaterchapter.

3DistributedComputing,M.L.LiuUnicastvs.Multicast4DistributedComputing,M.L.LiuInterprocessCommunicationsinDistributedComputing5DistributedComputing,M.L.LiuOperationsprovidedinanarchetypalInterprocessCommunicationsAPIReceive([sender],messagestorageobject)Connect(senderaddress,receiveraddress),forconnection-orientedcommunication.Send([receiver],message)Disconnect(connectionidentifier),forconnection-orientedcommunication.

6DistributedComputing,M.L.LiuInterprocessCommunicationinbasicHTTP

7DistributedComputing,M.L.LiuEventSynchronizationInterprocesscommunicationrequiresthatthetwoprocessessynchronizetheiroperations:onesidesends,thentheotherreceivesuntilalldatahasbeensentandreceived.Ideally,thesendoperationstartsbeforethereceiveoperationcommences.Inpractice,thesynchronizationrequiressystemsupport.8DistributedComputing,M.L.LiuSynchronousvs.AsynchronousCommunicationTheIPCoperationsmayprovidethesynchronizationnecessaryusingblocking.Ablockingoperationissuedbyaprocesswillblockfurtherprocessingoftheprocessuntiltheoperationisfulfilled.Alternatively,IPCoperationsmaybeasynchronousornonblocking.Anasynchronousoperationissuedbyaprocesswillnotblockfurtherprocessingoftheprocess.Instead,theprocessisfreetoproceedwithitsprocessing,andmayoptionallybenotifiedbythesystemwhentheoperationisfulfilled.9DistributedComputing,M.L.LiuSynchronoussendandreceive10DistributedComputing,M.L.LiuAsynchronoussendandsynchronousreceive11DistributedComputing,M.L.LiuSynchronoussendandAsync.Receive-112DistributedComputing,M.L.LiuSynchronoussendandAsync.Receive-213DistributedComputing,M.L.LiuSynchronoussendandAsync.Receive-314DistributedComputing,M.L.LiuAsynchronoussendandAsynchronousreceive15DistributedComputing,M.L.LiuEventdiagram16DistributedComputing,M.L.LiuBlocking,deadlock,andtimeoutsBlockingoperationsissuedinthewrongsequencecancausedeadlocks.Deadlocksshouldbeavoided.Alternatively,timeoutcanbeusedtodetectdeadlocks.17DistributedComputing,M.L.LiuUsingthreadsforasynchronousIPCWhenusinganIPCprogramminginterface,itisimportanttonotewhethertheoperationsaresynchronousorasynchronous.Ifonlyblockingoperationisprovidedforsendand/orreceive,thenitistheprogrammer’sresponsibilitytousingchildprocessesorthreadsifasynchronousoperationsaredesired.18DistributedComputing,M.L.LiuDeadlocksandTimeouts

ConnectandreceiveoperationscanresultinindefiniteblockingForexample,ablockingconnectrequestcanresultintherequestingprocesstobesuspendedindefinitelyiftheconnectionisunfulfilledorcannotbefulfilled,perhapsasaresultofabreakdowninthenetwork.Itisgenerallyunacceptableforarequestingprocessto“hang”indefinitely.Indefiniteblockingcanbeavoidedbyusingtimeout.Indefiniteblockingmayalsobecausedbyadeadlock

19DistributedComputing,M.L.LiuIndefiniteblockingduetoadeadlock20DistributedComputing,M.L.LiuDataRepresentationDatatransmittedonthenetworkisabinarystream.Aninterprocesscommunicationsystemmayprovidethecapabilitytoallowdatarepresentationtobeimposedontherawdata.Becausedifferentcomputersmayhavedifferentinternalstorageformatforthesamedatatype,anexternalrepresentationofdatamaybenecessary.Datamarshallingistheprocessof(I)flatterningadatastructure,and(ii)convertingthedatatoanexternalrepresentation.Somewellknownexternaldatarepresentationschemesare:SunXDRASN.1(AbstractSyntaxNotation)XML(ExtensibleMarkupLanguage)

21DistributedComputing,M.L.LiuDataEncodingProtocols22DistributedComputing,M.L.LiuSampleXMLfile

/xml/docs/tutorial/overview/1_xml.html#introXMLisatext-basedmarkuplanguagethatisfastbecomingthestandardfordatainterchangeontheWeb.XMLhassyntaxanalogustoHTML.UnlikeHTML,XMLtagstellyouwhatthedatameans,ratherthanhowtodisplayit.Example:<message>

<to>you@yourA</to>

<from>me@myA</from>

<subject>XMLIsReallyCool</subject>

<text>HowmanywaysisXMLcool?Letmecounttheways...</text>

</message>23DistributedComputing,M.L.LiuDataMarshalling24DistributedComputing,M.L.Liu

Text-basedprotocolsDatamarshallingisatitssimplestwhenthedataexchangedisastreamofcharacters,ortext.Exchangingdataintexthastheadditionaladvantagethatthedatacanbeeasilyparsedinaprogramanddisplayedforhumanperusal.Henceitisapopularpracticeforprotocolstoexchangerequestsandresponsesintheformofcharacter-strings.Suchprotocolsaresaidtobetext-based.Manypopularnetworkprotocols,includingFTP(FileTransferProtocol),HTTP,andSMTP(SimpleMailTransferProtocol),aretext-based.25DistributedComputing,M.L.LiuEventdiagramforaprotocolsession26DistributedComputing,M.L.LiuEventDiagramforaHTTPsession27DistributedComputing,M.L.LiuSequenceDiagram28DistributedComputing,M.L.LiusequencediagramforaHTTPsession29DistributedComputing,M.L.LiuProtocolInadistributedapplication,twoprocessesperforminterprocesscommunicationinamutuallyagreeduponprotocol.Thespecificationofaprotocolshouldinclude(i)thesequenceofdataexchange,whichcanbedescribedusingatimeeventdiagram.(ii)thespecificationoftheformatofthedataexchangedateachstep.30DistributedComputing,M.L.LiuHTTP:AsampleprotocolTheHypertextTransferProtocolisaprotocolforaprocess(thebrowser)toobtainadocumentfromawebserverprocess.Itisarequest/responseprotocol:abrowsersendsarequesttoawebserverprocess,whichreplieswitharesponse.31DistributedComputing,M.L.LiuTheBasicHTTPprotocol32DistributedComputing,M.L.LiuAsampleHTTPsession33DistributedComputing,M.L.LiuIPCparadigmsandimplementations ParadigmsofIPCofdifferentlevelsofabstractionhaveevolved,withcorrespondingimplementations.34DistributedComputing,M.L.LiuSummary Interprocesscommunications(IPC)isthebackboneofdistributedcomputing.InthischapterwehavelookedattheprinciplesofIPC,includingthefollowings:Whatisinterprocesscommunication?

Unicast?Multicast?WhatisanIPCapplicationprogramminginterface(API)andwhatprimitiveoperationsdoessuchaninterfaceprovide?Eventsynchronization:HowdoprocessesengagedinIPCsynchronizeorcoordinatethesendandreceiveoperations?Wh

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