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DISTRIBUTEDSYSTEMS,EDITION5CHAPTER1SOLUTIONSFM1LASTUPDATED12OCTOBER2011228PMGEORGECOULOURIS,JEANDOLLIMOREANDTIMKINDBERGANDGORDONBLAIR2012DISTRIBUTEDSYSTEMSCONCEPTSANDDESIGNEDITION5BYGEORGECOULOURIS,JEANDOLLIMOREANDTIMKINDBERGANDGORDONBLAIRADDISONWESLEYPEARSONEDUCATION2012CHAPTER1EXERCISESOLUTIONS11WEDEFINEADISTRIBUTEDSYSTEMASONEINWHICHHARDWAREORSOFTWARECOMPONENTSLOCATEDATNETWORKEDCOMPUTERSCOMMUNICATEANDCOORDINATETHEIRACTIONSONLYBYPASSINGMESSAGESWHATARETHECONSEQUENCESOFDEFININGADISTRIBUTEDSYSTEMINTHISMANNER11ANSACONCURRENCYBNOGLOBALCLOCKCINDEPENDENTFAILURESCHAPTER1SOLUTIONSFMPAGE1WEDNESDAY,OCTOBER12,2011230PMDISTRIBUTEDSYSTEMS,EDITION5CHAPTER1SOLUTIONSFM2LASTUPDATED12OCTOBER2011228PMGEORGECOULOURIS,JEANDOLLIMORE,TIMKINDBERGANDGORDONBLAIR201212HOWMIGHTTHECLOCKSINTWOCOMPUTERSTHATARELINKEDBYALOCALNETWORKBESYNCHRONIZEDWITHOUTREFERENCETOANEXTERNALTIMESOURCEWHATFACTORSLIMITTHEACCURACYOFTHEPROCEDUREYOUHAVEDESCRIBEDHOWCOULDTHECLOCKSINALARGENUMBEROFCOMPUTERSCONNECTEDBYTHEINTERNETBESYNCHRONIZEDDISCUSSTHEACCURACYOFTHATPROCEDURE12ANSSEVERALTIMESYNCHRONIZATIONPROTOCOLSAREDESCRIBEDINSECTION103ONEOFTHESEISCRISTIANSPROTOCOLBRIEFLY,THEROUNDTRIPTIMETTOSENDAMESSAGEANDAREPLYBETWEENCOMPUTERAANDCOMPUTERBISMEASUREDBYREPEATEDTESTSTHENCOMPUTERASENDSITSCLOCKSETTINGTTOCOMPUTERBBSETSITSCLOCKTOTT/2THESETTINGCANBEREFINEDBYREPETITIONTHEPROCEDUREISSUBJECTTOINACCURACYBECAUSEOFCONTENTIONFORTHEUSEOFTHELOCALNETWORKFROMOTHERCOMPUTERSANDDELAYSINTHEPROCESSINGTHEMESSAGESINTHEOPERATINGSYSTEMSOFAANDBFORALOCALNETWORK,THEACCURACYISPROBABLYWITHIN1MSFORALARGENUMBEROFCOMPUTERS,ONECOMPUTERSHOULDBENOMINATEDTOACTASTHETIMESERVERANDITSHOULDCARRYOUTCRISTIANSPROTOCOLWITHALLOFTHEMTHEPROTOCOLCANBEINITIATEDBYEACHINTURNADDITIONALINACCURACIESARISEINTHEINTERNETBECAUSEMESSAGESAREDELAYEDASTHEYPASSTHROUGHSWITCHESINWIDERAREANETWORKSFORAWIDEAREANETWORKTHEACCURACYISPROBABLYWITHIN510MSTHESEANSWERSDONOTTAKEINTOACCOUNTTHENEEDFORFAULTTOLERANCESEECHAPTER10FORFURTHERDETAILS13CONSIDERTHEIMPLEMENTATIONSTRATEGIESFORMASSIVELYMULTIPLAYERONLINEGAMESASDISCUSSEDINSECTION122INPARTICULAR,WHATADVANTAGESDOYOUSEEINADOPTINGASINGLESERVERAPPROACHFORREPRESENTINGTHESTATEOFTHEMULTIPLAYERGAMEWHATPROBLEMSCANYOUIDENTIFYANDHOWMIGHTTHEYBERESOLVED13ANSTHEADVANTAGESOFHAVINGASINGLESERVERMAINTAINAREPRESENTATIONOFTHEGAMEARETHAT1THEREISASINGLECOPYANDHENCENONEEDTOMAINTAINCONSISTENCYOFMULTIPLECOPIESAND2CLIENTSHAVEASINGLEPLACETOGOTODISCOVERTHISSTATETHEREMAYALSOBEADVANTAGESTOHAVINGAGLOBALVIEWOFTHEENTIRESYSTEMSSTATETHEPOTENTIALPROBLEMSARETHATTHISSINGLESERVERMAYFAILANDMAYALSOBECOMEABOTTLENECKAFFECTINGTHEPERFORMANCEANDSCALABILITYOFTHEAPPROACHTOHANDLEFAILURE,ITWOULDBENECESSARYTOINTRODUCEREPLICATION,WHICHINTURNWOULDREQUIREASOLUTIONTOMAINTAININGCONSISTENCYACROSSREPLICASTHEREAREANUMBEROFSOLUTIONSTOIMPROVINGPERFORMANCEANDSCALABILITYINCLUDINGRUNNINGTHESERVERONACLUSTERARCHITECTUREASDESCRIBEDINSECTION122,ORAGAINUSINGREPLICATIONANDLOADBALANCINGWITHINTHEDISTRIBUTEDENVIRONMENTALTERNATIVELY,APEERTOPEERSOLUTIONCANBEADOPTEDTHESETECHNIQUESARECOVEREDTHROUGHOUTTHEBOOK14AUSERARRIVESATARAILWAYSTATIONTHATSHEHASNEVERVISITEDBEFORE,CARRYINGAPDATHATISCAPABLEOFWIRELESSNETWORKINGSUGGESTHOWTHEUSERCOULDBEPROVIDEDWITHINFORMATIONABOUTTHELOCALSERVICESANDAMENITIESATTHATSTATION,WITHOUTENTERINGTHESTATIONSNAMEORATTRIBUTESWHATTECHNICALCHALLENGESMUSTBEOVERCOME14ANSTHEUSERMUSTBEABLETOACQUIRETHEADDRESSOFLOCALLYRELEVANTINFORMATIONASAUTOMATICALLYASPOSSIBLEONEMETHODISFORTHELOCALWIRELESSNETWORKTOPROVIDETHEURLOFWEBPAGESABOUTTHELOCALITYOVERALOCALWIRELESSNETWORKFORTHISTOWORK1THEUSERMUSTRUNAPROGRAMONHERDEVICETHATLISTENSFORTHESEURLS,ANDWHICHGIVESTHEUSERSUFFICIENTCONTROLTHATSHEISNOTSWAMPEDBYUNWANTEDURLSOFTHEPLACESSHEPASSESTHROUGHAND2THEMEANSOFPROPAGATINGTHEURLEGINFRAREDORAN80211WIRELESSLANSHOULDHAVEAREACHTHATCORRESPONDSTOTHEPHYSICALSPREADOFTHEPLACEITSELF15DISTRIBUTEDSYSTEMSAREGOINGTHROUGHAPERIODOFSIGNIFICANTCHANGE,WHICHCANBETRACEDBACKTOANUMBEROFINFLUENTIALTRENDSCANYOUIDENTIFYWHATTHESEMIGHTBE15ANSATHEEMERGENCEOFPERVASIVENETWORKINGTECHNOLOGYBTHEEMERGENCEOFUBIQUITOUSCOMPUTINGCOUPLEDWITHTHEDESIRETOSUPPORTUSERMOBILITYINDISTRIBUTEDSYSTEMSCHAPTER1SOLUTIONSFMPAGE2WEDNESDAY,OCTOBER12,2011230PMDISTRIBUTEDSYSTEMS,EDITION5CHAPTER1SOLUTIONSFM3LASTUPDATED12OCTOBER2011228PMGEORGECOULOURIS,JEANDOLLIMORE,TIMKINDBERGANDGORDONBLAIR2012CTHEINCREASINGDEMANDFORMULTIMEDIASERVICESDTHEVIEWOFDISTRIBUTEDSYSTEMSASAUTILITY16DUETOTHEINCREASINGMATURITYOFDISTRIBUTEDSYSTEMSINFRASTRUCTURE,ORGANIZATIONSAREMOVINGTOWARDSVIEWINGDISTRIBUTEDSYSTEMSASAUTILITYINTHISMODEL,RESOURCESAREPROVIDEDBYAPPROPRIATESERVICESUPPLIERSANDEFFECTIVELYRENTEDRATHERTHANOWNEDBYTHEENDUSEREXPLAINTHISMODELWITHRESPECTTOPHYSICALRESOURCESANDSOFTWARESERVICESCANYOUGIVEEXAMPLESOFSOMECOMPANIESTHATSUPPORTSUCHSOFTWARESERVICES16ANSAPHYSICALRESOURCESSUCHASSTORAGEANDPROCESSINGCANBEMADEAVAILABLETONETWORKEDCOMPUTERSREMOVINGTHENEEDTOOWNSUCHRESOURCESONTHEIROWNBSOFTWARESERVICESCANALSOBEMADEAVAILABLEACROSSTHEGLOBALINTERNETUSINGTHISAPPROACHEXAMPLEOFCOMPANIESSUPPORTINGSUCHSOFTWARESERVICESAREWWWGOOGLECOM,WWWYAHOOCOM17ASERVERPROGRAMWRITTENINONELANGUAGEFOREXAMPLECPROVIDESTHEIMPLEMENTATIONOFABLOBOBJECTTHATISINTENDEDTOBEACCESSEDBYCLIENTSTHATMAYBEWRITTENINADIFFERENTLANGUAGEFOREXAMPLEJAVATHECLIENTANDSERVERCOMPUTERSMAYHAVEDIFFERENTHARDWARE,BUTALLOFTHEMAREATTACHEDTOANINTERNETDESCRIBETHEPROBLEMSDUETOEACHOFTHEFIVEASPECTSOFHETEROGENEITYTHATNEEDTOBESOLVEDTOMAKEITPOSSIBLEFORACLIENTOBJECTTOINVOKEAMETHODONTHESERVEROBJECT17ANSASTHECOMPUTERSAREATTACHEDTOANINTERNET,WECANASSUMETHATINTERNETPROTOCOLSDEALWITHDIFFERENCESINNETWORKSBUTTHECOMPUTERSMAYHAVEDIFFERENTHARDWARETHEREFOREWEHAVETODEALWITHDIFFERENCESOFREPRESENTATIONOFDATAITEMSINREQUESTANDREPLYMESSAGESFROMCLIENTSTOOBJECTSACOMMONSTANDARDWILLBEDEFINEDFOREACHTYPEOFDATAITEMTHATMUSTBETRANSMITTEDBETWEENTHEOBJECTANDITSCLIENTSTHECOMPUTERSMAYRUNDIFFERENTOPERATINGSYSTEMS,THEREFOREWENEEDTODEALWITHDIFFERENTOPERATIONSTOSENDANDRECEIVEMESSAGESORTOEXPRESSINVOCATIONSTHUSATTHEJAVA/CLEVELACOMMONOPERATIONWOULDBEUSEDWHICHWILLBETRANSLATEDTOTHEPARTICULAROPERATIONACCORDINGTOTHEOPERATINGSYSTEMITRUNSONWEHAVETWODIFFERENTPROGRAMMINGLANGUAGESCANDJAVA,THEYUSEDIFFERENTREPRESENTATIONSFORDATASTRUCTURESSUCHASSTRINGS,ARRAYS,RECORDSACOMMONSTANDARDWILLBEDEFINEDFOREACHTYPEOFDATASTRUCTURETHATMUSTBETRANSMITTEDBETWEENTHEOBJECTANDITSCLIENTSANDAWAYOFTRANSLATINGBETWEENTHATDATASTRUCTUREANDEACHOFTHELANGUAGESWEMAYHAVEDIFFERENTIMPLEMENTORS,EGONEFORCANDTHEOTHERFORJAVATHEYWILLNEEDTOAGREEONTHECOMMONSTANDARDSMENTIONEDABOVEANDTODOCUMENTTHEMCHAPTER1SOLUTIONSFMPAGE3WEDNESDAY,OCTOBER12,2011230PMDISTRIBUTEDSYSTEMS,EDITION5CHAPTER1SOLUTIONSFM4LASTUPDATED12OCTOBER2011228PMGEORGECOULOURIS,JEANDOLLIMORE,TIMKINDBERGANDGORDONBLAIR201218WHATISCLIENTSERVERCOMPUTINGWHICHOFTHESEROLESISANACTIVEROLE,ANDWHICHISAPASSIVEONEEXPLAINREMOTEINVOCATIONINTHISCONTEXT18ANSCLIENTSERVERCOMPUTINGREFERSTOARUNNINGPROGRAMAPROCESSONANETWORKEDCOMPUTERTHATACCEPTSREQUESTSFROMPROGRAMSRUNNINGONOTHERCOMPUTERSTOPERFORMASERVICEANDRESPONDSAPPROPRIATELYCLIENTSAREACTIVEMAKINGREQUESTSANDSERVERSAREPASSIVETHEINTERACTIONBETWEENACLIENTANDASERVER,FROMTHEPOINTWHENTHECLIENTSENDSITSREQUESTTOWHENITRECEIVESTHESERVERSRESPONSE,ISCALLEDREMOTEINVOCATION19SUPPOSETHATTHEOPERATIONSOFTHEBLOBOBJECTARESEPARATEDINTOTWOCATEGORIESPUBLICOPERATIONSTHATAREAVAILABLETOALLUSERSANDPROTECTEDOPERATIONSTHATAREAVAILABLEONLYTOCERTAINNAMEDUSERSSTATEALLOFTHEPROBLEMSINVOLVEDINENSURINGTHATONLYTHENAMEDUSERSCANUSEAPROTECTEDOPERATIONSUPPOSINGTHATACCESSTOAPROTECTEDOPERATIONPROVIDESINFORMATIONTHATSHOULDNOTBEREVEALEDTOALLUSERS,WHATFURTHERPROBLEMSARISE19ANSEACHREQUESTTOACCESSAPROTECTEDOPERATIONMUSTINCLUDETHEIDENTITYOFTHEUSERMAKINGTHEREQUESTTHEPROBLEMSAREDEFININGTHEIDENTITIESOFTHEUSERSUSINGTHESEIDENTITIESINTHELISTOFUSERSWHOAREALLOWEDTOACCESSTHEPROTECTEDOPERATIONSATTHEIMPLEMENTATIONOFTHEBLOBOBJECTANDINTHEREQUESTMESSAGESENSURINGTHATTHEIDENTITYSUPPLIEDCOMESFROMTHEUSERITPURPORTSTOBEANDNOTSOMEOTHERUSERPRETENDINGTOBETHATUSERPREVENTINGOTHERUSERSFROMREPLAYINGORTAMPERINGWITHTHEREQUESTMESSAGESOFLEGITIMATEUSERSFURTHERPROBLEMSTHEINFORMATIONRETURNEDASTHERESULTOFAPROTECTEDOPERATIONMUSTBEHIDDENFROMUNAUTHORISEDUSERSTHISMEANSTHATTHEMESSAGESCONTAININGTHEINFORMATIONMUSTBEENCRYPTEDINCASETHEYAREINTERCEPTEDBYUNAUTHORISEDUSERS110THEINFOSERVICEMANAGESAPOTENTIALLYVERYLARGESETOFRESOURCES,EACHOFWHICHCANBEACCESSEDBYUSERSTHROUGHOUTTHEINTERNETBYMEANSOFAKEYASTRINGNAMEDISCUSSANAPPROACHTOTHEDESIGNOFTHENAMESOFTHERESOURCESTHATACHIEVESTHEMINIMUMLOSSOFPERFORMANCEASTHENUMBEROFRESOURCESINTHESERVICEINCREASESSUGGESTHOWTHEINFOSERVICECANBEIMPLEMENTEDSOASTOAVOIDPERFORMANCEBOTTLENECKSWHENTHENUMBEROFUSERSBECOMESVERYLARGE110ANSALGORITHMSTHATUSEHIERARCHICSTRUCTURESSCALEBETTERTHANTHOSETHATUSELINEARSTRUCTURESTHEREFORETHESOLUTIONSHOULDSUGGESTAHIERARCHICNAMINGSCHEMEEGTHATEACHRESOURCEHASANNAMEOFTHEFORMABCETCWHERETHETIMETAKENISOLOGNWHERETHEREARENRESOURCESINTHESYSTEMTOALLOWFORLARGENUMBERSOFUSERS,THERESOURCESAREPARTITIONEDAMONGSTSEVERALSERVERS,EGNAMESSTARTINGWITHAATSERVER1,WITHBATSERVER2ANDSOFORTHTHERECOULDBEMORETHANONELEVELOFPARTITIONINGASINDNSTOAVOIDPERFORMANCEBOTTLENECKSTHEALGORITHMFORLOOKINGUPANAMEMUSTBEDECENTRALISEDTHATIS,THESAMESERVERMUSTNOTBEINVOLVEDINLOOKINGUPEVERYNAMEACENTRALISEDSOLUTIONWOULDUSEASINGLEROOTSERVERTHATHOLDSALOCATIONDATABASETHATMAPSPARTSOFTHEINFORMATIONONTOPARTICULARSERVERSSOMEREPLICATIONISREQUIREDTOAVOIDSUCHCENTRALISATIONFOREXAMPLEITHELOCATIONDATABASEMIGHTBEREPLICATEDATMULTIPLEROOTSERVERSORIITHELOCATIONDATABASEMIGHTBEREPLICATEDINEVERYSERVERINBOTHCASES,DIFFERENTCLIENTSMUSTACCESSDIFFERENTSERVERSEGLOCALONESORRANDOMLYCHAPTER1SOLUTIONSFMPAGE4WEDNESDAY,OCTOBER12,2011230PMDISTRIBUTEDSYSTEMS,EDITION5CHAPTER1SOLUTIONSFM5LASTUPDATED12OCTOBER2011228PMGEORGECOULOURIS,JEANDOLLIMORE,TIMKINDBERGANDGORDONBLAIR2012111ADISTRIBUTEDSYSTEMISDESCRIBEDASSCALABLEWHENITREMAINSEFFECTIVEWHENTHEREISASIGNIFICANTINCREASEINTHENUMBEROFRESOURCESANDTHENUMBEROFUSERSHOWEVER,THESESYSTEMSSOMETIMESFACEPERFORMANCEBOTTLENECKSHOWCANTHESEBEAVOIDED111ANSCACHINGANDREPLICATIONMAYBEUSEDTOIMPROVETHEPERFORMANCEOFTHERESOURCESTHATAREVERYHEAVILYUSEDINADISTRIBUTEDSYSTEM112ASERVERPROCESSMAINTAINSASHAREDINFORMATIONOBJECTSUCHASTHEBLOBOBJECTOFEXERCISE17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