Chapter 6 Distributed Systems.pdf

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1、CHAPTER6DistributedSystemsYouknowyouhaveadistributedsystemwhenthecrashofacomputeryou’veneverheardofstopsyoufromgettinganyworkdone.—LeslieLamport6.1IntroductionWeveseeninthelastfewchaptershowpeoplecanauthenticatethemselvestosystems(andsystemscanauthenticatethemselvestoeachother)usingsecur

2、ityprotocols;howaccesscontrolscanbeusedtomanagewhichprincipalscanperformwhatoperationsinasystem;andsomeofthemechanicsofhowcryptocanbeusedtounderpinaccesscontrolindistributedsystems.Buttheresmuchmoretobuildingasecuredistributedsystemthanjustimplementingaccesscontrols,protocolsandcrypto.Wh

3、ensystemsbecomelarge,thescale-upproblemsarenotlinear;thereisoftenaqualitativechangeincomplexity,andsomethingsthataretrivialtodealwithinanetworkofonlyafewmachinesandprincipals(suchasnaming)suddenlybecomeabigdeal.Overthelast40years,computerscienceresearchershavebuiltmanydistributedsystemsa

4、ndstudiedissuessuchasconcurrency,failurerecoveryandnaming.Thetheoryissupplementedbyagrowingbodyofexperiencefromindustry,commerceandgovernment.Theseissuesarecentraltothedesignofeffectivesecuresystemsbutareoftenhandledratherbadly.Ivealreadydescribedattacksonsecurityprotocolsthatcanbeseenas

5、concurrencyfailures.Ifwereplicatedatatomakeasystemfault-tolerantthenwemayincreasetheriskofacompromiseofconfidentiality.Finally,namingisaparticularlythornyproblem.Manygovernmentsandorganisationsaretryingtobuild185186Chapter6■DistributedSystemslarger,flatternamespacesusingidentitycardstonumb

6、ercitizensandusingRFIDtonumberobjectsandyetnamingproblemsunderminedattemptsduringthe1990stobuildusefulpublickeyinfrastructures.6.2ConcurrencyProcessesaresaidtobeconcurrentiftheyrunatthesametime,andconcurrencygivesrisetoanumberofwell-studiedproblems.Processesmayuseolddata;theycanmakeincon

7、sistentupdates;theorderofupdatesmayormaynotmatter;thesystemmightdeadlock;thedataindifferentsystemsmightneverconvergetoconsistentvalues;andwhenitsimportanttomakethingshappenintherightorder,oreventoknowtheexacttime,thiscanbeharderthanyoumightthink.Systemsarenowrapidlybecomi