数据库系统概念第六版课后习题部分答案19s

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1、CHAPTER19DistributedDatabasesPracticeExercises19.1Howmightadistributeddatabasedesignedforalocal-areanetworkdifferfromonedesignedforawide-areanetwork?Answer:Datatransferonalocal-areanetwork(LAN)ismuchfasterthanonawide-areanetwork(WAN).Thusreplicationandfragmentationwillnotincreasethroughputandspeed-u

2、ponaLAN,asmuchasinaWAN.ButeveninaLAN,replicationhasitsusesinincreasingreliabilityandavailability.19.2Tobuildahighlyavailabledistributedsystem,youmustknowwhatkindsoffailurescanoccur.a.Listpossibletypesoffailureinadistributedsystem.b.Whichitemsinyourlistfrompartaarealsoapplicabletoacentral-izedsystem?

3、Answer:a.Thetypesoffailurethatcanoccurinadistributedsystemincludei.Sitefailure.ii.Diskfailure.iii.Communicationfailure,leadingtodisconnectionofoneormoresitesfromthenetwork.b.Thefirsttwofailuretypescanalsooccuroncentralizedsystems.19.3Considerafailurethatoccursduring2PCforatransaction.Foreachpos-sible

4、failurethatyoulistedinPracticeExercise19.2a,explainhow2PCensurestransactionatomicitydespitethefailure.Answer:Aproofthat2PCguaranteesatomiccommits/abortsinspiteofsiteandlinkfailures,follows.Themainideaisthatafterallsitesreplywithamessage,onlytheco-ordinatorofatransactioncanmakeacommitorabortd

5、ecision.Anysubsequentcommitorabortbya12Chapter19DistributedDatabasessitecanhappenonlyafteritascertainstheco-ordinator’sdecision,eitherdirectlyfromtheco-ordinator,orindirectlyfromsomeothersite.Letusenumeratethecasesforasiteaborting,andthenforasitecommitting.a.AsitecanabortatransactionT(bywritingan

6、ortT>logrecord)onlyunderthefollowingcircumstances:i.Ithasnotyetwrittenalog-record.Inthiscase,theco-ordinatorcouldnothavegot,andwillnotgetaormessagefromthissite.Thereforeonlyanabortdecisioncanbemadebytheco-ordinator.ii.Ithaswrittenthelogrecord,butoninquiryitfoundoutth

7、atsomeothersitehasanlogrecord.Inthiscaseitiscorrectforittoabort,becausethatothersitewouldhaveascertainedtheco-ordinator’sdecision(eitherdirectlyorindirectly)beforeactuallyaborting.iii.Itisitse