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ID:51622831
大小:1.58 MB
页数:95页
时间:2020-03-26
《数据库系统概念全套配套课件PPT ch19.ppt》由会员上传分享,免费在线阅读,更多相关内容在教育资源-天天文库。
1、Chapter19:DistributedDatabasesChapter19:DistributedDatabasesHeterogeneousandHomogeneousDatabasesDistributedDataStorageDistributedTransactionsCommitProtocolsConcurrencyControlinDistributedDatabasesAvailabilityDistributedQueryProcessingHeterogeneousDistributedDatabasesDirector
2、ySystemsDistributedDatabaseSystemAdistributeddatabasesystemconsistsoflooselycoupledsitesthatsharenophysicalcomponentDatabasesystemsthatrunoneachsiteareindependentofeachotherTransactionsmayaccessdataatoneormoresitesHomogeneousDistributedDatabasesInahomogeneousdistributeddatab
3、aseAllsiteshaveidenticalsoftwareAreawareofeachotherandagreetocooperateinprocessinguserrequests.EachsitesurrenderspartofitsautonomyintermsofrighttochangeschemasorsoftwareAppearstouserasasinglesystemInaheterogeneousdistributeddatabaseDifferentsitesmayusedifferentschemasandsoft
4、wareDifferenceinschemaisamajorproblemforqueryprocessingDifferenceinsoftwareisamajorproblemfortransactionprocessingSitesmaynotbeawareofeachotherandmayprovideonlylimitedfacilitiesforcooperationintransactionprocessingDistributedDataStorageAssumerelationaldatamodelReplicationSy
5、stemmaintainsmultiplecopiesofdata,storedindifferentsites,forfasterretrievalandfaulttolerance.FragmentationRelationispartitionedintoseveralfragmentsstoredindistinctsitesReplicationandfragmentationcanbecombinedRelationispartitionedintoseveralfragments:systemmaintainsseveralide
6、nticalreplicasofeachsuchfragment.DataReplicationArelationorfragmentofarelationisreplicatedifitisstoredredundantlyintwoormoresites.Fullreplicationofarelationisthecasewheretherelationisstoredatallsites.Fullyredundantdatabasesarethoseinwhicheverysitecontainsacopyoftheentiredata
7、base.DataReplication(Cont.)AdvantagesofReplicationAvailability:failureofsitecontainingrelationrdoesnotresultinunavailabilityofrisreplicasexist.Parallelism:queriesonrmaybeprocessedbyseveralnodesinparallel.Reduceddatatransfer:relationrisavailablelocallyateachsitecontainingarep
8、licaofr.DisadvantagesofReplicationIncreasedcostofupdates:eachreplicaofrelat
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