软件工程专业英语-Main Memory Database

我们毕业啦其实是答辩的标题地方软件工程专业英语SOFTWAREENGINEERINGESSENTIALS23.1AnOverviewofMainMemoryDatabases3.2TheEvolutionofMainMemoryDatabases3.3

TheKeyTechnologyofMainMemoryDatabaseSUMMARYCHPTER3

MAINMEMORYDATABASES33.1AnOverviewofMainMemoryDatabases43.1AnOverviewofMainMemoryDatabaseForalongtime,harddisksweretheonlytechnologythatcouldstoreenoughinformationtoholdadatabaseandofferedrandomaccessatthesametime.Therefore,conventionaldatabasemanagementsystemsweretunedtotakethistechnologytothemaximum.Butinrecentyears,mainmemorieshavebecomecheaperandgrowntoapointthatforsomefieldsofapplicationitallowsonetokeepthewholeinformationofadatabaseinmainmemoryandthereforespeedupoperation.3.1.1WhatisaMainMemoryDatabase?Amainmemorydatabasesystem(MMDB),alsoknownasanin-memorydatabase(IMDB)oramemoryresidentdatabase,isadatabasemanagementsystemthatprimarilyreliesonmainmemoryforcomputerdatastorage.Amainmemorydatabasesystemiscontrastedwithdatabasemanagementsystemsthatemployadiskstoragemechanism.Mainmemorydatabasesarefasterthandisk-optimizeddatabasessincetheinternaloptimizationalgorithmsaresimplerandexecutefewerCPUinstructions.Accessingdatainmemoryeliminatesquerytimewhenretrievingthedata,whichprovidesfasterandmorepredictableperformancethandisk.3.1.2TheDifferencesbetweenMMDBsandDRDBsBothconventionaldiskresidentdatabasemanagementsystems(DRDBMSs)andmainmemorydatabasemanagementsystems(MMDBMSs)processdatainmainmemory,andbothkeepa(backup)copyondisk.IfthecacheofaDRDBMSislargeenough,soonerorlaterthewholedatabasewillbeincache,andanMMDBMSneedstostoreabackupcopyondisksothatitisabletorecoverfromfailures.SowhatisthemaindifferencebetweenaDRDBMSandanMMDBMS?3.1.2TheDifferencesbetweenMMDBsandDRDBsThekeydifferenceisthatinanMMDBMStheprimarycopyofthedatabaselivespermanentlyinmainmemory,andthishasimportantimplicationsforthealgorithmsanddatastructuresused.EvenifthewholedatabaseofaDRDBMSiscachedinmainmemory,itwillnotprovidebestperformancesinceaDRDBMSisnottunedforthiscase.EachdataaccessinaDRDBMShastogotothebuffermanagertomakesuredataisstillinmainmemory.InMMDBMSsthesituationisdifferent:Sincedataisguaranteedtostaypresentinmainmemory,indexstructuresandalltheotherpartsofthedatabasedonotneedtoconsiderdiskaccessandcanbetunedforlowcomputationalcost.3.1.3FeaturesofMainMemoryDatabases

Themainmemorydatabasehasmanyexcellentfeatures,and

someimportantonesarelistedasfollows:HighthroughputrateandlowaccessdelayExcellentparallelprocessingpowerHighhardwaredependenciesConceptCheckWhatisaMainMemorydatabase?DescribetheDifferencesbetweenMainMemorydatabaseanddiskresidentdatabase.HowmanyfeaturesdoMMDBshave?3.2TheEvolutionofMainMemoryDatabasesThePeriodofMainMemoryDatabaseRudimentThePeriodofResearchandDevelopmentforMainMemoryDatabaseThePeriodofProductsGrowth3.2TheEvolutionofMainMemoryDatabasesResearchofthemainmemorydatabasebeganearly,anditsrudimentaroseattheendof1960s.Sincemainmemorydatabaseshadsmallcapacityandhighcost,theyweremostlyusedasthesecondarystorageforembeddedsystemsordiskresidentdatabaseandengineforspeedingup.ThemajorgoalofmainmemorydatabasesistostorehotdatafrequentlyusedintheDRDBatthemainmemory,whichcanoptimizequeryforthekeydataandsignificantlyimproveefficiencyinprocessingdata.Withthedecreasingcostandincreasingcapacityofmainmemorycontinuously,thepracticabilityofmainmemorydistinctlyincreased.Accordingly,thisalsopromotedtheresearchandevolutionofmainmemorytechnology.1、ThePeriodofProductsGrowthIn1969,IBMcorporationdevelopedIMS,thefirsthierarchicaldatabasemanagementsystemintheworld.ThemainthoughtsofdesignembodiedintheIMSinvolvestoringthedatadirectlyinthephysicalmemory,whichwasfrequentlyaccessedandneededrapidresponses.Themainmemorydatabaseevolvedfromthehierarchicaldatabases,andgraduallyturnedtotherelationalmainmemorydatabase.2.ThePeriodofResearchandDevelopmentforMainMemoryDatabaseIn1984,D.J.DeWittpublishedthepaper----TheImplementtechnologyofMainMemoryDatabase,inwhichhefirstputforwardtheconceptof“mainmemorydatabase”.Theexpertspredictedthatthehighpriceofcomputermemorywouldbedecreased,andthelargecapacitydatacouldfullybestoredatthemainmemory.Accordingly,thekeytechnologyofmainmemorydatabasebecametheresearchfocus,whichincludedmemorycomputations,hashalgorithm,usageofnonvolatilememoryandmainmemorydatabaserecoverymechanism,etc.3.ThePeriodofMainMemoryDatabaseRudimentWiththedevelopmentoftheInternet,peopleneeddatabasemanagementsystemswithhighperformanceandhighconcurrencytomeetmoreandmorenetworkrequirements.Suchapplicationneedscatalyzethemarketsofmainmemorydatabase.Inrecentyears,theadvancesofhardwareandsoftwaretechnologyhaveopenedupmanynewdatabaseapplicationdomains.Intheaspectofhardware,itisnodoubtthatthetechnologyadvanceofsemiconductor,theincreasingcapacityofmemory,largerandlargercapacityofDRAMsandthedecreasingpriceofDRAMsprovidehardwarefoundationforgradualexpansionofcomputermainmemory,andmakethetechnologiesofmainmemorydatabasemorefeasible,thepriceofmainmemorymorereasonable.Somecompaniesintroducedmanydifferentproductsgradually.153.3TheKeyTechnologyof

MainMemoryDatabase3.3.1PerformanceandDataStructureInDRDBMSsdiskaccesseshavetraditionallybeenthebottlenecksinceinthetimerequiredforonesinglediskaccess,aprocessorcanperformuptoseveralmillioninstructions.Therefore,themostimportantoptimizationsindisk-basedsystemsaretoreducethenumberofdiskaccesses,toprefersequentialaccess,andtokeeptheprocessorbusywhilewaitingforI/O.Toreducethenumberofdiskaccesses,cachingisused,

andspecialindexstructures,liketheB+tree,weredeveloped.Datathatisusedtogetherindatabaseisgroupedondisktobeabletoaccessitusingonesinglesequentialread.AhighdegreeofconcurrencyisemployedtokeeptheprocessorbusywhileothertransactionsarewaitingforI/O,andthereforeasmalllockinggranularitydowntorecordlevellockingisused.3.3.2CommitProcessingandBackup1.CommitProcessingFromauser’spointofview,MMDBMSsshouldnotdifferfromconventionaldatabasesystemsapartfromthehigherperformance.Therefore,MMDBMSsmustbeabletosupporttheACIDpropertiesknownfromconventionaldatabasesystems.Thishassomeimplicationsthatwillbedescribedinthefollowingsection.2.BackupMainmemoryisvolatileandislostincaseofapowerfailure.Unlessspecialcareistaken,likeinBressound,Clark,andKan(2001),dataisnotonlylostafterafailurebutevenduringasimplerestartofthesystem.Therefore,abackupcopyofthedatabasemustexist.Oneobvioussolutionistobackupdatatodisk(s).Incaseofafailurethisbackupcanbeusedtogetherwiththelogstorecoverthedatabase.Tokeepthetimeneededforarecoveryshort,thebackupmustbekeptup-to-date.Onecommonlyusedmechanismischeckpointing(Woo,Kim,&Lee,1997).SincethebackupdiskisonlyaccessedbythecheckpointerthatbacksupdatainthebackgroundandnoapplicationhastowaitforI/Otothisdisk,theI/Ocanbetailoredtomeettheneedofthecheckpointalone.Therefore,averylargeblocksizecanbeusedsincelargeblockscanbewrittentodiskmoreefficiently.3.3.3RecoveryandConcurrency

1.RecoveryAutomaticusesofcheckpointandredologArchive/RestoreFaultTolerant2.ConcurrencyControlAsitwasmentionedabove,inDRDBMSsahighdegreeofconcurrencyisdesirabletokeeptheprocessor(s)busywhileothertransactionsarewaitingfordiskI/O.Toachievemaximumparallelism,lockinggranularityshouldbeassmallaspossible.InanMMDBMSwaitingfor