众所周知原文Word文档格式.docx

众所周知原文Word文档格式.docx

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　　ADBMScanorganize，process，andpresentselecteddataelementsfromthedatabase．Thiscapabilityenablesdecisionmakerstosearch，probe，andquerydatabasecontentsinordertoextractanswerstononrecurringandunplannedquestionsthataren’tavailableinregularreports[3]．Thesequestionsmightinitiallybevagueand/orpoorlydefined，butpeoplecan“browse”throughthedatabaseuntiltheyhavetheneededinformation．Inshort，theDBMSwill“manage”thestoreddataitemsandassembletheneededitemsfromthecommondatabaseinresponsetothequeriesofthosewhoaren’tprogrammers．Inafile-orientedsystem，usersneedingspecialinformationmaycommunicatetheirneedstoaprogrammer，who，whentimepermits，willwriteoneormoreprogramstoextractthedataandpreparetheinformation[4]．TheavailabilityofaDBMS，however，offersusersamuchfasteralternativecommunicationspath（seeFig.7-1）．

　IftheDBMSprovidesawaytointeractivelyandupdatethedatabase,aswellasinterrogateitcapabilityallowsformanagingpersonaldata-Aceshowever,itdoesnotautomaticallyleaveanaudittrailofactionsanddoesnotprovidethekindsofcontrolanecessaryinamultiuserorganization.These-controlsareonlyavailablewhenasetofapplicationprogramsarecustomizedforeachdataentryandupdatingfunction. 

　　SoftwareforpersonalcomputerswhichperformmeoftheDBMSfunctionshavebeenverypopular 

arsenalcomputerswereintendedforusebyindividualsforpersonalinformationstorageandprocess-Thesemachineshavealsobeenusedextensivelysmallenterprises,professionalslikedoctors,acrylics,engineers,lawyersandsoon．Bythenatureof.intendedusage,databasesystemsonthesemachinesexceptfromseveraloftherequirementsoffulldogedatabasesystems．Sincedatasharingisnot：

Tended,concurrentoperationsevenlessso,the）fewercanbelesscomplex.Securityandintegrity7aintenancearede-emphasizedorabsent.Asdata-）limeswillbesmall,performanceefficiencyIsalsoimportant.Infact,theonlyaspectofadatabasesystemthatisImportant'

isdataIndependence.Data-.dependence,asstatedearlier.Meansthatapplicantprogramsanduserqueriesneednotrecognizant‘physicalorganizationofdataonsecondarystorage.Theimportanceofthisaspect,particularlyforthepersonalcomputeruser,isthatthisgreatlysimplifiesdatabaseusage.Theusercanstore,accessandmanipulatedataatahighlevel（closetotheapplication）andbetotallyshieldedfromthelowlevel（closetothemachine）detailsofdataorganization.WewillnotdiscussdetailsofspecificPCDBMSsoftwarepackageshere．Letussummarizeinthefollowingthestrengthsandweaknessesofpersonalcomputerdata-basesoftwaresystems：

　　Themostobviouspositivefactoristheuserfriendlinessofthesoftware.Auserwithnopriorcomputerbackgroundwouldbeabletousethesystemtostorepersonalandprofessionaldata,retrieveandperformrelayedprocessing.Theusershould,ofcourse,satietyhimselfaboutthequalityofsoftwareandthefreedomfromerrors（bugs）sothatinvest-meritsindataarcprotected. 

　　Fortheprogrammerimplementingapplicationswiththem,theadvantageliesinthesupportforapplicationsdevelopmentintermsofinputscreengenerations,outputreportgenerationetc．offeredbythesesstems． 

　　Themainnegativepointconcernsabsenceofdataprotectionfeatures．Unlessencrypted,datacaneaccessedbywhoeverhasaccesstothemachineDatacanbedestroyedthroughmistakesormaliciousintent.ThesecondweaknessofmanvofthePC-basedsystemsisthatofperformance.Ifdatavolumesgrowuptoafewthousandsofrecords，performancecouldbeabottleneck．

　　Fororganizationwheregrowthindatavolumesisexpected,availabilityof,thesameorcompatiblesoftwareonlargemachinesshouldbeconsidered. 

Thisisoneofthemostcommonmisconceptionsaboutdatabasemanagementsystemsthatareusedinpersonalcomputers．ThoroughlycomprehensiveandsophisticatedbusinesssystemscanbedevelopedindBASh,ParadoxandotherDBMSs．However,theyarecreatedbyexperiencedprogrammersusingtheDBMS'

sownprogramminglanguage.Thatisnotthesameasuserswhocreateandmanagepersonalfilesthatarenotpartofthemainstreamcompanysystem．

TransactionManagementofDatabase 

　　Theobjectiveoflong-durationtransactionsistomodellong-duration,interactiveDatabaseaccesssessionsinapplicationenvironments．Thefundamentalassumptionaboutshort-durationoftransactionsthatunderliesthetraditionalmodeloftransactionsisinappropriateforlong-durationtransactions．Theimplementationofthetraditionalmodeloftransactionsmaycauseintolerablylongwaitswhentransactionsalephtoacquirelocksbeforeaccessingdata,andmayalsocausealargeamountofworktobelostwhentransactionsarebackedoutinresponsetouser-initiatedabortsorsystemfailuresituations． 

　　Theobjectiveofatransactionmodelistopro-videarigorousbasisforautomaticallyenforcingcriterionfordatabaseconsistencyforasetofmultipleconcurrentreadandwriteaccessestothedatabaseinthepresenceofpotentialsystemfailuresituations．Theconsistencycriterionadoptedfortraditionaltransactionsisthenotionofscrializability．Scrializa-bilityisenforcedinconventionaldatabasesystemsthroughtheuseoflockingforautomaticconcurrencycontrol,andloggingforautomaticrecoveryfromsystemfailuresituations．A"

transaction”thatdoesn’tprovideabasisforautomaticallyenforcingdata-baseconsistencyisnotreallyatransaction.Tobesure,along-durationtransactionneednotadoptseri-alizabilityasitsconsistencycriterion．However.theremustbesomeconsistcricvcriterion．

　VersionSystemManagementofDatabase 

　　Despitealargenumberofproposalsonversionsupportinthecontextofcomputeraideddesignandsoftwareengineering,theabsenceofaconsensusonversionsemanticshasbeenakeyimpedimenttoversionsupportindatabasesystems．Becauseofthedifferencesbetweenfilesanddatabases,itisintuitivelyclearthatthemodelofversionsindatabasesystemscannotbeassimpleasthatadoptedinfilesystemstosupportsoftwareengineering．Fordata-bases,itmaybenecessarytomanagenotonlyversionsofsingleobjects（e.g.asoftwaremodule,document,butalsoversionsofacollectionofobjects（e.g.acompounddocument,ausermanual，etc．andperhapsevenversionsoftheschemaofdatabase（c.g.atableoraclass,acollectionoftablesorclasses）．

　　Broadly,therearethreedirectionsofresearchanddevelopmentinversioning．Firstisthenotionofaparameterizedversioning"

，thatis,designingandimplementingaversioningsystemwhosebehaviormaybetailoredbyadjustingsystemparametersThismaybetheonlyviableapproach,inviewofthefactthattherearevariousplausiblechoicesforvirtuallyeverysingleaspectofversioning．Thesecondistorevisittheseplausiblechoicesforeveryaspectofversioning,withtheviewtodiscardingsomeofthemeseitherimpracticalorflawed.Thethirdistheinvestigationintothesemanticsandimplementationofversioningcollectionsofobjectsandofversioningthedatabasesche

　Thereisnoconsensusofthedefinitionoftheterm"

managementinformationsystem"

.Somewriterspreferalternativeterminologysuchas"

informationprocessingsystem"

"

informationanddecisionsystem"

organizationalinformationsystem"

orsimply"

informationsystem"

torefertothecomputer-basedinformationprocessingsystemwhichsupportstheoperations,management,anddecision-makingfunctionsofanorganization.Thistextuses“MIS”becauseitisdescriptiveandgenerallyunderstood;

italsofrequentlyuses“informationsystem”insteadof“MIS”torefertoanorganizationalinformationsystem.

2、Adefinitionofamanagementinformationsystem,asthetermisgenerallyunderstood,isanintegrated,user-machinesystemforprovidinginformationtosupportoperations,management,anddecision-makingfunctionsinanorganization.Thesystemutilizescomputerhardwareandsoftware;

manualprocedures;

modelsforanalysisplanning,controlanddecisionmaking;

andadatabase.Thefactthatitisanintegratedsystemdoesnotmeanthatitisasingle,monolithicstructure;

rather,itmeansthatthepartsfitintoanoveralldesign.Theelementsofthedefinitionarehighlightedbelow.

Computer-baseduser-machinesystem

3、Conceptually,amanagementinformationcanexistwithoutcomputer,butitisthepowerofthecomputerwhichmakesMISfeasible.Thequestionisnotwhethercomputersshouldbeusedinmanagementinformationsystem,buttheextenttowhichinformationuseshouldbecomputerized.Theconceptofauser-machi