小区物业综合管理系统的设计与实现外文翻译.docx
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小区物业综合管理系统的设计与实现外文翻译
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本科毕业设计(外文翻译)
题目小区物业综合管理系统的设计与实现
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Databasespaceorganization
Spatialdatamanagementhasbeenanactiveareaofresearchinthedatabasefieldfortwodecades,withmuchoftheresearchbeingfocusedondevelop-ingdatastructuresforstoringandindexingspatialdata.However,nocommercialdatabasesystempro-videsfacilitiesfordirectlydefiningandstoringspa-tialdata,andforniulatingqueriesbasedonresearchconditionsonspatialdata.Webelievethefollowingaretherelevantissuesonwhichnear-termresearchshouldbefocused(intheorderofdecreasingimpor-tanceandurgency).
First,relationalqueryoptimizationtechniquesneedtobeextendedtodealwithspatialqueries,thatis,queriesthatcontainsearchconditionsonspatialpredicatestobedeveloped.
Second,moreworkneedstobedoneonexperi-mentalvalidationoftherelativeperformanceofsomeofthemorepromisingdatastructuresandindexingstructuresproposedthusfar,withconsiderationofamuchbroadersetofoperationsthanjustafewopera-tionsthathavetypicallybeenusedinthelimitedper-formancestudiesconductedthusfar.
Third,itisdifficulttobuildintoasingledata-basesystemmultipledatastructuresforspatialindex-ing,andallspatialoperatorsthatareusefulforawidevarietyofspatialapplications,assuch,itisdesirabletobuildadatabasesystemsothatitwillbeaseasyaspossibletoextendthesystemwithaddi-tionaldatastructuresandspatialoperators.
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.
Datasecuritypreventsunauthorizedusersfromviewingorupdatingthedatabase.Usingpasswords,the database, called subschema(pronounced"sub-scheme"),Forexample,anemployeedatabasecancontainallthedataaboutanindividualemployee,butonegroupofusersmaybeauthorizedtoviewonlypayrolldata,whileothersareallowedaccesstoonlyworkhistoryandmedicaldata.
TheDBMScanmaintaintheintegrityofthedatabasebynotallowingmorethanoneusertoup-datethesamerecordatthesametime,TheDBMScankeepduplicaterecordsoutofthedatabase,forexample,notwocustomerswiththesamecustomernumbers(Keyfields)canbeenteredintotheData-Base.
WhenaDBMSisused,thedetailedknowledgeofthephysicalorganizationofthedatadoesnothavetobebuiltintoeveryapplicationprogram.TheapplicationprogramaskstheDBMSfordatabyfieldpine,forexample,Icodedrepresentationof"give customernameandbalancedue"wouldbesenttoheDBMS.WithoutaDBMS,theprogrammermustsecrecyspaceforthefullstructureoftheintheprogram,Anychangeindatastructurerequires£hangsinalltheapplicationsprograms.
Themultiple-databasemodelisrepresentedbyproposalsforsharedandprivatedatabasearchitectrues,checkoutandcheckingofdatatoandfromsharedandprivatedatabases.Eachusermaypopulates/herprivatedatabasewithdatacheckedoutoftheshareddatabase,performupdatesagainstthedata.andcheckthembackintothesharedDatabase.
Themultiple-databasemodelcanbeusedto workaround"theconflictsituationsinherentinlonglunationdatabasesessions.Sinceeachuserfromtheshareddatabaseandworkagainst.is/herprivatedatabase,"disconnected"fromthe、hareddatabase(atleastonthesurface),theusersanavoidtheconflictsituations.Inparticular,multileusersmaybesimultaneouslyupdatingthesamethoughthavingtowaitforotheruserstocomatelatetheirupdates.However,whenupdatedistobeneckedintotheshareddatabase,itmayhavetobecheckedinasanewversion,necessitatingversionmanagement.Further,'whendatainaprivatedata-basereferencesdataintheshareddata,orviceversa,aprivatedatabaseisnotreallydisconnectedfromtheshareddatabase.Forexample,theevaluationofqueryingeneralwillrequirethedatabasesystemtoaccessbothaprivatedatabaseandtheshareddata-base,evenifthequerymayhavebeenformulatedagainstaprivatedatabase.Themultiple-databasemodelismoreappropriatethanthesingle-databasemodelinanenvironmentwhereitiseasytodetermineinadvancelogicalpartitionsofthedatabasethatcorrespondtoworktobeperformed.
Thesingle-databasemodelhasreceivedconsiderablymoreattentionthanthemultiple-databasemodel.Thefocusofresearchintosingle-databasemodelhasbeenonaddressingthetwinproblemoflongwaitsandlossofworkthatthelongdurationoftransactionsbringsabout.
Thesingle-databasemodelrequirestheintro-dictionofanotionofdatabaseconsistency,andprotocolsforconcurrencycontrolandrecovery,thataredifferentfromthosesupportedintraditionaldatabasesystems.Ifa"reasonable"notionofdatabaseconsistencyistobesupported(I.c.thedatabasesystemistoenforceitautomatically),thereareboundtobe”conflict"situationswhereonetransactioncomesintoanaccessconflictagainstsomeothertransaction.Ifawaitistobeavoided,somemeansofanegotiatedsettlementoftheconflictmustbeprovided,therebydraggingtheusersintothedetailsofconcurrencycontrol.Thesingle-databasemodelismoreappropriate-atethanthemultiple-databasemodelinanenvironmentwhereitisdifficulttodetermineinadvancelogicalpartitionsofthedatabasethatcorrespondtoworktobeperformed,andtheuserscloselycooperate.
Theobjectiveoflong-durationtransactionsistomodellong-duration,interactiveDatabaseaccesssessionsinapplicationenvironments.Thefundamentalassumptionaboutshort-durationoftransactionsthatunderliesthetraditionalmodeloftransactionsisinappropriateforlong-durationtransactions.Theimplementationofthetraditionalmodeloftransactionsmaycauseintolerablylongwaitswhentransactionsalephtoacquirelocksbeforeaccessingdata,andmayalsocausealargeamountofworktobelostwhentransactionsarebackedoutinresponsetouser-initiatedabortsorsystemfailuresituations.
Theobjectiveofatransactionmodelistopro-videarigorousbasisforautomaticallyenforcingcriterionfordatabaseconsistencyforasetofmultipleconcurrentreadandwriteaccessestothedatabaseinthepresenceofpotentialsystemfailuresituations.Theconsistencycriterionadoptedfortraditionaltransactionsisthenotionofscrializability.Scrializa-bilityisenforcedinconventionaldatabasesystemsthroughtheuse