翻译文献-分布式数据库的发展

外文原文DISTRIBUTEDDATABASEDEVELOPMENTCENTRALIZEDDATABASESHAVEBEENINLARGESCALEUSEINTHEDATAPROCESSINGINDUSTRYSINCETHEEARLY1970SDISTRIBUTEDDATABASESAREVERYMUCHMORESOPHISTICATEDTHANCENTRALIZEDDATABASES,ANDTHEYINVOLVECOMPLEXSOFTWARETOHANDLEPROBLEMSSUCHASCONCURRENCYCONTROL,DEADLOCKPREVENTIONANDTHEMAINTENANCEOFDATAINTEGRITYDURINGMULTISITEUPDATESITISONLYRECENTLYTHATTHESESOFTWAREPROBLEMSHAVEBEENUNDERSTOOD,ANDPRODUCTSWHICHINCORPORATETHISSOFTWAREHAVEONLYVERYRECENTLYSTARTEDTOAPPEARONTHEMARKETTHEPRESENTGENERATIONOFDDBSMAINLYINVOLVEREPLICATIONOFDATABASESATSEVERALSITES,RATHERTHANDISTRIBUTIONOFINDIVIDUALDATABASESTHENEXTGENERATIONOFDDBS,THOUGH,ISLIKELYTOINVOLVEDISTRIBUTIONOFDATAOVERSEVERALGEOGRAPHICNODES,INORDERTOPLACEDATAASCLOSEASPOSSIBLETOTHELOCATIONATWHICHTHEHIGHESTRATEOFACTIVITYTAKESPLACEWITHTHESEDATABASESITISLIKELYTHATMULTISITEUPDATESWILLBEMORECOMMONLYUSEDTHANATPRESENTTHISWILLINVOLVEVERYCAREFULPREPARATIONOFRECOVERYPROCEDURESINORDERTOENSURETHATTHERISKOFLOSSOFDATAINTEGRITYISDRIVERDOWNTOANABSOLUTEMINIMUMINSOMEINSTALLATIONSITWILLALSOBENECESSARYTOCAREFULLYMONITORBOTHTHENETWORKANDTHEDDBSONANONGOINGBASIS,INORDERTOENABLETHEOPERATIONSSTAFFTOREACTRAPIDLYTOANYSITUATIONINWHICHDATACORRUPTIONHASTAKENPLACESOMEWORKHASALREADYBEENDONEINTHISAREA,USINGEXPERTSYSTEMSTOHELPINDETERMININGWHETHERAGIVENPROBLEMISACOMMUNICATIONSPROBLEMORADATABASEPROBLEMHOWEVER,THEREISANEEDFORTHESEDIAGNOSTICTECHNIQUESTOMATURETOAPOINTWHERETHEYCANBEUSEDONANEVERYDAYBASISBYCOMPUTEROPERATIONSSTAFFINTHEIMMEDIATEFUTURE,THEMOSTINTENSIVEDEVELOPMENTWORKWILLBEINTHEAREAOFTHEOPTIMIZERTHISISTHEMODULEUSEDBYTHEDDBINORDERTOMINIMIZECOMPUTINGANDCOMMUNICATIONSCOSTSAREGENERALLYHIGHCOMPAREDWITHPROCESSINGCOSTSWITHTHEEMPHASISBEINGVERYMUCHONTHELATTERASCOMMUNICATIONSCOSTSAREGENERALLYHIGHCOMPAREDWITHPROCESSINGCOSTSASOPHISTICATEDOPTIMIZERWILLTAKEACCOUNTOFCOMMUNICATIONSCOSTS,DISCI/OANDPROCESSINGCOSTS,ANDWILLALSOEVALUATESEVERALWAYSOFTRANSPORTINGDATAACROSSTHENETWORKTOSATISFYTHEREQUIREMENTSOFAMULTISITETRANSACTIONITWILLALSOEVALUATESEVERALACCESSPATHSINTOTHEDATBASE,ANDINSOMECASESWILLSTOREINFORMATIONABOUTITSCOMPARISONSOF,DIFFERENTACCESSSTRATEGIESFORFUTUREUSEASAGENERALRULE,OPTIMIZATIONPROBLEMSBECOMECOMPLEXWHENTHENUMBEROFNODESINTHEDDBEXCEEDSFIVEANOTHERIMPORTANTFUTUREDEVELOPMENTISLIKELYTOOCCURINTHEAREAOFSYSTEMARCHITECTURESASCHOOLOFTHOUGHTHASRECENTLYDEVELOPEDWHICHADVOCATESTHATTHEMOSTATTRACTIVEMODELONWHICHTOBASEADDBISTHECLIENTSERVERMODELINTHISMODEL,THE“CLIENTS”THEDATAUSERS,ARESEPARATEDFROMTHE“SERVERS”THEDATAPROVIDERSANDARELINKEDTOTHEMBYAHIGHSPEEDLANTHEMODELHASTHEATTRACTIONTHATCLIENTSANDSERVERSCANEASILYBEADDEDINAMODULARFASHION,ANDTOSOMEEXTENTDIFFERENTHARDWARETYPESCANBEMIXEDHOWEVER,ITISVERYDEPENDENTONRELATIVELYSOPHISTICATEDFUTUREDEVELOPMENTSINLANTECHNOLOGY,PARTICULARLYIFHETEROGENEOUSCOMBINATIONSOFHARDWAREARETOBESUPPORTEDTHEMOSTNOTABLEPRODUCTSTHATCURRENTLYREFLECTTHECLIENTSERVERMODELARESYBASE,INGRES,ANDTANDEMSNONSTOPSQLDDBSARECURRENTLYTHESUBJECTOFCONSIDERABLEINTERESTAMONGSEVERALHARDWAREANDDBMSSUPPLIERS，ANDASSUMINGTHATTHEPROBLEMSINVOLVEDINRECOVERY,DIAGNOSTICS,OPTIMIZATINGANDLANARCHITECTURECANBESOLVED,THEDISTRIBUTEDDATADASEAREALOOKSSETTOBEAMAJORAREAOFGROWTHFORTHECOMPUTERINDUSTRYINTHE1990S。DATADASESDIVIDEINTOCENTRALSERVERSANDLOCALFRONTENDSWEVEHEARDALOTMAYBETOOMUCHABOUTTHECHANGESTAKINGPLACEINTHEDATABASEMARKETYOUKNOWTHELITANYSQL,DATABASESERVERS,CLIENT/SERVERRELATIONSHIPS,ETCLETSSTARTBYREVIEWINGTHEBASICEVOLUTIONOFPERSONALCOMPUTERDATABASESDATABASESONPERSONALCOMPUTERSHAVETYPICALLYBEENDIVIDEDINTOTWOMINGROUPSFLATFILEMANAGERSWORKWITHASINGLEFILEATATIMEANDAREUSUALLYMENUDASED,EASYTOUSE,ANDAIMEDATANENDUSERWHOKEEPSTRACKOFHISORHEROWNDATA,SUCHASATELEPHONEORADDRESSLISTRELATIONALDATABASECANWORKWITHMULTIPLEFILESATATIMEANDHAVETYPICALLYBEENAIMDATCONSULTANTSORINHOUSESUPPORTSTAFFWHOBUILDAPPLICATIONSINTHEPASTFEWYEARS,THESEDISTINCTIONSHAVEBEGUNTOBREAKDOWNONTHEONEHAND,THEFLATFILEMANAGERSHAVEGOTTENALOTMOREPOWERFULPROGRAMSANDHAVEBEGUNTOAPPROACHRELATIONALDATABASESINTERMSOFBASICCAPABILITIESRELATIONALDATABASESHAVEBEENCHANGINGALSOPROGRAMSLIKEPARADOXAREDESIGNEDSOTHATITSSIMPLEFORAUSERTOPERFORMADHOCQUERIESANDPRODUCTSLIKEDATAEASELETYOUDESIGNMULTIFILEAPPLICATIONSWITHLITTLEORNOPROGRAMMINGATTHESAMETIME,HOWEVER,OTHERDATABASEPRODUCTSAREEMPHASIZINGPOWERANDMULTIUSERFEATURESOVEREASEOFUSETHISISPARTICULARLYTRUEODTHEREALLYPOWERFULDATBASEENGINESORIGINALLYDESIGNEDFORMINICOMPUTERSANDMAINFRAMESTHATARENOWOFFEREDANDPROMOTEDINPCVERSIONSTHEMOSTSUCCESSFULEXAMPLEOFTISHASBEENORACLE,ANINCREDIBLYPOWERFULDATABASEENGINE,ANDONEDESIGNEDFORMULTIUSERENVIRONMENTS,BUTLACKINGINTHEEASEOFUSEFEATURESTHATMOSTPCUSERSEXPECTMARKERLEADERDBASEHASBEENTRYINGTOCHANGETOMATCHALLTHESEEXPECTATIONSDBASEIVADDSAFRONTENDTHATMAKESITEASIERTOUSE,AQUERYBYEXAMPLEINTERFACEFORADHOCQUERIES,ANDMOREPOWERINTHEFORMOFMOREADVANCEDINDEXINGANDRELATIONALITHYALLOFTHISISCOMINGTOAHEADASMOSTDATABASEVENDORSRECOGNIZETHATWEREMOVINGTOAWORLDINWHICHMANYBUSINESSUSERSOFDATABASESYSTEMSARECONNECTEDTOALOCALAREANETWORKANDPERHAPSVIAGATEWAYSTOMINICOMPUTERSANDMAINFRAMESCONSIDERHOWDATABASESHAVEPROGRESSEDWHENWEHADJUSTSINGLEUSERS,DATABASES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只与单个文件打交道，通常是菜单驱动，易用，供最终用户维持其自己的数据（如电话或地址表）管理。关系数据库一交可与多个文件打交道，它们主要供顾问或公司内编制应用程序的支持人员使用。在过去几年内，这些区别开始被打破。一方面，平文件管理程序变得更强，在基本功能方面开始向关系数据库靠拢。关系数据库也在。PARADOX一类的程序设计对执行特定询问的用户很简单。DATAEASE一类的产品让你很少或者不用编程就能设计多文件应用程序。同时，其它的数据库产品强调能力和多用户的特点超过过了易用性。这对当初为小型机和大型机设计，现在却由PC版提供各支持的功能真正强大的数据库驱动系统来说更是如此。其中最成功的例子是ORACLE，它是一个负有成名的强大数据库驱动系统，设计用于多用户环境，但缺乏多数PC用户期待的易用性。占据市场首位的DBASE也的试图变化，以满足各种期望。DBASEIV增加了一个前端，使之更易使用、供特定询问的按例询问接口以及以便晚高级的索引和关系的形式变得更强大。所有这些是出现在这样的时刻，多数数据库公司认识到，我们正在步入这样一个世界，即很多数据库系统的商业用户与局域网或者通过网关与小型机各大型机相连。考虑一下数据库是如何发展过来的。当我们只有单用户时，数据库是建在本地硬盘上。随着我们朝多用户数据库进展时，接下来的步伐是使数据库本身留驻在中央文件服务器上，现在所有主要的PC数据库都可以这样。虽然涉及的网络可以是基于服务器的（如NOVELL的NETWARE）或对等的（如10NET），但信息只存在网络磁盘上，本地工作站或个人机完成所有处理。今天带有LANPACK的DBASEPLUS或、RBASE和PARADOX就是这种情况。所有这一切并不太复杂，但它能啊作。然而现在，随着我们朝着更强的PC机和更复杂的网络操作系统发展，各公司开始采取新的方法。一个很大的差别是现在各公司把数据库看成两部分前端和数据库驱动系统。前端控制数据如何在屏幕上显示，用户如何与数据打交道；数据库驱动系统实际上保留数据和执行诸如查询、排序和索引等操作。这就是常常所说的客户/服务器方式，以各台个人机作为集中式数据库服务器的客户。“客户/服务器”意味着这个产品的运行在本质上是与网络相连的。SQL服务器是软件数据库“驱动系统”，它可以在网络服务器、专用网络工作站或者运行OS/2的非专用工作站上工作。这个机器完成所谓的“后端处理功能”，其中包括管理交易、安全、排序和读写数据。客户/服务器结构背后的思想是把处理负荷分散到适合它的地方。例如，服务器承担处理适用于每个助记词的任务（安全、存取共同数据等）。前端应用程序对用户接口来说可合性能最佳。这些应用的处理是在需要据处当地进行。这样，无用的网络信息流量减少了。这就是说随着使用SQL服务器的人数增加，造成的网络性能下降较小。一家市场研究公司说“随着符合这种新的客户/服务器结构产品的推出，供应商们将争夺市场份额，而用户可能旁观最初涌现的产品。”IBM推出了OS/2扩展版，它给OS/2增加了IBM自己的数据管理程序，本质上它是流行的大型机数据库DB2的微型机版本。ASHTONTATE公司各MICROSOFT公司开始交付它们的数据库服务器SQLSERVER,这是以SYBASE公司一种数据库驱动系统为基础的。这两具产品能以几种方法被寻址用一种语言（如C）写程序，采用SQL发送数据库查询，或者使用另一个起前端作用的数据库和程序。就它们本身来说，这些软件包不是作为前端或者另一种数据库编程语言设计的。ORACLE公司已开始使其数据库按同一方法发展，以作为一个由多种前端访问的数据库驱动系统。ORACLE已经有可供123、DBXL和MYPERCARD使用的接口。其他起始于小型机世界的数据库公司都采取类似的方法。MICRORIM公司已提议一个变种，即代号为STLAS的新产品系列，它被设计成能管理和协调以多种其它格式存放的数据、其它数据库服务器保持的数据和包括RBASE、DBASE和OS/2扩展版以及ORACLE、INGRES、INFORMIX和UNIFY等多数小型机数据库中的数据。ATLAS驱动系统也被设计成能通过多个前端进行访问（MICRORIM将前端称作”表面”）。在这样的环境中，数据库驱动系统（可以是OS/2扩展版、SQLSERVER、ORACLE、GUPTA公司的SQLBASE或者完全不现的其它产品）留驻在中央数据库服务器，通常是专用于此目的的计算机和硬磁盘。各个最终用户就能使用各种不同的前端或者用各种其它数据库语言开发的应用程序。各个前端如何与各个数据库驱动系统通信是有差别的。多数产品采用SQL的某个变型，SQL是通过IBM的DB2流行起来的数据库查询语言。有些产品将发送实际的SQL查询，有些将通过API（应用程序接口）进行通信，有些用服务器自有的程序发送信息。但这不应影响最终用户所做的事数据库驱动系统看管数据，前端管理查询和显示输出。如果前端写的正确，数据在任何情况都应可靠。然而，通信方法可能影响检索数据的速度。有些重要的事情要提醒。首先，这些前端还一个都没有上市。但很清楚，这些前端将会存在，可能在明年年底大量出现。其次，尽管有这些前端可以与数据库服务器相连这样一个事实，但多数前端将包含它们自己的本地数据库驱动系统。它们在用户工作站上工作。这是因为你不需要把所有的信息储存在共享数据库中（出于安全或只是出于合理），并且由于不是所有的用户都将与网络相连。现代数据字典在各级管理人员作决策时都需要完整