计算机专业外语(霍宏涛版)DistributedDatabase翻译.doc

Distributed databaseA distributed database is a database in which portions of the database are stored on multiple computers within a network.( 分布式数据库是一个把数据库的各个部分存放于网络上的多个不同计算机的数据库。)Users have access to the portion of the database at their location so that they can access the data relevant to their tasks without interfering with the work of others.( 用户只访问一部分位置的数据库,这样就可以访问和他们的任务相关的数据而不干扰别人的工作。)A distributed database system consists of a collection of sites ,connected together via some kind of communications network, in which（分布式数据库系统一个通过某种通信网络连接在一起的网站的集合）:(1).Each site is a full data base system site in its own right, but(每个节点是一个自成体系的数据库系统)(2).The sites have agreed to work together so that a user at any site can access data anywhere in the network exactly as if the data were all stored at the users own site.( 网站已经连接在一起工作, 以便在任何节点的用户都可以访问在网络任意地方的数据就像存储在用户自己的站点上一样。)It follows that a “distributed database” is really a kind of virtual database, whose component parts are physically stored in a number of distinct “real” databases at a number of distinct sites ( in effect , it is the logical union of those real databases).an example is shown in Figure 6.2.1.( 由此可见,一个“分布式数据库”真的是一种虚拟数据库,其组成部分物理上存储在许多不同的网站上“真正的”的数据库中 (实际上,它是逻辑连接了那些数据库)Note that, to repeat, each site is a database system site in its own right. (注意,重复一下,每个站点本身就是一个数据库系统网站。)in other words, each site ha its own local “real” databases ,its own local users ,its own local DBMS and transaction management software (including its own local locking ,logging ,recovery ,etc ,software),and its own local data communications manager(DC manager)( 换句话说,每个站点都有自己本地“真正的”数据库,自己的本地用户,自己的本地数据库和事务管理软件(包括其自己的本地锁定,日志记录、恢复等软件),和自己的本地数据通信管理器(DC manager)。).In particular ,a given user can perform operations on data at that users own local site exactly as if that site did not participate in the distributed system at all (at least, such a capability is an objective)( 特别是一个给定的用户可以对数据执行操作就像用户对的本地站点一样,就像网站根本没有存在于分布式系统上一样(至少,这种能力是客观的)。).The overall distributed system can thus be regarded as a kind of partnership among the individual local DBMSs at the individual local sites; a new software component at each sitelogically an extension of the local DBNSprovides the necessary partnership functionality ,and it is the combination of these new components together with the existing DBMSs that constitutes what is usually called the distribute database management system.( 整个分布式系统可以被视为由独立的本地节点数据库管理系统组成的一种合作关系；每个节点的新软件组件逻辑上是本地数据库的延伸提供了必要的合作功能，这些新组件连同现有数据库系统，同城被称为分布式数据库管理系统)Incidentally ,it is common to assume that the component sites are physically dispersedpossibly in fact geographically dispersed also ,as suggested by Figure 6.1.1although actually it is sufficient that they be dispersed logically. (顺便说一句,通常假定组件站点在物理上是分散的同时，实际上他们在逻辑上也是分散的)Two “sites” might even coexist on the same physical machine(especially during initial system testing )( 两个“网站”甚至可能共存于同一物理机器(尤其是在最初的系统测试中)。).In fact ,the emphasis in distributed systems has shifted back and forth over time; the earliest research tended to assume geographic distribution ,but most of the first few commercial installations involved local distribution instead, with (e.g.) several “sites” all in the same building and connected together by means of a local area network (LAN).( 事实上,分布式系统的重点已经随着时间的推移,最早的研究倾向于认为地理分布,但是大部分的几个商业巨头设施都涉及当地位置分布相反, (例如)几个“网站”都在同一座楼里相互连接在一起就是一个局域网(LAN)。)More recently ,however, the dramatic proliferation of wide area networks(WANs) has revived interested in the geographically distributed case . (然而，最近戏剧性的广域网络(广域网)已经重新扩散了感兴趣的地理分布情况。)Either way , it makes little differenceessentially the same problems have to be solved (at least ,this is true from a database perspective).( 无论哪种方式,它只有一点点的不同，而相同的问题以及解决(至少,这是真正的从数据库的角度来看)。)Why are distributed databases desirable ? (为什么分布式数据库可取的?)The basic answer to this question is that enterprises are usually distributed already , at least logically (into divisions , departments, workgroups, etc. ),and very likely physically too (into plants ,factories ,laboratories ,etc.)from which it follows that data is usually distributed already as well, because each organizational unit within the enterprise will naturally maintain data that is relevant to its own operations. (基本的答案是,企业通常是分布式的,至少在逻辑上(到部门,科室、工作组等),和很有可能物理上(到工厂、工厂、实验室等)数据通常是分布式,因为每个组织单元内的企业自然会保持与自己的业务相关的数据。)The total information asset of the enterprise is thus splintered into what are sometimes called islands of information.( 企业的总资产信息因此分裂成有时被称为信息的岛屿。) And what a distributed system does is provide the necessary “bridges” to connect those islands together. (分布式系统所做的就是为这些岛屿提供必要的“桥梁”,使这些岛屿连接在一起。)In other words , it enables structure of the database to mirror the structure of the enterpriselocal data can be kept locally, where it most logically belongs while at the same time remote data can be accessed when necessary.( 换句话说,它使数据库能反映企业的结构本地的数据可以保存在本地,大多数逻辑上属于本地而同时在必要时可以访问远程数据。)However , we should mention that there are some disadvantages too, of which the biggest is the fact that distribute systems are complex, at least from a technical point of view.( 然而,我们也应该提到有一些缺点,其中最大的是分步系统是复杂的,至少从技术的观点上来说。) Ideally, that complexity should be the implementers problem, not the users, but it is likely to be pragmaticthat some aspects of it will show through to users, unless very careful precautions are taken.( 理想情况下,这种复杂性应该是开发者的问题,不是用户的,实际上但它很可能会把的某些方面的问题显示到用户上,除非做好非常谨慎的预防措施。)If data is distributed but all servers run the same DBMS software, we have a homogeneous distributed database system. If different sites run under the control of different DBMSs, essentially autonomously, and are connected somehow to enable access to data from multiple sites, we have a heterogeneous distributed database system, also referred to as multidatabase system.( 如果数据是分布式的，但是所有的服务器运行相同的数据库管理系统软件，我们称之为同构分布式数据库系统。如果不同的节点分别为不同的数据库管理系统软件所控制