【正文】 rated , the munication and working concepts e into play . Distributed databases require software that resides partially in the larger puter. This software bridges the gap between personal and large puters and resolves the problems of inpatible data 揚(yáng)州大學(xué)計(jì)算機(jī)專業(yè) 畢業(yè)設(shè)計(jì) (外文翻譯 ) 4 formats. Ideally, it would make the mainframe databases appear to be large libraries of information, with most of the processing acplished on the personal puter. A drawback to some distributed systems is that they are often based on what is called a mainframeentire model , in which the larger host puter is seen as the master and the terminal or personal puter is seen as a slave. There are some advantages to this approach . With databases under centralized control , many of the problems of data integrity that we mentioned earlier are solved . But today’s personal puters, departmental puters, and distributed processing require puters and their applications to municate with each other on a more equal or peertopeer basis. In a database, the client/server model provides the framework for distributing databases. One way to take advantage of many connected puters running database applications is to distribute the application into cooperating parts that are independent of one anther. A client is an end user or puter program that requests resources across a work. A server is a puter running software that fulfills those requests across a work . When the resources are data in a database ,the client/server model provides the framework for distributing database. A file serve is software that provides access to files across a work. A dedicated file server is a single puter dedicated to being a file server. This is useful ,for example ,if the files are large and require fast access .In such cases, a miniputer or mainframe would be used as a file server. A distributed file server spreads the files around on individual puters instead of placing them on one dedicated puter. Advantages of the latter server include the ability to store and retrieve files on other puters and the elimination of duplicate files on each puter. A major disadvantage , however, is that individual read/write requests are being moved across the work and problems can arise when updating files. Suppose a user requests a record from a file and changes it while another user requests the same record and changes it too. The solution to this problems called record locking, which means that the first request makes others requests wait until the first request is satisfied . Other users may be able to read the record, but they will not be able to change it . A database server is software that services requests to a database across a work. For example, suppose a user types in a query for data on his or her personal puter . If the application is designed with the client/server model in mind ,the query language part on the personal puter simple sends the query across the work to the database server and requests to be notified when the data are found. Examples of distributed database systems can be found in the engineering world. Sun’s Network Filing System(NFS),for example, is used in puteraided engineering applications to distribute data among the hard disks in a work of Sun workstation. Distributing databases is an evolutionary step because it is logical that data should exist at the 揚(yáng)州大學(xué)計(jì)算機(jī)專業(yè) 畢業(yè)設(shè)計(jì) (外文翻譯 ) 5 location where they are being used . Departmental puters within a large corporation ,for example, should have data reside locally , yet those data should be accessible by authorized corporate management when they want to consolidate departmental data . DBMS software will protect the security and integrity of the database , and the distributed database will appear to its users as no different from the nondistributed database . In this information age, the data server has bee the heart of a pany. This one piece of software controls the rhythm of most organizations and is used to pump information lifeblood through the arteries of the work. Because of the critical nature of this application, the data server is also the one of the most popular targets for hackers. If a hacker owns this application, he can cause the pany39。(3)and an interface between the DBMS and its users. Several major trends are emerging that enhance the value and usefulness of database management systems。揚(yáng)州大學(xué)計(jì)算機(jī)專業(yè) 畢業(yè)設(shè)計(jì) (外文翻譯 ) 1 外文資料