【正文】 ance advantage over other database models. Today , such performance characteristics are only important in highvolume ,highspeed transaction processing such as automatic teller machine works or airline reservation system. Both hierarchical and work databases are application specific. If a new application is developed ,maintaining the consistency of databases in different applications can be very difficult. For example, suppose a new pension application is developed .The data are the same, but a new database must be created. Object Model The newest approach to database management uses an object model , in which records are represented by entities called objects that can both store data and provide methods or procedures to perform specific tasks. The query language used for the object model is the same objectoriented programming language used to develop the database application .This can create problems because there is no simple , uniform query language such as SQL . The object model is relatively new, and only a few examples of objectoriented database exist. It has attracted attention because developers who choose an objectoriented programming language want a database based on an objectoriented model. Distributed Database Similarly , a distributed database is one in which different parts of the database reside on physically separated puters . One goal of distributed databases is the access of information without regard to where the data might be stored. Keeping in mind that once the users and their data are separated , 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 揚州大學(xué)計算機專業(yè) 畢業(yè)設(shè)計 (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 qu