分布式dbms的概念與設(shè)計計算機(jī)等專業(yè)畢業(yè)設(shè)計（論文）外文翻譯-文庫吧
2025-01-03 13:35 本頁面
【正文】 number of fragments。Fragments may be replicated。Fragments/replicas are allocated to sites。The sites are linked by a munications network。The data at each site is under the control of a DBMS。The DBMS at each site can handle local applications, autonomously。Each DBMS participates in at least one global application。From the definition of the DDBMS, the system is expected to make the distribution transparent to the user. Thus, the fact that a distributed database is split tinto fragments that can be stored on different puters and perhaps replicated, should be hidden from the user. The objective of transparency is to make the distributed system appear like a centralized system. This is sometimes referred to as the fundamental principle of distributed DBMSs.Advantages and Disadvantages of DDBMSsThe distribution of data and applications has potential advantages over traditional centralized database systems. Unfortunately, there are also disadvantages. In this section, we review the advantages and disadvantages of the DDBMS.AdvantagesReflects organizational structureMany organizations are naturally distributed over several locations. For example, DreamHome has many officers in different cities. It is natural for databases used in such an application to be distributed over these locations. DreamHome may keep a database at each branch office containing details of such things as the staff who work at that location, the properties that are for rent, and the client’s whoown or wish to rent out these properties. The staff at a branch office will make local inquiries of the databases. The pany headquarters may wish to make global inquiries involving the access of data at all or a number of branches.Improved shareability and local autonomyThe geographical distribution of an organization can be reflected in the distribution of the data。 users at one site can access data stored at other sties. Data can be placed at the site close to the users who normally use that data. In this way, users have local control of the data, and they can consequently establish and enforce local policies regarding the use of this data. A global database administrator is responsible for the entire system. Generally, part of this responsibility is devolved to the local level, so that the local DBA can manage the local DBMS.Improved availabilityIn a centralized DBMS, a puter failure terminates the operations of the DBMS. However, a failure at one site of a DBMS, or a failure of a munication link making some sites inaccessible, does not make the entire system inoperable. Distributed DBMSs are designed to continue to function despit
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