畢業(yè)設(shè)計(jì)(論文)外文翻譯----分布式dbms的概念與設(shè)計(jì)-其他專業(yè)-展示頁
2025-01-31 05:04本頁面
【正文】 cations work. Each site is capable of independently processing user requests that require access to local data and is also capable of processing data stored on other puters in the work. Users access the distributed database via application. Applications are classified as those that do not require data from other sites and those that do require data from other sites. We require a DBMSs to have at least one global application. A DDBMS therefore has the 2 following characteristics: A collection of logically related shared data。 The data is split into a number of fragments。 Fragments/replicas are allocated to sites。 The data at each site is under the control of a DBMS。 Each DBMS participates in at least one global application。 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 3 responsibility is devolved to the local level, so that the local DBA can ma nage the local DBMS. Improved availability In 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 despite such failures. If a single node fails, the system may be able to reroute the failed node’s requests to another site. Improved reliability As data may be replicated so that it exists at more than one site, the failure of a node or a munication link does not necessarily make the data inaccessible. Improved performance As the data is located near the site of ‘greatest demand’, and given the inherent parallelism of distributed DBMSs, speed of database access may be better than that achievable form a remote centralized database. Furthermore, since each site handles only a part of the entire database, there may not be the same contention for CPU and I/O services as characterized by a centralized DBMS. Economics In the 1960s,puting power was calculated according to the square of the costs of the equipment: three times the cost would provide nine times the power. This was known as Grouch’s Law. However, it is now generally accepted that it costs much less to create a system of smaller puters with the equivalent power of a single large puter. This makes it more costeffective for corporate divisions and departments to obtain separate puters. It is also much more co
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