畢業(yè)設(shè)計(論文)外文翻譯----分布式dbms的概念與設(shè)計-其他專業(yè)-wenkub.com
2025-01-15 05:04 本頁面
【正文】 這些對于未來用戶來講是個很大的障礙。這些標準的缺乏嚴重的限制了分布式 DBMS 的發(fā)展?jié)摿Α？捎眯酝ǔＳ靡幌盗幸恢滦缘募s束條件來表述，數(shù)據(jù)庫不能違反這些規(guī)則。 安全性低 在集中式系統(tǒng)匯總，對數(shù)據(jù)的訪問是很容易控制的。 成本高 復(fù)雜性的增加就以為著獲得和維護 DDBMS 的成本會里集中式 DBMS 更高。 缺點 ： 復(fù)雜性高 分布式 DBMS 需要對用戶隱藏它分布式的本質(zhì)，且要給用戶提供令人滿意的性能，可靠性和可用性，這就注定了它比集中式 DBMS 更加復(fù)雜。 模塊化增長 在分布式環(huán)境匯總，擴展顯得更加容易一些。這將使各個合作部門配備 8 獨立的計算機更為經(jīng)濟。而且，由于每個結(jié)點只處理整個數(shù)據(jù)庫系統(tǒng)的部分工作，所以不會出現(xiàn)像集中式 DBMS 系統(tǒng)中 CPU服務(wù)和 I/O 服務(wù)之間的激烈競爭。分布式 DBMS 就是設(shè)計用來使系統(tǒng)在這些故障發(fā)生時仍然可以繼續(xù)工作。全局數(shù)據(jù)管理員（ DBA）對整個系統(tǒng)負責(zé)。本地的職員可以在本地數(shù)據(jù)庫上進行本地查詢，而公司的高層則可以訪問所有分支機構(gòu)的任何數(shù)據(jù)，進行全局查詢。 優(yōu)點 ： 反映了組織結(jié)構(gòu) 許多組織都是自然的分布與各個地方。透明性的目的就是要使用戶使用分布式系統(tǒng)如同使用集中式系統(tǒng)一樣。 ? 每個結(jié)點的數(shù)據(jù)都由 DBMS 控制， ? 每個結(jié)點的 DBMS 都能自主地處理本地應(yīng)用。 ? 數(shù)據(jù)是分段的。每個段在獨立的DBMS 的控制下，可以存儲在一個或多個通過通信網(wǎng)絡(luò)互聯(lián)起來的計算機上。 分布式數(shù)據(jù)庫：物理上分布于計算機網(wǎng)絡(luò)中，但在邏輯上相關(guān)的共享的集合。 分布式 DBMS 有助于結(jié)局信息孤島的問題，有時，數(shù)據(jù)庫被看做是孤立的，不可訪問的電子島，就像遙遠的島嶼一樣。 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 puter
高考資料相關(guān)推薦
鄂ICP備17016276號-1