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-預(yù)覽頁
【正文】 em that reflects this organizational structure, makes the data in all units accessible, and stores data proximate to the location where it is most frequently used. Distributed DBMSs should help resolve the islands of information problem. Databases are sometimes regarded, as electronic islands that are distinct and generally inaccessible places, like remote islands. This may be a result of geographical separation, inpatible puter architectures, inpatible munication protocols, and so on. Integrating the databases into a logical whole may prevent this way of thinking. 2．Concepts To start the discussion of distributed DBMSs, we first give a definition of a distributed database. Distributed database: a logically interrelated collection of shared data physically distributed over a puter network. Following on from this we have the definition of distributed DBMS. Distributed DBMS: the software system that permits the management of the distributed database and makes the distribution transparent to users.A distributed database management system consists of a single logical database that is split into a number of fragments. Each fragment is stored on one or more puters under the control of a separate DBMS, with the puters connected by a munications network. 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 network.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 following characteristics:A collection of logically related shared data。Fragments/replicas are allocated to sites。Each DBMS participates in at least one global application。這種分散的方式反映了許多公司的組織結(jié)果：在邏輯上分成多個分公司，部門，項目等，而在體制上分為辦公室，車間，工廠，每一個單元都維護(hù)著自己的操作數(shù)據(jù)。把數(shù)據(jù)庫綜合成一個邏輯整體可能會改變這種思考的方式。分布式DBMS：管理分布式數(shù)據(jù)庫并使分布性對用戶透明的軟件系統(tǒng)。用戶是通過聽用來訪問分布市數(shù)據(jù)庫的，應(yīng)用又可氛圍不需要從其他結(jié)點獲得數(shù)據(jù)的應(yīng)用和確實需要從其他結(jié)點獲得數(shù)據(jù)的應(yīng)用，一般要求DDBMS至少包含一個全局應(yīng)用。l 段/副本是分配在各個結(jié)點上的。從DDBMS的定義中可以看出，系統(tǒng)期望分布性對于用戶來講是透明的。這個要求為終端用戶提供了強(qiáng)大的功能。于是該應(yīng)用的數(shù)據(jù)庫將很自然的分布于這些不同的地方。數(shù)據(jù)會存放在靠近經(jīng)常是同這些數(shù)據(jù)的用戶的結(jié)點上。改進(jìn)了可用性對于集中式DBMS，計算機(jī)的一次故障會中斷所有的DBMS操作。提高了可靠性因為數(shù)據(jù)可以被復(fù)制而存在與多個結(jié)點之上，所以一個結(jié)點的鼓掌或通信鏈路故障不會妨礙對該數(shù)據(jù)的訪問。這就是著名的Grosch定律。當(dāng)數(shù)據(jù)庫地理上是遠(yuǎn)程的，并且應(yīng)用要求訪問分布式數(shù)據(jù)時，也潛在地存在著第二種節(jié)約開銷方式?？梢酝ㄟ^增強(qiáng)系統(tǒng)的處理和存儲能力來適應(yīng)日益增長的數(shù)據(jù)庫規(guī)模的需要。如果軟件不能完善的處理數(shù)據(jù)復(fù)制，那么分布式DBMS的可用性，可靠性和性能相對起集中式DBMS而言就會降低。而且，分布式DBMS需要額外的硬件設(shè)備來維持網(wǎng)絡(luò)統(tǒng)通信。過去往往認(rèn)為網(wǎng)絡(luò)是不安全的通信媒介，雖然現(xiàn)在在某種程度上這還是正確的，但是現(xiàn)在網(wǎng)絡(luò)的安全性已經(jīng)大大改善了。在分布式DBMS中，完整性約束對通信和處理開銷的要求使得它幾乎不可能實現(xiàn)。缺乏經(jīng)驗盡管人們已經(jīng)對多用途的分布式DBMS的協(xié)議以及相關(guān)問題理解得很透徹，但多用途的分布式DBMS仍然沒有被廣泛的接受。9
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