分布式DBMS的概念與設計計算機等專業(yè)畢業(yè)設計（論文）外文翻譯

1、大連交通大學2008屆本科生畢業(yè)設計(論文)外文翻譯The Concepts and Design of Distributed DBMS1. INTRODUCTION A major behind the development of database systems is the desire to integrate the operational data of an organization and to provide controlled access to the data. Although integration and controlled access may impl

2、y centralization, this is not the intention. In fact, the development of computer networks promotes a decentralized mode of work. This decentralized approach mirrors the organizational structure of many companies, which are logically distributed into divisions, departments, projects, and so on, and

3、physically distributed into offices, plants, factories, where each nit maintains its own operational data. The shareability of the data and the efficiency of data access should be improved by the development of a distributed database system that reflects this organizational structure, makes the data

4、 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 isl

5、ands. This may be a result of geographical separation, incompatible computer architectures, incompatible communication protocols, and so on. Integrating the databases into a logical whole may prevent this way of thinking. 2Concepts To start the discussion of distributed DBMSs, we first give a defini

6、tion of a distributed database. Distributed database: a logically interrelated collection of shared data physically distributed over a computer network. Following on from this we have the definition of distributed DBMS. Distributed DBMS: the software system that permits the management of the distrib

7、uted 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 computers under the control of a separate DBMS, with the computers connected b

8、y a communications 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 computers in the network.Users access the distributed database via application. Applications are classified as those tha

9、t 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;The data is split into a number of fragments;Fragments

10、 may be replicated;Fragments/replicas are allocated to sites;The sites are linked by a communications 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 defi

11、nition 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 computers and perhaps replicated, should be hidden from the user. The objective of transparency is to make

12、 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. Unfortunat

13、ely, 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

14、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 clients whoown or wish to rent out these properties. T

15、he staff at a branch office will make local inquiries of the databases. The company 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

16、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

17、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 computer failure terminates the operations of the DBMS

18、. However, a failure at one site of a DBMS, or a failure of a communication 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 fai

19、led nodes requests to another site.Improved reliabilityAs data may be replicated so that it exists at more than one site, the failure of a node or a communication link does not necessarily make the data inaccessible.Improved performance As the data is located near the site of greatest demand, and gi

20、ven 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 b

21、y a centralized DBMS.EconomicsIn the 1960s,computing 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 Grouchs Law. However, it is now generally accepted that it costs much less to create a system of

22、 smaller computers with the equivalent power of a single large computer. This makes it more cost-effective for corporate divisions and departments to obtain separate computers. It is also much more cost-effective to add workstations to a network than to update a mainframe system.The second potential

23、 cost saving occurs where databases are geographically remote and the applications require access to distributed data. In such cases, owing to the relative expense of data being transmitted across the network as opposed to the cost of local access, it may be much more economical to partition the app

24、lication and perform the processing locally at each site.Modular growthIn a distributed environment, it is much easier to handle expansion. New sites can be added to the network without affecting the operations of other sites. This flexibility allows an organization to expand relatively easily. Incr

25、easing database size can usually be handled by adding processing and storage power to the network. In a centralized DBMS, a growth may entail changes to both hardware and software.ComplexityA distributed DBMS that hides the distributed nature form the user and provides an acceptable level of perform

26、ance, reliability is inherently more complex than a centralized DBMS. The fact that data replication adequately, there will be degradation in availability, reliability, and performance compared with the centralized system, and the advantages we cited above will become disvantages.CostIncreased compl

27、exity means that we can expect the procurement and maintenance costs for a DDBMS to be higher than those for a centralized DBMS. Furthermore, a distributed DBMS requires additional hardware to establish a network between sites. These are ongoing communication costs incurred with the use of this netw

28、ork. There are also additional labor costs to manage and maintain the local DBMSs and the underlying network.SecurityIn a centralized system, access to the data can be easily controlled. However, in a distributed DBMS not only does access to replicated data have to be controlled in multiple location

29、s, but the network itself has to be made secure. In the past, networks were regarded as an insecure communication medium. Although this is still partially true, significant developments have been made to make network more secure.Integrity control more difficultDatabase integrity refers to the validi

30、ty and consistency of stored data. Integrity is usually expressed in terms of constraints, which are consistency rules that the database is not permitted to violate. Enforcing integrity constraints generally requires access to a large amount of data that defines the constraint but which is not invol

31、ved in the actual update operation itself. In a distributed DBMS, the communication and processing costs that are required to enforce integrity constraints may be prohibitive. We return to this problem in Section Lack of standards Although distributed DBMSs depend on effective communication, we are

32、only now starting to see the appearance of standard communication and data access protocols. This lack of standards has significantly limited the potential of distributed DBMSs. There are also no tools or methodologies to help users convert a centralized DBMS into a distributed DBMS.Lack of experien

33、ceGeneral-purpose distributed DBMSs have not been widely accepted, although many of the protocols and problems are well understood. Consequently, we do not yet have the same level of experience in industry as we have with centralized DBMSs, For a prospective adopter of this technology, this may be a

34、 significant deterrent.Database design more complexBesides the normal difficulties of designing a centralized database, the design of a distributed database has to take account of fragmentation of data, allocation of fragments to specific sites, and data replication. 分布式DBMS的概念與設計1 介紹推動數據庫系統發(fā)展的一個主要因

35、素是，人們希望將一個企業(yè)的操作數據綜合起來并能提供對數據的受控訪問，經管綜合和受控訪問意味著集中管理，但這并不是目的。實際上，計算機網絡的發(fā)展促進了分散式的作業(yè)模式。這種分散的方式反映了許多公司的組織結果：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，每一個單元都維護著自己的操作數據。數據的共享和數據訪問效率的提高以來與分布式數據庫系統的發(fā)展，分布式數據庫反映了上述組織結構，使得每個單元的數據都是可訪問的，并將最近的數據存放于最常用的位置。分布式DBMS有助于結局信息孤島的問題，有時，數據庫被看做是孤立的，不可訪問的電子島，就像遙遠的島嶼一樣。這可能是由于地理分割，計

36、算機體系結構不兼容或通信協議不兼容原因造成的。把數據庫綜合成一個邏輯整體可能會改變這種思考的方式。2. 概念在開始討論分布式DBMS之前，先給出分布式數據庫的定義。分布式數據庫：物理上分布于計算機網絡中，但在邏輯上相關的共享的集合。由此可以得到分布式DBMS的定義。分布式DBMS：管理分布式數據庫并使分布性對用戶透明的軟件系統。分布式數據庫管理系統是由一個被氛圍多段的邏輯數據庫構成的。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絡互聯起來的計算機上。每一個結點都可以獨立的處理用戶訪問本地數據的請求，并且也可以處理網絡上其他計算機存儲的數據。用戶是通過聽用來訪問分布市數據庫的，

37、應用又可氛圍不需要從其他結點獲得數據的應用和確實需要從其他結點獲得數據的應用，一般要求DDBMS至少包含一個全局應用。因此，DDBMS應當具有如下特征：l 邏輯上相關的共享數據的集合。l 數據是分段的。l 段是可以復制的。l 段/副本是分配在各個結點上的。l 結點是用通信網絡連接起來的。l 每個結點的數據都由DBMS控制，l 每個結點的DBMS都能自主地處理本地應用。l 每個DBMS至少參與一個全局應用。從DDBMS的定義中可以看出，系統期望分布性對于用戶來講是透明的。這樣，用戶就不需要知道分布式數據庫是分段的，存儲在多個不同計算機上，并且可能被復制等細節(jié)。透明性的目的就是要使用戶使用分布式系

38、統如同使用集中式系統一樣。這常常被稱為DDBMS的基本原則。這個要求為終端用戶提供了強大的功能。DDBMS的優(yōu)缺點數據分布以及應用分布比傳統的集中式數據庫具有潛在的優(yōu)勢，但也存在著不足。優(yōu)點：反映了組織結構許多組織都是自然的分布與各個地方。例如，DreamHome在許多城市都設有分支機構。于是該應用的數據庫將很自然的分布于這些不同的地方。DreamHome的每個分支機構都有一個數據庫用來記錄該機構的職員信息，出租房產信息以及房產所有者的信息。本地的職員可以在本地數據庫上進行本地查詢，而公司的高層則可以訪問所有分支機構的任何數據，進行全局查詢。改進了共享性和本地自主權數據的分布可以反映出一個組織

39、在地理上的分布，一個結點的用戶可以訪問其他結點上的數據。數據會存放在靠近經常是同這些數據的用戶的結點上。這樣，擁護就可以對數據擁有本地控制權，從而可以建立和執(zhí)行關于使用這些數據的本地策略。全局數據管理員（DBA）對整個系統負責。通常也可以把責任部分地下放到本地級，所以本地DBA可以管理本地DBMS。改進了可用性對于集中式DBMS，計算機的一次故障會中斷所有的DBMS操作。然而，在DDBMS系統中，一個結點的故障或通信鏈路的故障只會使某些結點不能被訪問，但決不會中斷整個系統的操作。分布式DBMS就是設計用來使系統在這些故障發(fā)生時仍然可以繼續(xù)工作。即使一個結點出現故障，系統也可以把對故障結點的訪問請求重定向到其他結點。提高了可靠性因為數據可以被復制而存在與多個結點之上，所以一個結點的鼓掌或通信鏈路故障不會妨礙對該數據的訪問。改進了性能因為數據是防止在離“最大需求“最近的結點上的，而且由于分布式DBMS內在的并行機制，所以訪問分布式數據庫的速度肯定比訪問遠程集中式