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)

1、<p>　　The Concepts and Design of Distributed DBMS</p><p>　　1. INTRODUCTION</p><p>　　A major behind the development of database systems is the desire to integrate the operational data of an

2、organization and to provide controlled access to the data. Although integration and controlled access may imply centralization, this is not the intention. In fact, the development of computer networks promotes a decentra

3、lized mode of work. This decentralized approach mirrors the organizational structure of many companies, which are logically distributed into divisions, departments, projects, </p><p>　　Distributed DBMSs shou

4、ld 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 s

5、eparation, incompatible computer architectures, incompatible communication protocols, and so on. Integrating the databases into a logical whole may prevent this way of thinking. </p><p>　　2．Concepts</p>

6、;<p>　　To start the discussion of distributed DBMSs, we first give a definition of a distributed database.</p><p>　　Distributed database: a logically interrelated collection of shared data physically

7、distributed over a computer network.</p><p>　　Following on from this we have the definition of distributed DBMS.</p><p>　　Distributed DBMS: the software system that permits the management of the

8、 distributed database and makes the distribution transparent to users.</p><p>　　A distributed database management system consists of a single logical database that is split into a number of fragments. Each f

9、ragment is stored on one or more computers under the control of a separate DBMS, with the computers connected by a communications network. Each site is capable of independently processing user requests that require acces

10、s to local data and is also capable of processing data stored on other computers in the network.</p><p>　　Users access the distributed database via application. Applications are classified as those that do n

11、ot 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:</p><p>　　A c

12、ollection of logically related shared data;</p><p>　　The data is split into a number of fragments;</p><p>　　Fragments may be replicated;</p><p>　　Fragments/replicas are allocated to

13、 sites;</p><p>　　The sites are linked by a communications network;</p><p>　　The data at each site is under the control of a DBMS;</p><p>　　The DBMS at each site can handle local app

14、lications, autonomously;</p><p>　　Each DBMS participates in at least one global application;</p><p>　　From the definition of the DDBMS, the system is expected to make the distribution transparen

15、t 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 mak

16、e the distributed system appear like a centralized system. This is sometimes referred to as the fundamental principle of distributed DBMSs.</p><p>　　Advantages and Disadvantages of DDBMSs</p><p>

17、;　　The 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 disadvantage

18、s of the DDBMS.</p><p>　　Advantages</p><p>　　Reflects organizational structure</p><p>　　Many organizations are naturally distributed over several locations. For example, DreamHome h

19、as 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

20、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 inquir</p><p>　　Improved s

21、hareability and local autonomy</p><p>　　The 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

22、 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 globa

23、l database administrator is responsible for the entire system. Generally, part of this responsibility is devolved</p><p>　　Improved availability</p><p>　　In a centralized DBMS, a computer failur

24、e terminates the operations of the DBMS. 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 de

25、signed 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.</p><p>　　Improved reliability</p><p>　

26、　As 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.</p><p>　　Improved performance </p>&l

27、t;p>　　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.

28、 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.</p><p><b>　　Economics<

29、/b></p><p>　　In 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 Grouch’s Law. How

30、ever, it is now generally accepted that it costs much less to create a system of smaller computers with the equivalent power of a single large computer. This makes it more cost-effective for corporate divisions and depar

31、tments to obtain separate computers. It is also much more cost-effective to add workstati</p><p>　　The second potential cost saving occurs where databases are geographically remote and the applications requi

32、re 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 application

33、 and perform the processing locally at each site.</p><p>　　Modular growth</p><p>　　In a distributed environment, it is much easier to handle expansion. New sites can be added to the network with

34、out affecting the operations of other sites. This flexibility allows an organization to expand relatively easily. Increasing database size can usually be handled by adding processing and storage power to the network. In

35、a centralized DBMS, a growth may entail changes to both hardware and software.</p><p>　　Complexity</p><p>　　A distributed DBMS that hides the distributed nature form the user and provides an acc

36、eptable level of performance, 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 wi

37、th the centralized system, and the advantages we cited above will become disvantages.</p><p><b>　　Cost</b></p><p>　　Increased complexity means that we can expect the procurement and

38、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 incur

39、red with the use of this network. There are also additional labor costs to manage and maintain the local DBMSs and the underlying network.</p><p><b>　　Security</b></p><p>　　In a cent

40、ralized 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 locations, but the network itself has to be made secur

41、e. 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.</p><p>　　Integrity cont

42、rol more difficult</p><p>　　Database integrity refers to the validity and consistency of stored data. Integrity is usually expressed in terms of constraints, which are consistency rules that the database is

43、not permitted to violate. Enforcing integrity constraints generally requires access to a large amount of data that defines the constraint but which is not involved in the actual update operation itself. In a distributed

44、DBMS, the communication and processing costs that are required to enforce integrity constraints may b</p><p>　　Lack of standards </p><p>　　Although distributed DBMSs depend on effective communic

45、ation, we are 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 m

46、ethodologies to help users convert a centralized DBMS into a distributed DBMS.</p><p>　　Lack of experience</p><p>　　General-purpose distributed DBMSs have not been widely accepted, although many

47、 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

48、 significant deterrent.</p><p>　　Database design more complex</p><p>　　Besides the normal difficulties of designing a centralized database, the design of a distributed database has to take accou

49、nt of fragmentation of data, allocation of fragments to specific sites, and data replication.</p><p>　　分布式DBMS的概念與設(shè)計(jì)</p><p><b>　　1 介紹</b></p><p>　　推動數(shù)據(jù)庫系統(tǒng)發(fā)展的一個(gè)主要因素是，人們希望

50、將一個(gè)企業(yè)的操作數(shù)據(jù)綜合起來并能提供對數(shù)據(jù)的受控訪問，經(jīng)管綜合和受控訪問意味著集中管理，但這并不是目的。實(shí)際上，計(jì)算機(jī)網(wǎng)絡(luò)的發(fā)展促進(jìn)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(jié)果：在邏輯上分成多個(gè)分公司，部門，項(xiàng)目等，而在體制上分為辦公室，車間，工廠，每一個(gè)單元都維護(hù)著自己的操作數(shù)據(jù)。數(shù)據(jù)的共享和數(shù)據(jù)訪問效率的提高以來與分布式數(shù)據(jù)庫系統(tǒng)的發(fā)展，分布式數(shù)據(jù)庫反映了上述組織結(jié)構(gòu)，使得每個(gè)單元的數(shù)據(jù)都是可訪問的，并將最近的數(shù)據(jù)存放

51、于最常用的位置。</p><p>　　分布式DBMS有助于結(jié)局信息孤島的問題，有時(shí)，數(shù)據(jù)庫被看做是孤立的，不可訪問的電子島，就像遙遠(yuǎn)的島嶼一樣。這可能是由于地理分割，計(jì)算機(jī)體系結(jié)構(gòu)不兼容或通信協(xié)議不兼容原因造成的。把數(shù)據(jù)庫綜合成一個(gè)邏輯整體可能會改變這種思考的方式。</p><p><b>　　2. 概念</b></p><p>　　在開始討論

52、分布式DBMS之前，先給出分布式數(shù)據(jù)庫的定義。</p><p>　　分布式數(shù)據(jù)庫：物理上分布于計(jì)算機(jī)網(wǎng)絡(luò)中，但在邏輯上相關(guān)的共享的集合。</p><p>　　由此可以得到分布式DBMS的定義。</p><p>　　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)。</p><p>　　分布式數(shù)據(jù)庫管理系統(tǒng)是由一個(gè)被氛圍多段的邏輯

53、數(shù)據(jù)庫構(gòu)成的。每個(gè)段在獨(dú)立的DBMS的控制下，可以存儲在一個(gè)或多個(gè)通過通信網(wǎng)絡(luò)互聯(lián)起來的計(jì)算機(jī)上。每一個(gè)結(jié)點(diǎn)都可以獨(dú)立的處理用戶訪問本地?cái)?shù)據(jù)的請求，并且也可以處理網(wǎng)絡(luò)上其他計(jì)算機(jī)存儲的數(shù)據(jù)。</p><p>　　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(diǎn)獲得數(shù)據(jù)的應(yīng)用和確實(shí)需要從其他結(jié)點(diǎn)獲得數(shù)據(jù)的應(yīng)用，一般要求DDBMS至少包含一個(gè)全局應(yīng)用。</p><p>　　因此

54、，DDBMS應(yīng)當(dāng)具有如下特征：</p><p>　　邏輯上相關(guān)的共享數(shù)據(jù)的集合。</p><p><b>　　數(shù)據(jù)是分段的。</b></p><p><b>　　段是可以復(fù)制的。</b></p><p>　　段/副本是分配在各個(gè)結(jié)點(diǎn)上的。</p><p>　　結(jié)點(diǎn)是用通信網(wǎng)絡(luò)

55、連接起來的。</p><p>　　每個(gè)結(jié)點(diǎn)的數(shù)據(jù)都由DBMS控制，</p><p>　　每個(gè)結(jié)點(diǎn)的DBMS都能自主地處理本地應(yīng)用。</p><p>　　每個(gè)DBMS至少參與一個(gè)全局應(yīng)用。</p><p>　　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。這樣，用戶就不需要知道分布式數(shù)據(jù)庫是分段的，存儲在多個(gè)不同計(jì)算機(jī)上，并

56、且可能被復(fù)制等細(xì)節(jié)。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。這常常被稱為DDBMS的基本原則。這個(gè)要求為終端用戶提供了強(qiáng)大的功能。</p><p><b>　　DDBMS的優(yōu)缺點(diǎn)</b></p><p>　　數(shù)據(jù)分布以及應(yīng)用分布比傳統(tǒng)的集中式數(shù)據(jù)庫具有潛在的優(yōu)勢，但也存在著不足。</p><p><b>　　優(yōu)點(diǎn)

57、：</b></p><p><b>　　反映了組織結(jié)構(gòu)</b></p><p>　　許多組織都是自然的分布與各個(gè)地方。例如，DreamHome在許多城市都設(shè)有分支機(jī)構(gòu)。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。DreamHome的每個(gè)分支機(jī)構(gòu)都有一個(gè)數(shù)據(jù)庫用來記錄該機(jī)構(gòu)的職員信息，出租房產(chǎn)信息以及房產(chǎn)所有者的信息。本地的職員可以在本地?cái)?shù)據(jù)庫上進(jìn)行本

58、地查詢，而公司的高層則可以訪問所有分支機(jī)構(gòu)的任何數(shù)據(jù)，進(jìn)行全局查詢。</p><p>　　改進(jìn)了共享性和本地自主權(quán)</p><p>　　數(shù)據(jù)的分布可以反映出一個(gè)組織在地理上的分布，一個(gè)結(jié)點(diǎn)的用戶可以訪問其他結(jié)點(diǎn)上的數(shù)據(jù)。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(diǎn)上。這樣，擁護(hù)就可以對數(shù)據(jù)擁有本地控制權(quán)，從而可以建立和執(zhí)行關(guān)于使用這些數(shù)據(jù)的本地策略。全局?jǐn)?shù)據(jù)管理員（DBA）對整個(gè)系統(tǒng)負(fù)責(zé)。

59、通常也可以把責(zé)任部分地下放到本地級，所以本地DBA可以管理本地DBMS。</p><p><b>　　改進(jìn)了可用性</b></p><p>　　對于集中式DBMS，計(jì)算機(jī)的一次故障會中斷所有的DBMS操作。然而，在DDBMS系統(tǒng)中，一個(gè)結(jié)點(diǎn)的故障或通信鏈路的故障只會使某些結(jié)點(diǎn)不能被訪問，但決不會中斷整個(gè)系統(tǒng)的操作。分布式DBMS就是設(shè)計(jì)用來使系統(tǒng)在這些故障發(fā)生時(shí)仍然可

60、以繼續(xù)工作。即使一個(gè)結(jié)點(diǎn)出現(xiàn)故障，系統(tǒng)也可以把對故障結(jié)點(diǎn)的訪問請求重定向到其他結(jié)點(diǎn)。</p><p><b>　　提高了可靠性</b></p><p>　　因?yàn)閿?shù)據(jù)可以被復(fù)制而存在與多個(gè)結(jié)點(diǎn)之上，所以一個(gè)結(jié)點(diǎn)的鼓掌或通信鏈路故障不會妨礙對該數(shù)據(jù)的訪問。</p><p><b>　　改進(jìn)了性能</b></p>

61、<p>　　因?yàn)閿?shù)據(jù)是防止在離“最大需求“最近的結(jié)點(diǎn)上的，而且由于分布式DBMS內(nèi)在的并行機(jī)制，所以訪問分布式數(shù)據(jù)庫的速度肯定比訪問遠(yuǎn)程集中式數(shù)據(jù)庫快。而且，由于每個(gè)結(jié)點(diǎn)只處理整個(gè)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。</p><p><b>　　節(jié)約開銷</b></p><p>　　20世紀(jì)60年代，

62、計(jì)算能力是通過設(shè)備趁本的平方來衡量的：3倍的成本能產(chǎn)生9倍的能力。這就是著名的Grosch定律。然而，現(xiàn)在通常認(rèn)為用相對低得多的成本建立的小型計(jì)算機(jī)系統(tǒng)就能夠獲得與大型計(jì)算機(jī)相當(dāng)?shù)挠?jì)算能力。這將使各個(gè)合作部門配備獨(dú)立的計(jì)算機(jī)更為經(jīng)濟(jì)。而且在網(wǎng)絡(luò)中添加一個(gè)工作站也會比升級大型計(jì)算機(jī)系統(tǒng)更加經(jīng)濟(jì)。</p><p>　　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(shí)，也潛在地存在著第二種節(jié)約開銷方式。在這種情況下

63、，在網(wǎng)絡(luò)中驚醒數(shù)據(jù)傳送需要的開銷比本地訪問的開銷更大，將應(yīng)用進(jìn)行劃分并且在每個(gè)結(jié)點(diǎn)中執(zhí)行開銷將更少。</p><p><b>　　模塊化增長</b></p><p>　　在分布式環(huán)境匯總，擴(kuò)展顯得更加容易一些。新結(jié)點(diǎn)可以添加到網(wǎng)絡(luò)中，而不會影響其他結(jié)點(diǎn)的操作，這種適應(yīng)性使得組織的擴(kuò)展相對同意一些。可以通過增強(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。集中式

64、DBMS的擴(kuò)展會使硬件和軟件都需要升級。</p><p><b>　　缺點(diǎn)：</b></p><p><b>　　復(fù)雜性高</b></p><p>　　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。而且數(shù)據(jù)的可復(fù)制性更增加了分布式DBMS的復(fù)

65、雜性。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。這是，上述的優(yōu)點(diǎn)就會全部變成缺點(diǎn)。</p><p><b>　　成本高</b></p><p>　　復(fù)雜性的增加就以為著獲得和維護(hù)DDBMS的成本會里集中式DBMS更高。而且分布式DBMS需要額外的硬件開銷以建立連接各個(gè)結(jié)點(diǎn)的網(wǎng)絡(luò)。而且，分布式DBMS需要額

66、外的硬件設(shè)備來維持網(wǎng)絡(luò)統(tǒng)通信。設(shè)置在管理和維護(hù)本地DBMS和地層網(wǎng)絡(luò)時(shí)也需要一定的人力開銷。</p><p><b>　　安全性低</b></p><p>　　在集中式系統(tǒng)匯總，對數(shù)據(jù)的訪問是很容易控制的。而在分布式DBMS中，不僅需要對各個(gè)接上復(fù)制數(shù)據(jù)的訪問驚醒控制，而且網(wǎng)絡(luò)本身也同樣如此。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)

67、在網(wǎng)絡(luò)的安全性已經(jīng)大大改善了。</p><p><b>　　更難以控制的完整性</b></p><p>　　數(shù)據(jù)庫的完整性就是指存儲數(shù)據(jù)的可用性和一致性?？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。強(qiáng)制的完整性約束需要訪問大量用來定義這些約束的數(shù)據(jù)，但這些數(shù)據(jù)在更新操作中并不是真正的涉及。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

68、幾乎不可能實(shí)現(xiàn)。</p><p><b>　　缺乏標(biāo)準(zhǔn)</b></p><p>　　分布式DBMS的實(shí)現(xiàn)基于有效的通信網(wǎng)絡(luò)的支持，但直到現(xiàn)在才逐漸出現(xiàn)了標(biāo)準(zhǔn)的通信和數(shù)據(jù)訪問協(xié)議。這些標(biāo)準(zhǔn)的缺乏嚴(yán)重的限制了分布式DBMS的發(fā)展?jié)摿Α６?，直到現(xiàn)在還沒有一種工具或方法能將集中式DBMS轉(zhuǎn)換為分布式DBMS。</p><p><b>　　缺

69、乏經(jīng)驗(yàn)</b></p><p>　　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。因此，在分布式DBMS方面所積累的工業(yè)經(jīng)驗(yàn)和集中式DBMS是不能相比的。這些對于未來用戶來講是個(gè)很大的障礙。</p><p>　　數(shù)據(jù)庫的設(shè)計(jì)更加復(fù)雜</p><p>　　分布式數(shù)據(jù)庫的設(shè)計(jì)除了要考慮集中式