外卖平台的设计与制作外文翻译资料

英语原文共 1 页，剩余内容已隐藏，支付完成后下载完整资料

毕业设计(论文)外文翻译

2020 年 2 月 23 日

Database System Concepts

Silberschatz A, Korth H F, Sudarshan S

CHAPTER19

Distributed Databases

Unlike parallel systems, in which the processors are tightly coupled and constitute a single database system, a distributed database system consists of loosely coupled sites that share no physical components. Furthermore, the database systems that run on each site may have a substantial degree of mutual independence.We discussed the basic structure of distributed systems in Chapter 17.

Each site may participate in the execution of transactions that access data at one site, or several sites. The main difference between centralized and distributed database systems is that, in the former, the data reside in one single location, whereas in the latter, the data reside in several locations. This distribution of data is the cause of many difficulties in transaction processing and query processing.In this chapter, we address these difficulties.

We start by classifying distributed databases as homogeneous or heterogeneous,in Section 19.1. We then address the question of how to store data in a distributed database in Section 19.2. In Section 19.3, we outline a model for transaction processing in a distributed database. In Section 19.4, we describe how to implement atomic transactions in a distributed database by using special commit protocols. In Section 19.5, we describe concurrency control in distributed databases. In Section 19.6, we outline how to provide high availability in a distributed database by exploiting replication, so the system can continue processing transactions even when there is a failure. We address query processing in distributed databases in Section 19.7. In Section 19.8, we outline issues in handling heterogeneous databases. In Section 19.10, we describe directory systems, which can be viewed as a specialized form of distributed databases.In this chapter, we illustrate all our examples using the bank database of Figure 19.1.

19.1 Homogeneous and Heterogeneous Databases

In a homogeneous distributed database system, all sites have identical database management system software, are aware of one another, and agree to cooperate in processing usersrsquo; requests. In such a system, local sites surrender a portion of their autonomy in terms of their right to change schemas or database-management

branch(branch name, branch city, assets)

account (account number, branch name, balance)

depositor (customer name, account number)

Figure 19.1 Banking database.

system software. That software must also cooperate with other sites in exchanging information about transactions, to make transaction processing possible across multiple sites.

In contrast, in a heterogeneous distributed database, different sites may use different schemas, and different database-management system software. The sites may not be aware of one another, and they may provide only limited facilities for cooperation in transaction processing. The differences in schemas are often a major problem for query processing, while the divergence in software becomes a hindrance for processing transactions that access multiple sites.

In this chapter, we concentrate on homogeneous distributed databases. However, in Section 19.8 we briefly discuss issues in heterogeneous distributed database systems.

19.2 Distributed Data Storage

Consider a relation r that is to be stored in the database. There are two approaches to storing this relation in the distributed database:

bull; Replication. The system maintains several identical replicas (copies) of the relation, and stores each replica at a different site. The alternative to replication is to store only one copy of relation r.

bull; Fragmentation. The system partitions the relation into several fragments, and stores each fragment at a different site.

Fragmentation and replication can be combined: A relation can be partitioned into several fragments and there may be several replicas of each fragment. In the following subsections, we elaborate on each of these techniques.

19.2.1 Data Replication

If relation r is replicated, a copy of relation r is stored in two or more sites. In the most extreme case, we have full replication, in which a copy is stored in every site in the system.

There are a number of advantages and disadvantages to replication.

bull; Availability. If one of the sites containing relation r fails, then the relation r can be found in another site. Thus, the system can continue to process queries involving r, despite the failure of one site.

bull; Increased parallelism. In the case where the majority of accesses to the relation r result in only the reading of the relation, then several sites can process queries involving r in parallel. The more replicas of r there are, the greater the chance that the needed data will be found in the site where the transaction is executing. Hence, data replication minimizes movement of data between sites.

lt;

剩余内容已隐藏，支付完成后下载完整资料

资料编号：[253971]，资料为PDF文档或Word文档，PDF文档可免费转换为Word