Need for distributed database systems, Database Management System

Need For Distributed Database Systems

A distributed database is a set of database kept on multiple computers that appears to applications as a one database. As a result, an application can concurrently access and modify the data in various databases in a network. Every database in the system is controlled by its local server but cooperates to maintain the consistency of the global distributed database. Computers in a distributed system communicate with each other by various communication media, such as telephone line or high-speed buses. They don't share main memory, nor a clock, while, to work properly many applications on dissimilar computers may have to synchronise their clocks. In some cases the absolute time may be significant and the clocks may have to be synchronised with atomic clocks.

The processors in a distributed system may differ in size and function such as workstation, small microcomputers, minicomputers, and large general-purpose computer system. These processors are referred to by nodes, sites, computers, and so on, depending on the context in which they are mentioned. We mostly use the term site, in order to emphasise the physical distribution of these systems.

A distributed database system has of a collection of sites, every of which may participate in the implementation of transactions, which access data at one site, or various sites. The main difference among centralised and distributed database systems is that, in the former, the data resides in one Centralised control, while in the latter; the data resides in various sets under the control of local

distributed DBMS components which are under the control of single DBMS. As we shall see, this distribution of data is the cause of many problems that will be addressed in this unit.

Independent or decentralised systems were normally used in previous days. There was duplication of hardware and other facilities. Evolution of computer systems is also led to incompatible procedures and is short of of management control. A centralised database system was then evolved. In the centralised database the DBMS and data reside at a one database instance. Though for recovery purposes we keep redundant database information yet it is under the control of a one DBMS. A more enhancement of the centralised database system may be to give access to centralised data from a number of distributed locations by a network. In such a system a site failure except the central site will not result in total system failure. Though, communication technology has greatly better yet the centralised approach may create troubles for an organization that has geographically dispersed operations and data has to be accessed from a centralised database. Some of the troubles may be:

a. loss of messages among a local and central site;

b. failure of communication links among local and central site. This would make the system unreliable;

c. excessive load on the central site would delay accesses and queries. A one site would have to bear a large number of transactions and hence would need large computing systems.

The troubles as above could be addressed using distributed database systems. It improves the sharability and reliability of data and the efficiency of data access. Distributed Database Systems can be taken a system connected to intelligent remote devices each of which can itself act as a local database repository. All data is accessible from each site. The distributed system increases the effectiveness of access because multiple of sites can co-ordinate efficiently to respond to a query and control & processing is limited to this DBMS.

Posted Date: 3/12/2013 5:42:55 AM | Location : United States







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