In today’s world of universal dependence on information systems, all sorts of people need access to companies’ databases. In addition to a company’s own employees, these include the company’s customers, potential customers, suppliers, and vendors of all types. It is possible for a company to have all of its databases concentrated at one mainframe computer site with worldwide access to this site provided by telecommunications networks, including the Internet. Although the management of such a centralized system and its databases can be controlled in a well-contained manner and this can be advantageous, it poses some problems as well. For example, if the single site goes down, then everyone is blocked from accessing the databases until the site comes back up again. Also the communications costs from the many far-flung PCs and terminals to the central site can be expensive. One solution to such problems, and an alternative design to the centralized database concept, is known as distributed database. The idea is that instead of having one, centralized database, we are going to spread the data out among the cities on the distributed network, each of which has its own computer and data storage facilities. All of this distributed data is still considered to be a single logical database. When a person or process anywhere on the distributed network queries the database, it is not necessary to know where on the network the data being sought is located. The user just issues the query, and the result is returned. This feature is known as location transparency. This can become rather complex very quickly, and it must be managed by sophisticated software known as a distributed database management system or distributed DBMS. A distributed database is a Data Collection which satisfies the following assumptions: resides on more than one machine with computational power; machines are connected by a communication network; it benefits of a distributed database management system which allows users to feel they work on the entire database and gives users the opportunity to declare what they want not how they want. The practical experience has demonstrated that there are powerful reasons for a distributed system to be feasible it has to be relational. So we suppose we deal only with this type of DBMSs. Distributed database management system has to ensure local applications for each computational station as well as global applications on more computational machines; to develop applications it has to provide a high level query language with distributed query building means. Transparency levels must confer the image of a unique database. TYPES OF DISTRIBUTED DATABASES
Homogeneous and Heterogeneous Distributed Database Systems
A homogenous distributed database system is a network of two or more databases that reside on one or more machines that uses, locally, the same DBMS product. An application can simultaneously access or modify the data in several databases in a single distributed environment. For a client application, the location and platform of the databases are transparent. In a heterogeneous system, sites may run different DBMS products, which need not be based on the same underlying data model, and so the system may be composed of relational, network, hierarchical, and object oriented DBMSs. Homogeneous systems are much easier to design and manage. This approach provides incremental growth, making the addition of a new site to the DDBMS easy, and allows increased performance by exploiting the parallel processing capability of multiple sites. Heterogeneous systems usually result where individual sites have implemented their own databases and integration is considered at a later stage. In a heterogeneous system, translations are required to allow communication between different DBMSs. The typical solution used by some relational systems that are part of a heterogeneous DDBMS is to use gateways, which convert the language and...
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