Distributed database management system

A distributed database management system ('DDBMS') is a software system that permits the management of a distributed database and makes the distribution transparent to the users. A distributed database is a collection of multiple, logically interrelated databases distributed over a computer network. Sometimes "distributed database system" is used to refer jointly to the distributed database and the distributed DBMS. A distributed database system consists of loosely coupled sites that share no physical component.Database systems that run on each site are independent of each other.

Overview

Distributed database management systems is a software for managing databases stored on multiple computers in a network. A distributed database is a set of databases stored on multiple computers that typically appears to applications on a single database. Consequently, an application can simultaneously access and modify the data in several databases in a network.A DDBMS mainly classified into two types

• Homogeneous Distributed database management systems
• Heterogeneous Distributed database management systems

Characteristics

• All sites are interconnected.
• Fragments can be replicated.
• Logically related shared data can be collected.
• Data at each and every site is controlled by the DBMS.
• Each Distributed Database Management System takes part in at least one global application.

Functionality

• Security
• Keeping track of data
• Replicated data management
• System catalog management
• Distributed transaction management
• Distributed query processing
• Distributed database recovery

Homogeneous DDBMS

In a homogeneous distributed database all sites have identical software and are aware of each other and agree to cooperate in processing user requests. Each site surrenders part of its autonomy in terms of right to change schema or software. Homogeneous DDBMS appears to user as a single system. The homogeneous system is much easier to design and manage. The following conditions must be satisfied for homogeneous database. The operating system used, at each location must be same or compatible. The data structures used at each location must be same or compatible. The database application (or DBMS) used at each location must be same or compatible.

Heterogeneous DDBMS

In a heterogeneous distributed database different sites may use different schema and software. Difference in schema is a major problem for query processing and transaction processing. Sites may not be aware of each other and may provide only limited facilities for cooperation in transaction processing. In heterogeneous system, different nodes may have different hardware & software and data structures at various nodes or locations are also incompatible. Different computers and operating systems, database applications or data models may be used at each of the locations. For example, one location may have the latest relational database management technology, while another location may store data using conventional files or old version of database management system. Similarly, one location may have the Windows NT operating system, while another may have UNIX. Heterogeneous systems are usually used when individual sites use their own hardware and software. On heterogeneous system, translations are required to allow communication between different sites (or DBMS). In this system, the users must be able to make requests in a database language at their local sites. Usually the SQL database language is used for this purpose. If the hardware is different, then the translation is straightforward, in which computer codes and word-length is changed. The heterogeneous system is often not technically or economically feasible. In this system, a user at one location may be able to read but not update the data at another location.

See also

• Heterogeneous Database System

References

• M. T. Özsu and P. Valduriez, Principles of Distributed Database Systems (2nd edition), Prentice-Hall, 1999. ISBN 0-13-659707-6.