With the rising need for secure, reliable and accessible information in today’s technology environment, the need for distributed databases and client/server applications is also increasing. A distributed database is a single logical database that is spread physically across computers in multiple locations that are connected by data network links. Distributed database is a kind of virtual database whose component parts are physically stored in a number of distinct real databases at a number of distinct locations. The users at any location can access data at anywhere in the network as if the data were all stored at the user’s own location. The objective of a distributed database management system (DDBMS) is to control the management…show more content… III. DISTRIBUTED DATABASE DESIGN
Distributed Database Systems are needed for the applications where data and its accesses are inherently distributed and to increase the availability during failures.
However, some additional factors have been considered for a Distributed Database:
A. Data Fragmentation: In Distributed Databases, we need to specify and understand the logical unit of Database Distribution and allocation. The database may be broken up into logical units called fragments which will be stored at various sites. The easiest logical units are the tables themselves. Two Types of Data Fragmentation are:
• Horizontal fragmentation: A horizontal fragment of a table is a subset of rows in it. So horizontal fragmentation spilts a table 'horizontally' by selecting the relevant rows and these fragments can be assigned to different sides in the distributed system • Vertical fragmentation: A vertical fragment of a table keeps only some attributes of it. It spilts a table vertically by columns. It is essential to include the primary key of the table in each vertical fragment so that the full table can bere constructed if…show more content… Data Replication: A popular option for data distribution as well as for fault tolerance of a database is to store a separate copy of the database at each of two or more sites.A replicate of each fragment can be maintained at several sites. Data replication is the process of deciding which fragments will be replicated.
IV. FUNDAMENTALS OF TRANSACTION MANAGEMENT AND CONCURRENCY CONTROL
Transaction: A transaction consists of a series of operations performed on a database.
The major issue in transaction management is that if a database was in a consistent state prior to the initiation of a transaction, then the database should return to a consistent state after the transaction is completed.
Properties of Transaction: A Transaction has four properties that lead to the consistency and reliability of a distributed database. These are Atomicity, Consistency, Isolation, and Durability.