07-05-2014, 04:15 PM
[u]Distributed DBMS Concepts and Design [/u]
Distributed DBMS .ppt (Size: 644.5 KB / Downloads: 16)
Introduction
Distributed Database changes the way of data sharing, conceptually from centralization into decentralization.
Development of computer networks promotes a decentralized mode of work.
Development of distributed systems should improve the sharing ability of the data and the efficiency of data access
Distributed systems should help resolve the "islands of information" problem
Concepts
Distributed database
A logically interrelated collection of shared data and description of this data, physically distribute over a computer network.
Distributed DBMS
The software system that permits the management of the distributed databases and makes the distribution transparent to users.
DBMS have following characteristics:
A collection of logically related shared data
The data is split into number of fragments
Fragments may be replicated
Fragments/replicas are allocated to sites.
The sites are linked with computer network.
The data at each site is under the control of a DBMS
The DBMS at each site can handle local applications autonomously.
Each DBMS participates in at least one global application.
Distributed Database Management System
It is not necessary for every site in the system to have its own local database as shown
The system is expected to make the distribution transparent to the user
Distributed database is split into fragments that can be stored on different computers and perhaps replicated
The objective of the transparency is to make the distributed system to appear like a centralized system
Types of Fragmentation
Horizontal Fragmentation:
each tuple of r is assigned to one or more fragments
Vertical Fragmentation:
the schema for relation r is split into several smaller schemas. All schemas must contain a common candidate key (or superkey) to ensure lossless join property.
Example : relation account with following schema
Summary
A relation may be divided into a number of sub relations called fragments, which may be horizontal, vertical, mixed.
The three correctness rules of fragmentation are: completeness, reconstruction, and disjoitness.
There are four allocation strategies regarding the
placement of data: centralized, partitioned, complete replication and selected replication.