Distributed Database

Distributed Database

Prepared by Asma Halabeia Date: 3/7/2003

Objectives ●To understand concept of DDB ●To know architecture for DDB ●To know the Rules of DDB ●To know types of DDB ●Compare adva & disadva ●Show example of DDBQ

Distributed database concept Distributed database is collection of different data which belong logically to the same system but spread over the Site of .computer network

Example NEW YORK

PARIS

DBMS

Personal Computer

Database

Workstation

DBMS

Communications Network

FileServer

Personal Computer

Database

FileServer Workstation Laser printer

DBMS Pesonal Computer

Database

TOKYO

Why need distributed DB ●Organizational and economic reason. ●Interconnection of existing database. ●Reduced communication overhead. ●Performance considerations. ●Reliability and availability.

A reference architecture of DDB :A reference architecture consists of .global schema● .fragmentation schema● .allocation schema● .local mapping schema● Some levels may be missing depend .on levels of transparency supported

Reference Architecture for DDB

Reference Architecture for DDB

●●global schema: defines all the data which are contained in distribut database. ●fragment: each global relation can be split into several non overlapping portion. ●●fragmentation schema: the mapping between global relation and fragment. ●●allocation schema: define at which site a fragment located. ●●local mapping schema: depends on the type of DBMS .

Rules for a DDB 12 • 0. –

• 1. • 2. • 3. • 4. • 5. • 6.

Fundamental Principle To the user, a distributed system should look exactly like a non-distributed system.

Local Autonomy No Reliance on a Central Site Continuous Operation Location Independence Fragmentation Independence Replication Independence

Rules for a DDB 12 • 7.Distributed Query Processing • 8.Distributed Transaction Processing • 9.Hardware Independence • 10.Operating System Independence • 11.Network Independence • 12.Database Independence • Last four rules are ideals.

Types of distributed database Homogeneous DB ♦ all sites use the same DBMS sites. ♦ approach provides incremental growth and allows increased performance.

Heterogeneous DB ♦ sites may run different DBMS product. ♦ occur when sites have implemented their own database.

Homogeneous Database

Identical DBMSs

Source: adapted from Bell and Grimson, 1992.

Heterogeneous Database

Non-identical DBMSs

Source: adapted from Bell and Grimson, 1992.

Advantages & disadvantages of DDB Advantages: ●increased reliability availability. ●capacity and growth. ●efficient & flexibility. ●distributed database sharing.

Disadvantages: ●software cost & complexity. ●slow response. ●security. ●integrity

Example of DDBQ select * from scott.emp; select * from scott.emp@DBII;