1. Chapter 9 :
Distributed Database

2. Chapter 9 - Objectives Concepts.
Advantages and disadvantages of distributed databases.
Functions and architecture for a DDBMS.
Comparison criteria for DDBMSs.

3. Concepts Distributed Database
A logically interrelated collection of shared data (and a description of this data), physically distributed over a computer network.
Distributed DBMS
Software system that permits the management of the distributed database and makes the distribution transparent to users.

4. Centralized & Distributed Database
Single site database – centralized database
A database is located at a single site
Multi-site database – distributed database
A database is stored on multiple computers (not connected)

5. Centralised DB – DB located at single site or location
Dumb Terminal
Central Computer DB
Dumb Terminal
Remote Terminal

6. Distributed DB – DB distributed over several sites
DBMS 1
DBMS 3
Network
DBMS 2

7. Centralized & Distributed Database
Ad/Disadvantages of centralized database
Performance degradation (no. of users)
High cost (when no. of access , communication transmission cost
Fast access as all at one site (advantage)
Reliability issues (if one fails, all process stop)

8. Centralized & Distributed Database
Advantages of distributed database
Reduced cost – data needed at the local site, so reduce cost
High availability – if one site fails, data can be still accessed from other sites
Support graphical interfaces – more user friendly interfaces
Shared workload – spread over several sites

9. Data Distribution Strategies
Data allocation or distribution – a process of deciding where to locate the data
Three strategies
Centralized – at a single site
Partitioned – split the database (vertical/horizontal)
Replication – duplicates the database/copy

10. Data Replication Two types of replication :
Fully replicated – same data store at several site
Partially replicated – part of database at another site

11. Data Replication Advantages (available locally) :
Fast response (queries processed faster)
Reduced transmission cost (DB close to user)
High reliability (got copies)
Disadvantages (duplicate/redundant data):
Increased storage requirement
Increased cost (changes/update must be done to all the DB)
Reduced security (poor security at one site, allows attacks to all the data in DB)

12. Partitioning
Two types of partitioning
i. Horizontal partitioning – rows of table are distributed
Advantages :
Fast access
Reduced cost
Security
Disadvantages :
Backup problem
Varying access speed
Student_ID
Student_Name
Age A1
Ali 22 A2
Siti 46 A3
Raju 44 A4
Karen 21
Row 1 and 2 are split from the original table and located in different location

13. Partitioning Fast access Reduced cost security Backup problem
ii. Vertical partitioning – columns of table are distributed
Advantages :
Fast access
Reduced cost
security
Disadvantages :
Backup problem
Speed difference
Student_ID
Student_Name
Age
Column 1 and 2 are split from the original table and located in different location

14. Distributed DBMS Distributed processing for distributed database
Must provide the following transparencies :
Transaction transparency?
Distribution transparency?
Failure transparency?
Performance transparency?
Heterogeneity transparency?

15. Transaction transparency?
Transaction maintains consistency and the transaction completed entirely or it is aborted
Distribution transparency?
Gives an impression to user that there is only one DB when in actual fact it is split and scattered at several sites
Failure transparency?
if any site fails, it is not known by other users at other sites, their transaction goes on as usual

16. Performance transparency?
The overall performance of the DDBMS is not significantly different from the centralized DB
Heterogeneity transparency?
Gives an impression that there is only one DBMS being used when in actual fact there are several DBMS being used

17. Client/server is normally a 2-tier architecture
Tier-1 : Client
Tier-2 : DB server
More complex client/server or web-based systems may require 3-tier architecture
Tier 1 : Client
Tier 2 : Application server
Tier 3 : DB server

18. ODBC (Open DB Connectivity) Standard
Is an interface by which application programs can access and process SQL databases in a DBMS-independent manner
Any application that uses the ODBC interface will be able to access any other DB without changing the program coding
If you have an application program that uses ODBC interface, you should be able to access an Oracle, Informix, Access or other DB.

19. END