1. DATABASE LINK
DISTRIBUTED DATABASE

2. Overview database link
Some DBMS like Oracle have a way to communicate each other
This communication can be built by means database link
A database link is a pointer that defines a one- way communication path from an Oracle Database server to another database server
The link pointer is actually defined as an entry in a data dictionary table

3. Overview database link (cont’d)
A database link connection allows local users to access data on a remote database
Each database in distributed system must have unique global database name in a network domain

4. Database link

5. Database link type Private database link Public database link
Only Db link creator can have access to database link – only user
Public database link
All user can have access to database link
Global database link
Can access data and db object
User access a remote database through the following of link
Connected user link – connected as them self – same user name and password
Fixed user link – connect from different user
Current user link – connects as global user

6. Shared database link
Link between local server process and remote database

7. Global database name
Each database in a distributed database is uniquely identified by its global database name
The database forms a global database name by prefixing the database network domain
Specified by the DB_DOMAIN initialization parameter at database creation

8. Global database name

9. Global database link sintax

10. Oracle Connectivity concepts
There are two main objects in database connectivity
Database Service
Database Instance

11. Database service
Oracle database is represented to clients as a service
Database can have one or more services associated with it

12. Database service (cont’d)
A database can have multiple services associated with it

13. Database service (cont’d)
Multiple service can have following functionalities
A single database can be identified in a number of different ways by different clients
A database administrator can limit or reserve system resources

14. Database Instances A database has at least one instance
An instance is comprised of a memory area called the System Global Area (SGA) and Oracle background processes
The memory and processes of an instance efficiently manage the associated database's data and serve the database users

15. Database instance (cont’d)
Single Instance

16. Database instance (cont’d)
Multiple Instance

17. How client and server communicate
To connect to a database service, clients use a connect descriptor that provides the location of the database and the name of the database service
Example of configuration

18. Oracle net service
Stack Communication for client/server connection

19. Oracle net service (cont’d)
Stack communication for java application

20. Oracle net service (cont’d)
Stack communication for web client

21. Listener architecture
The database server receives an initial connection from a client application through the listener. The listener is an application positioned on top of the Oracle Net foundation