A database is a collection of data treated as a unit. The purpose of a
database is to store and retrieve related information.
An Oracle Database reliably manages a large amount of data in a multiuser
environment so that many users can concurrently access the same data. This is
accomplished while delivering high performance. At the same time, the database
prevents unauthorized access and provides efficient solutions for failure
recovery.
An Oracle Database consists of an instance and its associated databases.
The instance consists of memory structures and background processes. Every time
an instance is started, a shared memory area called the System Global Area
(SGA) is allocated and the background processes are started.
The database consists of both physical structures and logical structures.
Because the physical and logical structures are separate, the physical storage
of data can be managed without affecting access to logical storage structures.
Connecting
to the Database
Connection
and session are closely related to user process but are very different in
meaning.
A
connection is a communication pathway between a user process and an Oracle
Database instance. A communication pathway is established using available
interprocess communication mechanisms (on a computer that runs both the user
process and Oracle Database) or network software (when different computers run
the database application and Oracle Database, and communicate through a
network).
A session represents
the state of a current user login to the database instance. For example, when a
user starts SQL*Plus, the user must provide a valid username and password, and
then a session is established for that user. A session lasts from the time the
user connects until the time the user disconnects or exits the database
application.
In the case
of a dedicated connection, the session is serviced by a permanent dedicated
process. The session is serviced by an available server process selected from a
pool, either by the middle tier or by the Oracle shared server architecture.
Multiple
sessions can be created and exist concurrently for a single Oracle Database
user using the same username. For example, a user with the username/password of
HR/HR can connect to the same Oracle Database instance several times.
Database
Structures
After
starting an instance, the Oracle software associates the instance with a
specific database. This is called mounting the database. The database is then
ready to be opened, which makes it accessible to authorized users. Multiple
instances can execute concurrently on the same computer, each accessing its own
physical database.
You can
look at the Oracle Database architecture as various interrelated structural
components.
An Oracle
instance uses memory structures and processes to manage and access the
database. All memory structures exist in the main memory of the computers that
constitute the database server. Processes are jobs that work in the memory of
these computers. A process is defined as a “thread of control” or a mechanism
in an operating system that can run a series of steps.
