What Is Oracle Server 10g Architecture?

The Oracle Server architecture can be described in three categories:

1 User-related processes
2 Logical memory structures that are collectively called an Oracle instance
3 Physical file structures that are collectively called a database

User Processes:
At the user level, two processes allow a user to interact with the instance and, ultimately, with the database: the User Process and the Server Process. Whenever a user runs an application, such as a human resources or order-taking application, Oracle starts a User Process to support the user's connection to the instance. Depending on the technical architecture of the application, the User Process exists either on the user's own PC or on the middle-tier application server. The User Process then initiates a connection to the instance. Oracle calls the process of initiating and maintaining communication between the User Process and the instance a connection. Once the connection is made, the user establishes
a session in the instance.

After establishing a session, each user then starts a Server Process on the host server itself. It is this Server Process that is responsible for performing the tasks that actually allow the user to interact with the database.

In addition to the User and Server processes that are associated with each user connection, an additional memory structure called the Program Global Area (PGA) is also created for each user. The PGA stores user-specific session information such as bind variables and session variables. Every Server Process on the server has a PGA memory area.

The Oracle Instance:
An Oracle Server instance is made up of Oracle's main memory structure, called the System Global Area (SGA), and several Oracle background processes. It is with the SGA that the Server Process communicates when the user accesses the data in the database.

The System Global Area:
The SGA is made up of three required components and three optional components.

Required Components
Shared Pool (Required) :Caches the most recently used SQL statements that have been issued by database users
Database Buffer Cache (Required) : Caches the data that has been most recently accessed by database users
Redo Log Buffer (Requires) : Stores transaction information for recovery purposes
Java Pool (Optional):  Caches the most recently used Java objects and application code when Oracle's JVM option is used
Large Pool (Optional):: Caches data for large operations such as Recovery Manager (RMAN) backup and restore activities and Shared Server components
Streams Pool (Optional): Caches the data associated with queued message requests when Oracle's Advanced Queuing option is used