What is an oracle database instance

Oracle database instance is a running environment of the Oracle database system. It is a process running on the physical server. It is composed of multiple related memory structures and background processes and is responsible for managing and controlling all operations of the database. It provides high-performance and reliable database services through functions such as memory management, connection management, data caching, transaction management, and fault recovery.

The operating environment of this tutorial: Windows 10 system, Oracle version 19c, DELL G3 computer.

Oracle database instance is a running environment of the Oracle database system. It is a process running on the physical server and consists of multiple related memory structures and background processes. Instance is one of the core components of the Oracle database system, which is responsible for managing and controlling all operations of the database.

An Oracle database can contain multiple instances, each instance is independent and has its own memory and process. Each instance has a unique instance name that is used to distinguish different instances. Instances are independent of each other and can run different versions and configurations at the same time.

In an Oracle database system, each instance is associated with a database. The database is the place where data is physically stored, and the instance is the tool for managing and controlling the database. Instances serve application and user needs by reading and writing to the database.

Oracle database instance includes the following main components:

SGA (System Global Area): SGA is the memory structure of the instance, used to store data and metadata in the database. It includes shared pool, data buffer, redo log buffer, etc. The size of the SGA can be configured as needed to meet the performance needs of the database.

PGA (Program Global Area): PGA is a memory area used by each user process to store user session information and temporary data. Each user process has its own PGA, which is independent of other user processes.

Background process: The background process is the main working process of the instance and is responsible for performing various tasks of the database. Common background processes include background writing process (DBWn), background reading process (DBRn), log writing process (LGWR), etc. These processes ensure the normal operation and efficient performance of the database.

Control file: The control file is an important part of the instance and is used to record the structure information and status information of the database. It includes the name of the database, the location of data files and log files, the opening mode of the database, etc. The control file is used to restore the state of the database when the instance is restarted.

The main functions of the Oracle database instance include:

Connection management: The instance is responsible for managing the connection between the user and the database. It is responsible for authenticating users, assigning and managing users' sessions, and controlling user access rights.

Memory management: The instance is responsible for managing the memory allocation and use of SGA and PGA. It loads data and metadata into SGA as needed to improve database performance and responsiveness.

Data cache: The instance uses the data buffer in the SGA to store data blocks in the database. The data buffer can reduce access to physical disks and increase data reading speed.

Transaction management: The instance is responsible for managing transactions in the database. It uses redo logs to record transaction modifications and manages transaction consistency and isolation through rollback segments.

Fault recovery: The instance is responsible for handling database failures and exceptions. It can repair database damage by restarting the instance, restoring control files and log files, etc.

In short, the Oracle database instance is the core component of the Oracle database system and is responsible for managing and controlling all operations of the database. It provides high-performance and reliable database services through functions such as memory management, connection management, data caching, transaction management, and fault recovery.

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