Oracle Architecture

An Oracle Database consists of at least one database instance and one database. The database instance handles memory and processes. The database consists of physical files called data files and can be a non-container database or a multi-tenant container database.

A single-instance database architecture consists of one database instance and one database. single-instance databases can be installed on the same server machine. There are separate database instances for each database. This configuration is useful to run different versions of Oracle Database on the same machine.

An Oracle Real Application Clusters (Oracle RAC) database architecture consists of multiple instances that run on separate server machines. All of them share the same database. This configuration is designed for high availability, scalability, and high-end performance.

The listener is a database server process. It receives client requests, establishes a connection to the database instance, and then hands over the client connection to the server process.

A database instance contains a set of Oracle Database background processes and memory structures. The main memory structures are the System Global Area (SGA) and the Program Global Areas (PGAs). A database instance exists only in memory.

When an instance is started, Oracle Database allocates a memory structure and starts one or more background processes. The instance continues to function when server processes terminate.

The background processes operate on the stored data (data files) in the database and use the memory structures to do their work.

A database is a set of physical files on the disk.