Search notes:

Oracle Architecture

Big Picture

Oracle distinguishes between

Database

The database is the set of (physical) files that store the data or are used for the operation.
The files that store data are called data files.
The files used for operation are called system files.

Instance

An Oracle instance consists of memory and processes.
Thus, when an Oracle Database is shut down, there is no instance.

Single instance and RAC databases

Single instance

In a single instance database, the database is attached to at most one instance.

Real Application Clusters

On the other hand, in a Real Application Cluster (RAC) environment, there are multiple instances that share the same database.
These instances (usually) run on different (hardware) servers.
For a client application, there is no difference between a RAC environment and a single instance: the cluster feels like a single server.

Listener

The listener is needed for establishing connections between client applications and an Oracle server.
When a client application wants to open an Oracle connection, it contacts the listener. The listener then creates a user process and hands the connection details to the client application.

Index