The Instance

The Oracle instance is the collection of processes that handle requests from a client for a data.

An Oracle instance is either started as part of the process of booting a server or can be started explicitly
with commands. Although you start an instance with a single command, there are actually three distinct
steps in the startup process:
❑ Starting the instance process itself
❑ Mounting the database, which consists of opening the control files for the instance
❑ Opening the database, which makes the database available for user requests

An instance can be stopped with another command or via the system console, which follows the same
sequence of events in reverse. You can stop an instance gracefully by stopping users from logging on to
the database and only shutting down when the last active user has logged off, or you can simply stop the instance, which may result in incomplete transactions.