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Oracle instance/database states
An Oracle instance is/has either
The instance has opened the control files, but not yet opened the database. In order to find the control files, the instance uses the init parameter control_files which it has read when it started.
A database can be mounted by any number of instances at the same time. If it is mounted by two more instances, this is a Oracle Parallel Server environment.
An opened database is available for normal operations. When the database is opened, the instance opens the redo log files and the online database files.