After starting an instance, Oracle associstes the instance with the specified databases. This is called mounting the database. The database is then ready to be opened. which makes it accessible to authorized users.
Multiple instances can run concurrently on the same computer, each accessing its own physical databse. In clustered and massively parallel systems(MPS). Real Application clusters enables instances to mount a single database. Only the database administrator can start up an instance and open the database. If a database is open, then the database administrator can shut down the database so that it is closed .When a database is closed, users can not access the information that it contains.Security for database startup and shutdown is controlled through connections to Oracle with administrator prvilliges. normal users do not have control over the current status of an oracle database
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