Oracle basic terms and concepts

Instance:
Oracle database is associated with an Oracle instance. While database getting started on serve, Oracle allocates a memory area called the System Global Area (SGA) and starts one or more Oracle process (depends on configuration)

The combination of the SGA and the Oracle process is called an Oracle instance.

SGA= Context areas, Database Buffer Cache and Redo log and Oracle processes

States of instances
1.Mount
2.Opened
3.Unmount

Parameter file
To start any Oracle instance, Oracle must read an initilization parameter file: A file

containing configuration parameters for that intance and databse.
Initialisation parameter categorrized into different groups

Parameter related to Name
Parameter for limit setting maximum and minimum
Parameter for capacity : SGA (variable)

Parameter file tells Oracle :
The name of the database for which to start up an instance
How much memory to use for memory structures in the SGA
What to do with filled online redo log files
The names and locations of the database control files
The names of undo tablespaces or private rollback segments in the database

How to change Parameter values:
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Variable parameters for example related to SGA can be changed by dynamically by using ALTER SESSION or ALTER SYSTEM

Note:
If we are not using server parameter file, change made using the ALTER SYSTEM statement are only in effect for the current instance, it means if we reboot server, changes will be revert back to previous values. We must manually update the text initialization parameter file