dedicated/shared mode


you can have on the same instance sessions in dedicated or shared mode depending on your Oracle Net configurations.

SQL> select username, server from v$session;

USERNAME SERVER
—————————— ———
SYS DEDICATED
SYS DEDICATED
SYS DEDICATED
SYS DEDICATED
SYS DEDICATED
DEDICATED
SYS DEDICATED
POWER_USER DEDICATED
DBSNMP DEDICATED
DEDICATED
DEDICATED

USERNAME SERVER
—————————— ———
SYS DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DBSNMP DEDICATED
DEDICATED
DEDICATED

USERNAME SERVER
—————————— ———
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED

USERNAME SERVER
—————————— ———
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED

USERNAME SERVER
—————————— ———
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED
DEDICATED

50 rows selected.

The choice of configuring shared vs dedicated server depends on number of concurent sessions.

Ideally, If you have less than 1000 concurent sessions to database then stay on dedicated server but if you have more than 1000 concurent sessions choose shared server.
There are many features in shared server such as connection pooling.
Check documentation about features and advantages of shared server architecture.

if your TNSNAMES.ora (located on the client machines) files list DEDICATED in them specifically, it’ll override your SHARED SERVER settings. 

To find dedicated or shared server

We can easily find whether your database is running in dedicated or shared server by the following command:

ps -ef|grep $ORACLE_SID|grep d000

If the above command shows any output then the database is running in shared server mode. Otherwise its running in dedicated mode.
Note:
*Replace $ORACLE_SID with your instance name
*d000 is the dispatcher process

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