Quiescing a Database

Quiesce is used to describe pausing or altering the state of running processes on a computer, particularly those that might modify information stored on disk during a backup, in order to guarantee a consistent and usable backup.
There are times when a DBA needs to perform work on the database that requires no other ACTIVE sessions. This is very common in development databases, where a DBA tries to run a package or a modify a table when the application is till running.
There are also frequent occurrences where too much IT support folks time is spent in bringing lot of application servers down to perform a deployment or some database maintenance. That is where Quiescing comes in.
To put a database(run on one instance and it affects all the instances of the RAC database).

ALTER SYSTEM QUIESCE RESTRICTED;

Quiescing puts the database in mode that does not allow any further connections to become ACTIVE state. It would however wait for all ACTIVE connections to become INACTIVE or terminate. Gladly Oracle provides a query to find sessions which are stopping it from enter into QUIESCE mode.

 select bq.sid, ss.user, ss.osuser, ss.type, ss.program
from v$blocking_quiesce bq, v$session ss
where bq.sid = ss.sid;

Note:

In this discussion of quiesce database, a DBA is defined as user SYS or SYSTEM. Other users, including those with the DBA role, are not allowed to issue the ALTER SYSTEM QUIESCE DATABASE statement or proceed after the database is quiesced.

Once in QUIESCE mode, DBA connections are the only ones that are allowed to be in ACTIVE state. The DBA can then easily lock any object to perform his tasks.

Once done to release the QUIESCE

ALTER SYSTEM UNQUIESCE;
You can query V$INSTANCE view to see active state .

SQL> select active_state from v$instance;

You can see 3 situation.  NORMAL – Database is normal state (unquiescing). QUIESCING – Being quiesced, but some non-DBA sessions are still active. There are active sessions QUIESCED – Quiesced. There is no non-DBA sessions are active.