Seed Database (pdb$seed) - Read Write OR Read Only Mode in Oracle Database 12c (12.1.0.1.0)

Connect to container database (cdb1)

SQL> connect sys/oracle@cdb1 as sysdba
Connected.

SQL> show con_name

CON_NAME
------------------------------
CDB$ROOT

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
READ WRITE

Perform close instance operation in seed database (pdb$seed)

SQL> alter pluggable database pdb$seed close;
alter pluggable database pdb$seed close
                         *
ERROR at line 1:
ORA-65017: seed pluggable database may not be dropped or altered

SQL> alter session set "_oracle_script"=TRUE;
Session altered.

Perform close instance operation in seed database (pdb$seed)

SQL> alter pluggable database pdb$seed close immediate instances=all;
Pluggable database altered.

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
READ WRITE

SQL> show con_name

CON_NAME
------------------------------
CDB$ROOT

Open the pdb$seed database in read write mode

SQL> alter pluggable database pdb$seed open read write;
Pluggable database altered.

SQL> show con_name

CON_NAME
------------------------------
CDB$ROOT

SQL> alter session set container=PDB$SEED;
Session altered.

SQL> show con_name

CON_NAME
------------------------------
PDB$SEED

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
READ WRITE

SQL> show pdbs

    CON_ID CON_NAME  OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
2 PDB$SEED  READ WRITE NO

SQL> connect sys/oracle@cdb1 as sysdba
Connected.

Create Pluggable Database (cdb1_pdb4) in Container Database (cdb1)

SQL> show pdbs

    CON_ID CON_NAME        OPEN MODE  RESTRICTED
---------- ------------------------------ -----------------   ----------
2 PDB$SEED        READ WRITE NO
3 CDB1_PDB3        MOUNTED

create pluggable database cdb1_pdb4 admin user user1 identified by oracle
*
ERROR at line 1:
ORA-65081: database or pluggable database is not open in read only mode

Close the pdb$seed and open it in read only mode

SQL> alter pluggable database pdb$seed close immediate instances=all;
Pluggable database altered.

SQL> alter pluggable database pdb$seed open read only;
Pluggable database altered.

SQL> show con_name

CON_NAME
------------------------------
PDB$SEED

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
READ ONLY

Set the parameter "_oracle_script" and create pluggable database (cdb1_pdb4)

SQL> alter session set "_oracle_script"=FALSE;
Session altered.

SQL> create pluggable database cdb1_pdb4 admin user user1 identified by oracle;
Pluggable database created.

SQL> alter pluggable database cdb1_pdb4 open;
Pluggable database altered.

SQL> show pdbs

    CON_ID CON_NAME        OPEN MODE   RESTRICTED
---------- ------------------------------ ----------------     ----------
2 PDB$SEED        READ ONLY   NO
3 CDB1_PDB3        MOUNTED
4 CDB1_PDB4        READ WRITE  NO


SQL> alter session set container=cdb1_pdb4;
Session altered.

Conclusion: pdb$seed is template database for new pluggable databases in container database (cdb).