In this Document

APPLIES TO:

PURPOSE

This document addresses the frequently asked questions related to Oracle Multitenant achitecture which includes container database (CDB) and pluggable database (PDB).

QUESTIONS AND ANSWERS

General CDB / PDB Concept in  12c Multitenant Architecture

What is a pluggable database (PDB) in Multitenant Architecture?

Pluggable Databases (PDBs) is new in Oracle Database 12c Release 1 (12.1). You can have many pluggable databases inside a single Oracle Database occurrence. Pluggable Databases are fully backwards compatible with an ordinary pre-12.1 database.

Why would I consider using the Multitenant option ?

You should consider following Database Consolidation Goals to achieve:

• Reduce Total Cost of Operation

           - Administration costs

           - Operations costs

           - Data center costs

           - Storage costs

           - Contingency costs

• Improve Functionalities

            - Resource utilization

            - Manageability

            - Integration

            - Service management

• Must not need to change applications
• Must not harm performance
• Must provide resource management and isolation
  between applications
• Must simplify patching and upgrade
  of Oracle Database

What other benefits will I get from Multitenant option ?

The benefits of mulitenant Pluggable Databases are:

• Fast provisioning of a new database or of a copy of an existing database.
• Fast redeployment, by unplug and plug, of an existing database to a new platform.
• Quickly patch or upgrade the Oracle Database version for many databases and for the cost of doing it once.
• Patch or upgrade by unplugging a PDB and plugging it into a different container database (CDB) in a later version.
• A machine can run more database instances in the form of PDBs than as individual, monolithic databases.
• Separate the duties of the application administrator from the duties of the administrator of the Oracle-supplied system.

How easy to migrate an existing pre 12.1 database to 12c Multitenant database ?

Migrate to 12c Pluggable database is very simple and easy. You can evaluate and adopt which one best suit  for you.

Plan A.

• Upgrade an existing pre 12.1 database to 12.1
• Plug-in the database post upgrade into a CDB

Plan B.

• Provision empty PDBs per database that need to consolidated
• Use datapump or golden gate replication to migrate a Database into a PDB

You can take a look at Doc ID 2051130.1 - Database upgrade to 12c Pluggable database (Multitenant) using TTS method. It talks about upgrading 11.2.0.4.0 database directly to 12.1.0.2.0 pluggable database through TTS method. This method won't require intermediate 12c upgrade.

Which Oracle Database features are currently not supported in a Multitenant architecture ?

The following Oracle Database features are currently not supported in a CDB:

• Continuous Query Notification

• Flashback Data Archive

• Heat Maps

• Automatic Data Optimization

If you must use one or more of these features, then create a non-CDB.

Does each non-CDB need its own CDB if I do not have a license for Multitenant?

Yes

Can multiple CDBs run on the same server?

Yes

Can multiple CDBs run out of the same ORACLE_HOME installation?

Yes

Basic Multitenant CDB / PDB Operations

How do I know if my database is Multitenant or not ?

Establish a SQL Session and run this Query:

SQL> select NAME, DECODE(CDB, 'YES', 'Multitenant Option enabled', 'Regular 12c Database: ') "Multitenant Option ?" , OPEN_MODE, CON_ID from V$DATABASE;

NAME                        Multitenant Option ?                  OPEN_MODE              CON_ID
---------           ------------------------------           --------------------           ----------
CDB2              Multitenant Option enabled                      MOUNTED                       0

What Pluggable databases do we have in this container database ?

SQL>  Show pdbs;

CON_ID CON_NAME OPEN MODE RESTRICTED
---------- ------------------------------ --------------- ---------------
2 PDB$SEED READ ONLY NO
3 PDB1 READ WRITE NO
4 PDB2 MOUNTED
5 PDB3 READ WRITE YES

How do I connect to a Pluggable Database , say, PDB6 ?

You can switch to PDB6 from other PDB or Root Container using following command.

SQL> alter session set container = pdb6;

Connecting to a PDB directly Using the SQL*Plus CONNECT Command:

You can use the following techniques to connect to a PDB with the SQL*Plus CONNECT command:

A) Database connection using easy connect

      Ex: CONNECT username/password@host[:port][/service_name][:server][/instance_name]

Examples of SQLPLUS from Os prompt:

$ sqlplus username/password@//<HOST>:1521/pdb2
OR
$ sqlplus username/password@//localhost:1521/pdb2
OR
$ sqlplus username/password@//localhost/pdb2

SQL> show con_name

CON_NAME
------------------------------
PDB2

B) Database connection using a net service name

Example TNSNAMES.ora:

=======

LISTENER_CDB1 =
  (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))


CDB1 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = cdb1)
    )
  )

PDB2 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = www.example.com)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = pdb2)
    )
  )
=======

Example of SQLPLUS from Os prompt:

$ sqlplus username/password@pdb2

How do I switch to main container Database ?

SQL> ALTER SESSION SET CONTAINER = CDB$ROOT;

How do I determine which PDB or CDB I am currently connected to ?

SQL> show con_name

CON_NAME
------------------------------
PDB6

OR

SQL>  select sys_context ( 'Userenv', 'Con_Name') "Container DB" from dual;

Container DB
--------------------------------------------------------------------------------
PDB6

How do I start up a Pluggable database ?

When connect to current PDB:

SQL> alter pluggable database open;

When connect to root:

SQL> alter pluggable database pdb6 open;

How do I shutdown / close a Pluggable database ?

When connect to current PDB:

SQL> alter pluggable database close;

When connect to root:

SQL> alter pluggable database pdb6 close;

How do I shut down / Startup the Container Database ?

Use startup / Shutdown command similar to startup / shutdown of Non CDB.

When the container database is shutdown  , no PDB is accessible.

In a CDB, the root and all of the PDBs share a single instance, or, when using Oracle RAC, multiple concurrent database instances. You start up and shut down an entire CDB, not individual PDBs. However, when the CDB is open, you can change the open mode of an individual PDB by using the ALTER PLUGGABLE DATABASE statement.

Which parameters are modifiable at PDB level ?

select NAME, ISPDB_MODIFIABLE from V$PARAMETER;

What common users do I have in my cdb ?

How do I create a common user ?

SQL> create user c##xxxidentified by <password> container=all;

How do I create a local user ?

 SQL> create user <username> identified by <password> container=current; 

Multitenant  Architecture

What is the difference between Container ID Zero and One ?

CON_ID "0" means data does not pertain to any particular Container but to the CDB as a whole. For example, a row returned by fetching from V$DATABASE pertains to the CDB and not to any particular Container, so CON_ID is set to "0".  A CONTAINER_DATA object can conceivably return data pertaining to various Containers (including the Root which has CON_ID==1) as well as to the CDB as a whole, and CON_ID in the row for the CDB will be set to 0.

Following table describes  various values of CON_ID Column in Container Data Objects.
0 = The data pertains to the entire CDB
1=  The data pertains to the root
2= The data pertains to the seed
3 - 254 = The data pertains to a PDB, Each PDB has its own container ID.

Are there any background processes ex, PMON, SMON etc associated with PDBs ?

No. There is one set of background processes shared by the root and all PDBs. 

Are there separate control file required for each PDB ?

No. There is a single redo log and a single control file for an entire CDB.

Are there separate Redo log file required for each PDB ?

No. There is a single redo log and a single control file for an entire CDB.

Can I monitor SGA usage on a PDB by PDB basis?

There are single SGA shared by all pluggable databases. However, you can determine SGA consumptions by all containers i.e, root and PDB.

SQL> alter session set container=CDB$ROOT;

SQL> select POOL, NAME, BYTES from V$SGASTAT where CON_ID = '&con_id';

SQL> select CON_ID, POOL, sum(bytes) from  v$sgastat
group by CON_ID, POOL order by  CON_ID, POOL;

Can I monitor PGA usage on a PDB by PDB basis?

select CON_ID, sum(PGA_USED_MEM), sum(PGA_ALLOC_MEM), sum(PGA_MAX_MEM)
from  v$process
group by CON_ID order by  CON_ID;

alter session set container =CDB$ROOT;
select NAME , value from  v$sysstat  where NAME like 'workarea%';

alter session set container = <targetPDB>;
select NAME , value from  v$sysstat  where NAME like 'workarea%';

Do I need separate UNDO tablespaces for  for each of my PDB ?

There is one active undo tablespace for a single-instance CDB. For an Oracle RAC CDB, there is one active undo tablespace for each instance. Only a common user who has the appropriate privileges and whose current container is the root can create an undo tablespace.

Please refer the below note for various modes of Undo available from 12.2 onwards

Undo Modes in 12.2 Multitenant Databases - Local and Shared Modes (Doc ID 2169828.1)

Do I need separate SYSTEM tablespaces for each of my PDB ?

There is a separate SYSTEM  tablespace for the root and for each PDB.

Do I need separate SYSAUX  tablespaces for  for each of my PDB ?

There is a separate SYSAUX tablespace for the root and for each PDB.

Do I need Temporary tablespaces for  for each of my PDB ?

There is one default temporary tablespace for the entire CDB. However, you can create additional temporary tablespaces in individual PDBs. One active temporary tablespace is needed for a single-instance CDB, or one active temporary tablespace is needed for each instance of an Oracle RAC CDB.

Can I specify a separate default tablespace for the root and for each PDB ?

Yes. You can specify a separate default tablespace for the root and for each PDB

Are all physical datafiles separate for root and PDB?

There are separate datafiles for the root, the seed, and each PDB.

Where is user data stored in CDB ?

In a CDB, most user data is in the PDBs. The root contains no user data or minimal user data.

Does Pluggable database support separate database characterset ? 

A CDB uses a single character set. All of the PDBs in the CDB use this character set.

Oracle recommends the following:  

• For all new deployments and if all PDBs are created empty, Oracle strongly recommends AL32UTF8 for the CDB database character set and AL16UTF16 for the CDB national character set.
• If you can migrate your existing databases to AL32UTF8 before consolidation, Oracle recommends that you do so and consolidate into one or more AL32UTF8 CDBs, depending on your needs. You can use Oracle Database Migration Assistant for Unicode to migrate a non-CDB to AL32UTF8. You cannot migrate the CDB using Oracle Database Migration Assistant for Unicode, after creation.
• If you cannot migrate your existing databases prior to consolidation, then you have to partition them into sets with plug-in compatible database character sets and plug each set into a separate CDB with the appropriate superset character set.

Reference: Oracle Database Globalization Support Guide, 12c Release 1 (12.1)

How do I configure Net Files in a Pluggable database environment ? 

There is a single listener.ora, tnsnames.ora, and sqlnet.ora file for an entire CDB. All of the PDBs in the CDB use these files.

Advance CDB / PDB Operations

How can I install and setup Pluggable Database ?

Use runInstaller to install the Oracle Database software

Use dbca to create databases. You can create many pluggable databases in a single operation.

DBCA enables you to specify the number of PDBs in the CDB when it is created.After a CDB is created, you can use DBCA to plug PDBs into it and unplug PDBs from it.

What Operations act on PDBs as entities ?

These operations act on PDBs as entities:

• create PDB (brand-new, as a clone of an existing PDB, by plugging in an unplugged PDB)

• unplug PDB

• drop PDB

• set the Open_Mode for a PDB

How can I create a pluggable database ?

create pluggable database x admin user a identified by p;

create pluggable database y admin user a identified by p file_name_convert = ('pdbseed', 'y');

How to drop a PDB irrevocably ?

drop pluggable database x

including datafiles;

How easy is it   to manage the provisioning of PDBs using PL/SQL ?

Following an Example of PL/SQL Code to show this.

-- Using Oracle-Managed Files

declare

t0 integer not null := -1;

procedure Show_Time(What in varchar2) is

t varchar2(10);

begin

t := Lpad((DBMS_Utility.Get_Time() - t0), 5);

DBMS_Output.Put_Line('create PDB:'||t||' centiseconds');

end Show_Time;

begin

t0 := DBMS_Utility.Get_Time();

execute immediate '

create pluggable database x

admin user a identified by p

';

Show_Time('create PDB:');

t0 := DBMS_Utility.Get_Time();

execute immediate '

drop pluggable database x

including datafiles

';

Show_Time('drop PDB: ');

end;

How to clone a PDB from an existing PDB ?

 The clonee must be open in read only mode.

 -- Using Oracle-Managed Files

create pluggable database x2

from x;

How to unplug a PDB ?

alter pluggable database x unplug into '/some_directory/x_description.xml' ;

The into keyword must be followed by the full path for a

description of the PDB, in XML, generated by the operation.

Scalability & RAC

How  to add or modify a user-managed service ?

srvctl add service … –pdb <pdb_name>

Starting a user-managed service using

srvtcl will open the PDB automatically in all the instances in which the service is started.

 Specifying the empty string ("") as the <pdb_name> will cause the pluggable database

attribute of a service to be set to null. The service can then be used only to connect to the root.

How can I view which service is attached to my Pluggable database ? 

 SQL> column NAME format a30

SQL> select PDB, INST_ID, NAME from gv$services order by 1;

PDB                                    INST_ID    NAME
-------------------------------- ---------- --------------------------------
CDB$ROOT                                  1 cdb1XDB
CDB$ROOT                                  1 SYS$BACKGROUND
CDB$ROOT                                  1 SYS$USERS
CDB$ROOT                                  1 cdb1
PDB1                                           1 pdb1
PDB2                                           1 pdb2

Diagnosibility

Where can I find Alert log for my pluggable Database ?

A Single copy of Alert log is generated which contains warnings and alert information for all PDBs.

XML version of alert can be found in "Diag Alert" and text formatted Aler log can be found in "Diag Trace" of the container database.

 You can find  details by selecting from v$diag_info dynamic view.

Where can I find trace files related for my pluggable Database ?

All traces generated from all PDBs are currently found in "Diag Trace" of the container database.

 You can find  details by selecting from v$diag_info dynamic view.

Miscellaneous

If a user-defined, common user creates schema objects in a PDB, and if later that PDB is unplugged
and plugged into a different CDB in which that common user does not exist,
then what happens to the schema objects?
By which user will they be owned?  
Will other users within the PDB, which had been granted privileges on those schema objects, still retain those privileges?

If you plug a PDB that contains a common user into a CDB, then the following actions take place:
The common user accounts in this PDB lose commonly granted privileges that they may have had,
including the SET CONTAINER privilege.

If the target CDB has a common user with the same name as a common user in a newly plugged-in PDB,
then the new common user is merged with the target CDB common user. The password of the target CDB
common user takes precedence. Otherwise, a common user in a newly plugged in PDB becomes a locked account. In this case, you can do one of the following:

        Leave the user account locked and use the objects of its schema.

        Use Oracle Data Pump to copy these objects to another schema,
        and then drop the locked user account.

        Close the PDB, connect to the root, and then create a common user with the same name
        as the locked account. When you re-open the PDB, Oracle Database resolves the differences
        in the roles and privileges that were commonly granted to the locked user. Afterward,
        you can unlock this user account in the PDB. Privileges and roles that were
        locally granted to the user will remain unchanged.

Is the multitenant option available in Standard Edition?

Yes, but you may only create one PDB, per CDB

Can a transaction span across PDBS ?

No, though "alter session set container" is allowed after starting a transaction in a PDB,
only select is allowed in the second PDB.Transaction is preserved and you can do
commit or rollback after switch back to original PDB

What data can be seen in CDB_ and V$ views from each container?

CDB_* views are container data objects. When a user connected to the root queries a CDB_* view,
the query results will depend on the CONTAINER_DATA attribute for users for the view.
The CONTAINER_DATA clause of the SQL ALTER USER statement is used to set and modify
users' CONTAINER_DATA attribute.

In the root of a multitenant container database (CDB), CDB_* views can be used to
obtain information about tables, tablespaces, users, privileges, parameters, and so on
contained in the root and in pluggable databases (PDBs).

The CDB_* views are owned by SYS, regardless of who owns the underlying DBA_* view.
By default, a user connected to the root will only see data pertaining to the root.

May the database timezone be set on a per-PDB basis?

Yes.

May NLS currency settings (NLS_CURRENCY et al) be set on a per-PDB  basis?

Yes.

How to monitor the undo usage of each container /database in CDB/PDB ?

select NAME,MAX(TUNED_UNDORETENTION), MAX(MAXQUERYLEN), MAX(NOSPACEERRCNT), MAX(EXPSTEALCNT)
from V$CONTAINERS c , V$UNDOSTAT u
where c.CON_ID=u.CON_ID
group by NAME;

select NAME,SNAP_ID,UNDOTSN,UNDOBLKS,TXNCOUNT,MAXQUERYLEN,MAXQUERYSQLID
from V$CONTAINERS c , DBA_HIST_UNDOSTAT u
where c.CON_ID=u.CON_ID
and u.CON_DBID=c.DBID
order by NAME;

What is the difference between schema-based consolidation and the multitenant architecture?

    1. Name collision might prevent schema-based consolidation.

    2. Schema-based consolidation brings weak security.

    3. Per-application, back-end, point-in-time recovery is prohibitively difficult.

    4. Resource management between application back-ends is difficult.

    5. Patching the Oracle version for a single application back-end is not possible.

    6. Cloning a single application back-end is difficult.