【Oracle】-【TWO_TASK】-TWO_TASK环境变量的作用

当前配置的两个连接串(一个本地数据库，一个远程数据库)：

[oracle@liu admin]$ more tnsnames.ora
BISAL =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = bisal)
    )
  )
TEST =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = x.x.x.x)(PORT = 1524))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = star)
    )
  )

当前TWO_TASK环境变量为空：

[oracle@liu Desktop]$ echo $TWO_TASK

[oracle@liu Desktop]$

当未指定服务名连接数据库时，默认是bisal这个本地数据库，即由系统的ORACLE_SID指定：

[oracle@liu Desktop]$ echo $ORACLE_SID
bisal
[oracle@liu Desktop]$ env | grep SID
ORACLE_SID=bisal
[oracle@liu Desktop]$

使用/的方式连接：

[oracle@liu Desktop]$ sqlplus / as sysdba
SQL*Plus: Release 10.2.0.1.0 - Production on Sun Aug 11 14:38:08 2013
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> select * from global_name;
GLOBAL_NAME
--------------------------------------------------------------------------------
BISAL
SQL>

设定TWO_TASK=TESTTKT：

[oracle@liu Desktop]$ export TWO_TASK=TESTTKT
[oracle@liu Desktop]$ echo $TWO_TASK
TESTTKT
[oracle@liu Desktop]$ 
[oracle@liu Desktop]$

使用TESTTKT的用户名登陆，但不指定服务名：

[oracle@liu Desktop]$ sqlplus star/star
SQL*Plus: Release 10.2.0.1.0 - Production on Sun Aug 11 14:39:31 2013
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.7.0 - 64bit Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.7.0 - Production

SQL> select * from global_name;
GLOBAL_NAME
--------------------------------------------------------------------------------
ALL9IT2
SQL>

此时默认连接的就是TESTTKT这个远程库了。

此时关闭本地库bisal的监听：

LSNRCTL> stop bisal

验证监听状态：

[oracle@liu Desktop]$ tnsping bisal
TNS Ping Utility for Linux: Version 10.2.0.1.0 - Production on 11-AUG-2013 15:06:18
Copyright (c) 1997, 2005, Oracle.  All rights reserved.
Used parameter files:
/opt/oracle/102/network/admin/sqlnet.ora
Used TNSNAMES adapter to resolve the alias
Attempting to contact (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521)) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = bisal)))
TNS-12541: TNS:no listener

再次连接bisal库：

[oracle@liu Desktop]$ sqlplus bisal/xxx@bisal
SQL*Plus: Release 10.2.0.1.0 - Production on Sun Aug 11 15:04:54 2013
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
ERROR:
ORA-12541: TNS:no listener
Enter user-name:

打开监听，再用@bisal的方式连接bisal本地库：

[oracle@liu Desktop]$ sqlplus bisal/xxx@bisal
SQL*Plus: Release 10.2.0.1.0 - Production on Sun Aug 11 14:40:08 2013
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
Error accessing PRODUCT_USER_PROFILE
Warning:  Product user profile information not loaded!
You may need to run PUPBLD.SQL as SYSTEM
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> select * from global_name;
GLOBAL_NAME
--------------------------------------------------------------------------------
BISAL
SQL>

但之前可以通过操作系统验证登陆的本地库sys账户：

[oracle@liu Desktop]$ sqlplus / as sysdba
SQL*Plus: Release 10.2.0.1.0 - Production on Sun Aug 11 14:40:37 2013
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
ERROR:
ORA-01031: insufficient privileges
Enter user-name:

禁止访问了。

只能通过如下方式访问之前的本地库sys账户：

[oracle@liu Desktop]$ sqlplus bisal/bisal@bisal
SQL*Plus: Release 10.2.0.1.0 - Production on Sun Aug 11 14:43:52 2013
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
Error accessing PRODUCT_USER_PROFILE
Warning:  Product user profile information not loaded!
You may need to run PUPBLD.SQL as SYSTEM
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> conn sys/oracle as sysdba
Connected.
SQL> show user
USER is "SYS"
SQL>

总结：

1、未设置TWO_TASK环境变量时，不指定服务名连接的默认是ORACLE_SID的值。

2、当设置TWO_TASK环境变量时，不指定服务名连接的默认是TWO_TASK的值。

3、当使用@服务名方式连接时，会忽略ORACLE_SID或TWO_TASK的值。

4、当使用TWO_TASK环境变量时，操作系统验证登录数据库的方式被禁止，提示ORA-01031: insufficient privileges。

5、因为TWO_TASK设置后，屏蔽了ORACLE_SID的作用，因此即使对于本地库，也需要通过SQLNet的方式连接，也就是需要监听，一般对于从本机访问本地库可能不打开监听，但此时需要打开监听，才能通过@服务名的方式连接到本地库。

6、补充：tnsnames.ora文件默认采用的路径如下：

$HOME(~)/.tnsnames.ora
$TNS_ADMIN/tnsnames.ora
/var/opt/oracle/tnsnames.ora or /etc/tnsnames.ora (depending on platform)
$ORACLE_HOME/network/admin/tnsnames.ora

7、关于TWO和TASK的来源，这个词来源于20世纪80年代，TWO TASK表示两个一起工作的含义，仅仅是意会，并不局限于两个。

8、个人理解，设置TWO_TASK，能够覆盖ORACLE_SID的作用，改变不带@服务名默认连接的数据库(本地or远程)，要求所有的sqlplus连接都是通过SQLNet的方式，禁止操作系统验证。仅此而已。