以Oracle Net看，数据库服务器可能也是客户端
          因为tnsnames.ora可以有服务器的描述
          也就是，只有监听器才会一直是服务器
         
          实例：监听=n：m
          
          ㈠ 双监听器的好处：
                        ① 客户端故障转移--CF
                        ② 客户端负载均衡--CLB
         
          ㈡ 注意事项：
               
                ① 要保证1521端口可用，否则，OEM将不可用，须emca重配
                ② 非标端口使用静态注册是个比较好的实践

          配置案例：

[oracle@localhost admin]$ cat listener.ora 
think =
  (DESCRIPTION =
    (load_balance = on)
    (failover = on)
    (ADDRESS_LIST =
      (ADDRESS=(PROTOCOL = tcp)(HOST= 192.168.1.113)(PORT= 1521)
      (ADDRESS=(PROTOCOL = tcp)(HOST= 192.168.1.113)(PORT= 1522)
     )
    )
  )

SID_LIST_think =
  (SID_LIST =
    (SID_DESC =
      (GLOBAL_DBNAME = emrep.us.oracle.com)
      (SID_NAME = emrep)
      (ORACLE_HOME = /u01/app/oracle/product/10.2.0/db_1)))

water =
  (DESCRIPTION =
    (load_balance = on)
    (failover = on)
    (ADDRESS_LIST =
      (ADDRESS=(PROTOCOL = tcp)(HOST= 192.168.1.113)(PORT= 1521))
      (ADDRESS=(PROTOCOL = tcp)(HOST= 192.168.1.113)(PORT= 1522))
     )
    )
  )

SID_LIST_water =
  (SID_LIST =
    (SID_DESC =
      (GLOBAL_DBNAME = emrep.us.oracle.com)
      (SID_NAME = emrep)
      (ORACLE_HOME = /u01/app/oracle/product/10.2.0/db_1)))
      

[oracle@localhost admin]$ lsnrctl status water

LSNRCTL for Linux: Version 10.2.0.1.0 - Production on 10-DEC-2012 21:47:22

Copyright (c) 1991, 2005, Oracle.  All rights reserved.

Connecting to (DESCRIPTION=(load_balance=on)(failover=on)(ADDRESS=(PROTOCOL=tcp)(HOST=192.168.1.113)(PORT=1521)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 10.2.0.1.0 - Production
Start Date                10-DEC-2012 20:02:23
Uptime                    0 days 1 hr. 44 min. 58 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Parameter File   /u01/app/oracle/product/10.2.0/db_1/network/admin/listener.ora
Listener Log File         /u01/app/oracle/product/10.2.0/db_1/network/log/listener.log
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=localhost.localdomain)(PORT=1521)))
Services Summary...
Service "emrep.us.oracle.com" has 1 instance(s).
  Instance "emrep", status READY, has 1 handler(s) for this service...
Service "emrep_XPT.us.oracle.com" has 1 instance(s).
  Instance "emrep", status READY, has 1 handler(s) for this service...
The command completed successfully

  如果1521和1522并存，在业务不繁忙时，listener进程只会使用1521  
  
[oracle@localhost admin]$ lsnrctl status water

LSNRCTL for Lin