ORA-12523: TNS: 监听程序无法找到适用于客户机连接的例程

今天使用PL/SQL Developer连接到一台新的测试服务器时，遇到ORA错误：ORA-12523: TNS: 监听程序无法找到适用于客户机连接的例程。对应的监听日志文件里面错误为TNS-12523: TNS:listener could not find instance appropriate for the client connection

首先使用oerr命令查看ORA-12523错误提示的详细内容

[oracle@DB-Server admin]$ oerr ora 12523

12523, 00000, "TNS:listener could not find instance appropriate for the client connection"

// *Cause:  The listener could not find any available (database) instances, 

// that are appropriate for the client connection.

// *Action: Run "lsnrctl services" to ensure that the instance(s) are

// registered with the listener, and have status READY.

按照提示信息，我首先运行"lsnrctl services" 查看数据库实例是否注册了监听服务，并且监听服务是否处于就绪状态。

[oracle@DB-Server admin]$ lsnrctl services

LSNRCTL for Linux: Version 10.2.0.4.0 - Production on 06-JAN-2015 14:24:23

Copyright (c) 1991, 2007, Oracle.  All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.16.22)(PORT=1521)))

Services Summary...

Service "EPPS" has 1 instance(s).

  Instance "EPPS", status UNKNOWN, has 1 handler(s) for this service...

    Handler(s):

      "DEDICATED" established:0 refused:0

         LOCAL SERVER

The command completed successfully

然后检查了一下本地的tnsnames.ora的配置情况,发现其使用共享服务器模式(SHARED)连接到数据库

TEST =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.16.22)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = SHARED)
      (SERVICE_NAME = epps)
    )

检查查看测试服务器，确认其配置是否开启了共享服务器(Shared Server)模式,如下所示，数据库为开启共享服务器模式

SQL> show parameter shared_server

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------------------------

max_shared_servers                   integer     40

shared_server_sessions               integer

shared_servers                       integer     1

SQL> 

SQL> show parameter dispatchers

NAME                                 TYPE                             VALUE

------------------------------------ -------------------------------- ------------------------------

dispatchers                          string                           (protocol=TCP)

max_dispatchers                      integer

SQL> 

但是为什么数据库启用了共享服务器模式,客户端无法以SHARED模式登陆数据库？ 那么我先修改客户端的连接方式为专用(DEDICATED)模式

TEST =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.16.22)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = epps)
    )
  )

此时以使用PL/SQL Developer登录数据库，发现OK，不会出现ORA-12523错误哦。查看当前会话，你可以通过下面SQL发现使用专用方式连接数据库。

SQL> select server from v$session where sid = (select sid from v$mystat where rownum < 2);

SERVER

---------

DEDICATED

当然，你可以用下面SQL语句查看，其中GET253194为本人计算机名称

COL USERNAME FOR A20

COL OSUSER FOR A10

COL MACHINE FOR A20

COL TERMINAL FOR A20;

SELECT SID, USERNAME, OSUSER, MACHINE,TERMINAL, SERVER 

FROM V$SESSION

WHERE TERMINAL='GET253194'; 

修改tnsnames.ora，将(SERVER = DEDICATED)删除（如下所示），依然可以连接到数据库，查看其连接方式，发现它会自己选择专用连接方式。也就是说默认为专用模式连接，除非指定为共享服务器模式

TEST =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.16.22)(PORT = 1521))
    (CONNECT_DATA =
      (SERVICE_NAME = epps)
    )
  )

SQL> select server from v$session where sid = (select sid from v$mystat where rownum < 2);

SERVER

---------

DEDICATED

那么问题出在哪里呢？ 很是纳闷，明明服务器为共享服务器模式，应该可以以共享服务连接方式连接数据库的。最后在折腾中发现，这台服务器本来是生产服务器，后来退下来被我当做测试服务器，于是我将IP地址改为了192.168.16.22， 但是当时只是修改了/etc/sysconfig/network-scripts/ifcfg-eth0系统文件. 忘记修改/etc/hosts对应的IP地址。我将/etc/hosts中的IP地址修改过来后，发现ORA-12523错误不见了，问题完满解决。但是本质的原因呢？看来还需要深入了解监听服务原理。多去学习、了解一些相关资料。留待后面补充。