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。