SQLNET.EXPIRE_TIME参数—之防火墙短连接

DCD: Dead Connection Detection ，可以用于检测、标记僵死而没有断开会session，再由PMON进行清理，释放资源。
开启DCD，只需要在服务端的sqlnet.ora文件中添加SQLNET.EXPIRE_TIME参数，单位为分钟：

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SQLNET.EXPIRE_TIME = 10

如果时间达到这个值，server端就是发出一个”probe” packet 给客户端，如要客户断是正常的，这个packet就被忽略，timer重新计时；如果客户端异常中断，则server端就会收到一个消息，用以释放连接。

DCD还可以用于防止防火墙的timeout，例如：

某个系统RMAN备份，在结束时，报：

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RMAN-00571: ===========================================================

RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============

RMAN-00571: ===========================================================

RMAN-06004: ORACLE error from recovery catalog database:

ORA-03135: connection lost contact
ORACLE error from recovery catalog database:

ORA-03114: not connected to ORACLE

参考文章(ID 729811.1)：

+ Is the rman catalog Database running on a different system than the Target Database?
If so, verify with the System Administrators what the TCP/IP timeout is set to.
If there is any firewall between two systems, set the value of SQLNET.EXPIRE_TIME 
to less than the TCP timeout value of the firewall.

+ Increase the value of SQLNET.EXPIRE_TIME in the sqlnet.ora file on both

target servers if no firewall exists between two systems.

+ Increase the value of keepalive interval.

以上这个RMAN报错例子就是由于防火墙设置timeout原因，当client和server在timeout时间内没有数据传输的时候，会话就会被防火墙断开。而设置SQLNET.EXPIRE_TIME参数，使其小于防火墙的timeout时间，就可以避免这一情况的发生。

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