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DCD: Dead Connection Detection ，可以用于檢測、標記僵死而沒有斷開會session，再由PMON進行清理，釋放資源。
開啟DCD，只需要在服務端的sqlnet.ora文件中添加SQLNET.EXPIRE_TIME參數，單位為分鐘：
SQLNET.EXPIRE_TIME = 10 |
如果時間達到這個值，server端就是發出一個”probe” packet 給客戶端，如要客戶斷是正常的，這個packet就被忽略，timer重新計時；如果客戶端異常中斷，則server端就會收到一個消息，用以釋放連接。
DCD還可以用于防止防火墻的timeout，例如：
某個系統RMAN備份，在結束時，報：
released channel: dev_0 released channel: dev_1 released channel: dev_2 released channel: dev_3 released channel: dev_4 released channel: dev_5 released channel: dev_6 released channel: dev_7 released channel: dev_8 released channel: dev_9 released channel: dev_10 released channel: dev_11 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 |
參考文章(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. |
以上這個RMAN報錯例子就是由于防火墻設置timeout原因，當client和server在timeout時間內沒有數據傳輸的時候，會話就會被防火墻斷開。而設置SQLNET.EXPIRE_TIME參數，使其小于防火墻的timeout時間，就可以避免這一情況的發生。
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