《【TAF】使用Oracle RAC的TAF技術實現不間斷查詢》（http://space.itpub.net/519536/viewspace-676654）文中談到當FAILOVER_MODE型別為SELECT時可以實現查詢語句不間斷的返回結果。另外一種型別是SESSION，看一看SESSION型別會給我們帶來什麼樣的效果。

1.在tnsnames.ora檔案中新增測試連線串
RACDB1@rac1 /home/oracle$ vi $ORACLE_HOME/network/admin/tnsnames.ora
testractafsession =
(DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = rac1-vip)(PORT = 1521))
    (ADDRESS = (PROTOCOL = TCP)(HOST = rac2-vip)(PORT = 1521))
    (LOAD_BALANCE = yes)
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = RACDB)
      (FAILOVER_MODE =
        (TYPE = SESSION)
        (METHOD = BASIC)
        (RETRIES = 180)
        (DELAY = 5)
      )
    )
)

注意，此處配置的FAILOVER_MODE中的TYPE是“SESSION”。

2.使用testractaf連線串連線到資料庫
RACDB1@rac1 /home/oracle$ sqlplus sec/sec@testractafsession

SQL*Plus: Release 10.2.0.1.0 - Production on Mon Oct 25 21:16:47 2010

Copyright (c) 1982, 2005, Oracle. All rights reserved.

Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, Real Application Clusters, OLAP and Data Mining options

SQL> select instance_name from v$instance;

INSTANCE_NAME
----------------
RACDB1

可見，此時sqlplus連線到了RAC的第一個例項RACDB1上。

3.手工停止RACDB1例項
[root@rac2 ~]# srvctl stop instance -d RACDB -i RACDB1

4.重新確認查詢語句所在例項
SQL> select instance_name from v$instance;
select instance_name from v$instance
*
ERROR at line 1:
ORA-25408: can not safely replay call

SQL> select instance_name from v$instance;

INSTANCE_NAME
----------------
RACDB2

此時在原sqlplus介面下提交查詢SQL語句後立即收到“ORA-25408: can not safely replay call”錯誤，但是，再次提交同樣的查詢便會得到結果，結果表明此時已經切換到另外一個例項RACDB2上。
這裡的報錯和不使用TAF技術的報錯是有本質區別的，這裡最重要的優勢是：不需要重新發起連線。

5.恢復例項RACDB1，繼續
[root@rac2 ~]# srvctl start instance -d RACDB -i RACDB1
SQL> select instance_name from v$instance;

INSTANCE_NAME
----------------
RACDB2

6.此處我們模擬一個長時間無法返回全部資訊的查詢語句
SQL> select * from all_objects a,all_objects b,all_objects c where a.object_id=b.object_id and a.object_id=c.object_id;

…… 此處省略輸出資訊 ……

7.手工停止RACDB2例項
[root@rac2 ~]# crs_stat -t
Name           Type           Target    State     Host
------------------------------------------------------------
ora....B1.inst application    ONLINE    ONLINE    rac1
ora....B2.inst application    ONLINE    ONLINE    rac2
ora.RACDB.db   application    ONLINE    ONLINE    rac2
ora....DB1.srv application    ONLINE    ONLINE    rac1
ora....DB2.srv application    ONLINE    ONLINE    rac2
ora.....taf.cs application    ONLINE    ONLINE    rac1
ora....SM1.asm application    ONLINE    ONLINE    rac1
ora....C1.lsnr application    ONLINE    ONLINE    rac1
ora.rac1.gsd   application    ONLINE    ONLINE    rac1
ora.rac1.ons   application    ONLINE    ONLINE    rac1
ora.rac1.vip   application    ONLINE    ONLINE    rac1
ora....SM2.asm application    ONLINE    ONLINE    rac2
ora....C2.lsnr application    ONLINE    ONLINE    rac2
ora.rac2.gsd   application    ONLINE    ONLINE    rac2
ora.rac2.ons   application    ONLINE    ONLINE    rac2
ora.rac2.vip   application    ONLINE    ONLINE    rac2
[root@rac2 ~]# srvctl stop instance -d RACDB -i RACDB2
[root@rac2 ~]# crs_stat -t
Name           Type           Target    State     Host
------------------------------------------------------------
ora....B1.inst application    ONLINE    ONLINE    rac1
ora....B2.inst application    OFFLINE   OFFLINE
ora.RACDB.db   application    ONLINE    ONLINE    rac2
ora....DB1.srv application    ONLINE    ONLINE    rac1
ora....DB2.srv application    ONLINE    OFFLINE
ora.....taf.cs application    ONLINE    ONLINE    rac1
ora....SM1.asm application    ONLINE    ONLINE    rac1
ora....C1.lsnr application    ONLINE    ONLINE    rac1
ora.rac1.gsd   application    ONLINE    ONLINE    rac1
ora.rac1.ons   application    ONLINE    ONLINE    rac1
ora.rac1.vip   application    ONLINE    ONLINE    rac1
ora....SM2.asm application    ONLINE    ONLINE    rac2
ora....C2.lsnr application    ONLINE    ONLINE    rac2
ora.rac2.gsd   application    ONLINE    ONLINE    rac2
ora.rac2.ons   application    ONLINE    ONLINE    rac2
ora.rac2.vip   application    ONLINE    ONLINE    rac2

8.結論
當RACDB2例項終止後，持續返回的結果資訊被中斷，緊隨其後的是報錯資訊“ORA-25401: can not continue fetches”，在報錯的同時該連線已經重新連線到其他可用的資料庫例項上，後續的SQL語句不受影響。

錯誤提示資訊如下：
…… 省略輸出資訊 ……
…… 省略輸出資訊 ……
ERROR:
ORA-25401: can not continue fetches

2085 rows selected.

SQL>
SQL> select instance_name from v$instance;

INSTANCE_NAME
----------------
RACDB1

9.小結
本文給出了當故障來臨時在FAILOVER_MODE中TYPE選擇“SESSION”時的應對特點，此時與“SELECT”型別有一些區別。例項故障出現時，這裡選擇了報錯並終止。在這種配置下可以結合具體應用來應對這些報錯。在具體應用設計的過程中需要充分考慮Failover不同模式下的應對方案。

Good luck.

secooler
10.10.25

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【TAF】使用Oracle RAC的TAF技術之SESSION型別

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