理論背景：

TAF（ Transparent Application Failover ） allows oracle clients to reconnect to a surviving instance in the event of a failure of the instance to which it is connected. There are two types of TAF available, SESSION and SELECT.

TAF允許oracle客戶端重新連線到一個可持續的例項，當客戶端連線的例項出現失敗時。有2種有效的TAF型別，session and select 。有2種模式在TAF建立的故障轉移連線，basic 和preconnect 。

session: 使用session方式，所有select查詢相關的結果在重新建立新的連線後將全部丟失，需要重新發布select命令。

select:select：使用select方式，Oracle net會跟蹤事務期間的所有select語句，並跟蹤每一個與當前select相關的遊標已返回多少行給客戶端。此時，假定select查詢已返回500行，客戶端當前連線的節點出現故障，Oracle Net自動建立連線到倖存的例項上並繼續返回剩餘的行數給客戶端。假定總行數為1500，行，則1000行從剩餘節點返回。

BASIC：客戶端透過地址列表成功建立連線後，即僅當客戶端感知到節點故障時才建立到其他例項的連線

PRECONNECT: 預連線模式，是在最初建立連線時就同時建立到所有例項的連線，當發生故障時，立刻就可以切換到其他鏈路上

說明：上述兩種方式適用於不同的情形，對於select方式，通常使用與OLAP資料庫，而對於session方式則使用與OLTP資料庫。因為 select 方式，Oracle 必須為每個session儲存更多的內容，包括遊標，使用者上下文等，需要更多的資源。其次，兩種方式期間所有未提交的DML事務將自動回滾且必須重啟啟動。alter session語句不會failover。臨時物件不會failover也不能被重新啟動。
配置Service-Side TAF 示例（oracle使用者操作）

1 建立TAF Service
node1-> pwd
/u01/app/11.2.0/grid/bin
node1-> ./srvctl add service -d devdb -s server_taf -r "devdb1" -P BASIC

說明：
Srvctl add service -d -s -r "preferred-instance-list" -a "available-instance-list" -P
srvctl add service中，只有perferred才會建立服務。即在OCR中註冊一個ora.raw.dmm.rac1.Srv的服務。

2 啟動 server_taf服務
node1-> ./srvctl start service -d devdb -s server_taf

3 檢查service 執行情況
node1-> ./srvctl config service -d devdb
Service name: server_taf
Service is enabled
Server pool: devdb_server_taf
Cardinality: 1
Disconnect: false
Service role: PRIMARY
Management policy: AUTOMATIC
DTP transaction: false
AQ HA notifications: false
Failover type: NONE
Failover method: NONE
TAF failover retries: 0
TAF failover delay: 0
Connection Load Balancing Goal: LONG
Runtime Load Balancing Goal: NONE
TAF policy specification: BASIC
Edition:
Preferred instances: devdb1
Available instances:

4 確認service ID
SQL> select name,service_id from dba_services where name = 'server_taf';

NAMESERVICE_ID
---------------------------------------------------------------- ----------
server_taf 3

5 給service 新增引數

SQL> execute dbms_service.modify_service (service_name => 'server_taf' -
, aq_ha_notifications => true -
, failover_method => dbms_service.failover_method_basic -
, failover_type => dbms_service.failover_type_select -
, failover_retries => 180 -
, failover_delay => 5 -
, clb_goal => dbms_service.clb_goal_long);
> > > > > >
PL/SQL procedure successfully completed.

6 確認引數修改
col name format a15
col failover_method format a11 heading 'METHOD'
col failover_type format a10 heading 'TYPE'
col failover_retries format 9999999 heading 'RETRIES'
col goal format a10
col clb_goal format a8
col AQ_HA_NOTIFICATIONS format a5 heading 'AQNOT'
SQL> select name, failover_method, failover_type, failover_retries,goal, clb_goal,aq_ha_notifications from dba_services where service_id = 3;
NAMEMETHOD TYPERETRIES GOAL CLB_GOAL AQNOT
--------------- ----------- ---------- -------- ---------- -------- -----
server_tafBASIC SELECT 180 NONE LONG YES

7 檢查service 註冊情況
node1-> lsnrctl services

LSNRCTL for Linux: Version 11.2.0.4.0 - Production on 26-FEB-2017 04:43:55
Copyright (c) 1991, 2013, Oracle. All rights reserved.
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=LISTENER)))
Services Summary...
Service "+ASM" has 1 instance(s).
Instance "+ASM1", status READY, has 1 handler(s) for this service...
Handler(s):
"DEDICATED" established:2 refused:0 state:ready
LOCAL SERVER
Service "devdb" has 1 instance(s).
Instance "devdb1", status READY, has 1 handler(s) for this service...
Handler(s):
"DEDICATED" established:0 refused:0 state:ready
LOCAL SERVER
Service "devdbXDB" has 1 instance(s).
Instance "devdb1", status READY, has 1 handler(s) for this service...
Handler(s):
"D000" established:0 refused:0 current:0 max:1022 state:ready
DISPATCHER
(ADDRESS=(PROTOCOL=tcp)(HOST=node1.localdomain)(PORT=26677))
Service "server_taf" has 1 instance(s).
Instance "devdb1", status READY, has 1 handler(s) for this service...
Handler(s):
"DEDICATED" established:0 refused:0 state:ready
LOCAL SERVER
The command completed successfully

8 在客戶端就可以使用Service-Side TAF了

在客戶端TNS 配置：

server_taf =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = scan-cluster.localdomain)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = server_taf)
)
)

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,
Data Mining and Real Application Testing options

9 停止service
node1-> ./srvctl stop service -d devdb -s server_taf -i devdb1

10 刪除service
node1-> ./srvctl remove service -d devdb -s server_taf -i devdb1

Oracle RAC TAF 無縫failover

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