理论背景：

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允许oracle客户端重新连接到一个可持续的实例，当客户端连接的实例出现失败时。有2种有效的TAF类型，session and select 。有2种模式在TAF建立的故障转移连接，basic 和preconnect 。

session: 使用session方式，所有select查询相关的结果在重新建立新的连接后将全部丢失，需要重新发布select命令。

select:select：使用select方式，Oracle net会跟踪事务期间的所有select语句，并跟踪每一个与当前select相关的游标已返回多少行给客户端。此时，假定select查询已返回500行，客户端当前连接的节点出现故障，Oracle Net自动建立连接到幸存的实例上并继续返回剩余的行数给客户端。假定总行数为1500，行，则1000行从剩余节点返回。

BASIC：客户端通过地址列表成功建立连接后，即仅当客户端感知到节点故障时才创建到其他实例的连接

PRECONNECT: 预连接模式，是在最初建立连接时就同时建立到所有实例的连接，当发生故障时，立刻就可以切换到其他链路上

说明：上述两种方式适用于不同的情形，对于select方式，通常使用与OLAP数据库，而对于session方式则使用与OLTP数据库。因为 select 方式，Oracle 必须为每个session保存更多的内容，包括游标，用户上下文等，需要更多的资源。其次，两种方式期间所有未提交的DML事务将自动回滚且必须重启启动。alter session语句不会failover。临时对象不会failover也不能被重新启动。
配置Service-Side TAF 示例（oracle用户操作）

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

说明：
Srvctl add service -d -s -r "preferred-instance-list" -a "available-instance-list" -P
srvctl add service中，只有perferred才会创建服务。即在OCR中注册一个ora.raw.dmm.rac1.Srv的服务。

2 启动 server_taf服务
node1-> ./srvctl start service -d devdb -s server_taf

3 检查service 运行情况
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 确认service ID
SQL> select name,service_id from dba_services where name = 'server_taf';

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

5 给service 添加参数

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 确认参数修改
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 检查service 注册情况
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 在客户端就可以使用Service-Side TAF了

在客户端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 删除service
node1-> ./srvctl remove service -d devdb -s server_taf -i devdb1

ITPub博客

Oracle RAC TAF 无缝failover

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