Transparent Application Failover (TAF)

一、什么是TAF

Transparent Application Failover ,透明应用程序故障转移，当客户端和oracle数据库建立连接后，正在执行某个应用，此时客户端连接的某个实例突然宕机，那么客户端的连接就会被转移到其他健康实例上去， 对于客户端的taf，如果执行的是update，insert等事物语句，那么就会回退，如果是select语句，那么会转移到新的实例继续执行，这个过程不需要用户的的介入，是透明的。

TAF 是和下列类型的故障转移

• Oracle Real Application Clusters

• Replicated systems

• Standby databases

• Single instance Oracle database

二：net service参数：

ParameterDescription

FAILOVER

在连接监听的时候，如果FAILOVER=on,在连接地址列表中的第一个地址，监听连接失败，指示net去连接地址列表中的其他监听，当FAILOVER=on=off 时，只会连接地址列表中的第一地址的监听，

Connect-time failover is turned on by default for multiple address lists (ADDRESS_LIST), connect descriptors (DESCRIPTION), and multiple connect descriptors (DESCRIPTION_LIST).

When using a connect descriptor with a SERVICE_NAME, ensure that the value is not a GLOBAL_DBNAME in any SID_DESCentry, or a SID_NAME in any SID_DESC entry without a GLOBAL_DBNAME set.

LOAD_BALANCE            

当LOAD_BALANCE   =on 那么指示net progress  以随机的方式连接地址列表中地址，在不同监听之间平衡负载压力。当LOAD_BALANCE   =off

是让net progress 安顺序访问地址列表的监听，直到连接成功为止.

Client load balancing is turned on by default for multiple connect descriptors (DESCRIPTION_LIST).

SOURCE_ROUTE

When set to on, instructs Oracle Net to use each address in the order presented until the destination is reached. This parameter is required for reaching the destination using a specific route, that is, by specific computers. This parameter is used to enable connections to Oracle Connection Manager.

Creating a List of Listener Protocol Addresses

sales.us.example.com= (DESCRIPTION=  (ADDRESS_LIST=    (ADDRESS=(PROTOCOL=tcp)(HOST=sales1-server)(PORT=1521))    (ADDRESS=(PROTOCOL=tcp)(HOST=sales2-server)(PORT=1521)))  (CONNECT_DATA=    (SERVICE_NAME=sales.us.example.com)))

OptionParameter Setting

Try each address, in order, until one succeeds.

FAILOVER=on

Try each address, randomly, until one succeeds.

LOAD_BALANCE=on

FAILOVER=on

Try one address, selected at random.

LOAD_BALANCE=on

Use each address in order until destination reached.

SOURCE_ROUTE=on

Use only the first address.

LOAD_BALANCE=off

FAILOVER=off

SOURCE_ROUTE=off

sales.us.example.com= (DESCRIPTION=  (ADDRESS_LIST=   (LOAD_BALANCE=off)   (FAILOVER=ON)   (ADDRESS=(PROTOCOL=tcp)(HOST=sales1-server)(PORT=1521))    (ADDRESS=(PROTOCOL=tcp)(HOST=sales2-server)(PORT=1521)))  (CONNECT_DATA=(SERVICE_NAME=sales.us.example.com)))

三、TAF 参数

BACKUP  

A different net service name for backup connections. A backup should be specified when using preconnect to pre-establish connections.

DELAY

每次重试的时间间隔. 如果 RETRIES 已经指定，那么DELAY默认是1秒，If a callback function is registered, then this parameter is ignored.

METHOD

设置快速转移从  primary node 到 backup node:

• basic:  在故障转移的时间间隔建立连接。此选项是故障转移前是不去对backup node做连接的，只有在故障发生时，才开始去连接backup node。

• preconnect: 建立预连接。此选项比basic方式提供了更快的故障转移，。用户建立connect，对primary node和backup node 节点都会建立连接

  BASIC方式在Failover时会有时间延迟，PRECONNECT方式虽然没有时间延迟，但是建立多个冗余连接会消耗更多资源
  

RETRIES

故障转移重试的次数，如果指定了DELAY，那么默认重试5次，If a callback function is registered, then this parameter is ignored.

TYPE

故障转移的类型. Three types of Oracle Net failover functionality are available by default to Oracle Call Interface (OCI) applications:

• session: Set to failover the session. If a user's connection is lost, then a new session is automatically created for the user on the backup. This type of failover does not attempt to recover select operations.

• select: Set to enable users with open cursors to continue fetching on them after failure. However, this mode involves overhead on the client side in normal select operations.

• none: This is the default. No failover functionality is used. This can also be explicitly specified to prevent failover from happening.

  对于select和session方式对于未提交的事务都会自动回滚，区别在于对select 语句的处理，对于select，用户正在执行的select语句会被转移到新的实例上，在新的节点上继续返回后续结果集，而已经返回的记录集则抛弃。

  假设用户正在节点1上执行查询，整个结果集共有100条记录，现在已从节点1上返回10条记录，这时节点1宕机，用户连接被转移到节点2上，如果是session模式，则需要重新执行查询语句；如果是select方式，会从节点2上继续返回剩下的90天记录，而已经从节点1返回的10条记录不会重复返回给用户，对于用户而言，感受不到这种切换，显然为了实现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，需要更多的资源也是用资源换时间的方案。

  
  

四、举例

TAF with Connect-Time Failover and Client Load Balancing

在客户端的tnsname.ora里添加如下

sales.us.example.com= (DESCRIPTION=  (LOAD_BALANCE=on)    ----随机访问一个监听，直到成功  (FAILOVER=on)   (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales1-server)         (PORT=1521))   (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales2-server)  ---ip地址或者主机名       (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (FAILOVER_MODE=       (TYPE=select)               --select方式的故障转移       (METHOD=basic))))

此方式建立的的故障转移，只能故障转移select语句，例如  updateinsert 语句在failover后操作会回退。

update t1 set sal=sal+1

此时数据库出现故障，那么就会被故障转移到另外一个节点，这是提交就会保存哦

SQL> commit;
commit
*
第 1 行出现错误:
ORA-25402: 事务处理必须重新运行

以此方式建立的taf，是建立在客户端的tnsname.ora里，这种方式建立的taf有一定的弊端，就是当客户端连接的时候，如果某个实例是在mount状态，那么客户端也是可能连接上去，由于是mount状态，所以会报错，而且现在应用程序都是用中间层，在客户端配置容易出错误，所以 有了服务端的taf，能解决此问题。

TAF Retrying a Connection

sales.us.example.com= (DESCRIPTION=  (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales1-server)         (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (FAILOVER_MODE=       (TYPE=select)        (METHOD=basic)       (RETRIES=20)       (DELAY=15))))   ----TAF重新连接的时间，每次重试连接间隔15秒，总共重试20次，故障转移类型为select方式

TAF Pre-establishing a Connection

sales1.us.example.com= (DESCRIPTION=  (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales1-server)         (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (INSTANCE_NAME=sales1)      (FAILOVER_MODE=       (BACKUP=sales2.us.example.com)  -----此backup指向下面红色字体       (TYPE=select)        (METHOD=preconnect))))sales2.us.example.com= (DESCRIPTION=  (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales2-server)         (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (INSTANCE_NAME=sales2)     (FAILOVER_MODE=       (BACKUP=sales1.us.example.com)        (TYPE=select)        (METHOD=preconnect))))

通过测试此预连接方式，只能故障转移已经连接到数据库上的连接select。如果已经发生故障，再去连接sales1.us.example.com是连接不上的. 发生故障前连接到发生故障的数据库的update ，在发生故障时没有提交的会回退。

Specifying the Instance Role for Primary and Secondary Instance Configurations

instance_role  在下列情况使用：

           1、你想明确指定连接到第一个实例还是第二个实例上去，默认是连接到第一个实例，主实例上。

           2、你想使用taf预连接到第二实例上去。

The INSTANCE_ROLE 值有：

• primary: 指定连接到 primary instance.

• secondary: 指定连接到secondary instance.

• any: 可以连接到连接地址里的任何一个实例。

Connection to Instance Role Type

sales_primary= (DESCRIPTION=  (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales1-server)         (PORT=1521))   (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales2-server)         (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (INSTANCE_ROLE=primary)))sales_secondary= (DESCRIPTION=  (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales1-server)         (PORT=1521))   (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales2-server)         (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (INSTANCE_ROLE=secondary)))

Connection To a Specific Instance

sales1= (DESCRIPTION=  (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales1-server)         (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (INSTANCE_ROLE=any)     (INSTANCE_NAME=sales1)     (SERVER=dedicated)))sales2= (DESCRIPTION=  (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales2-server)         (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (INSTANCE_ROLE=any)     (INSTANCE_NAME=sales2)     (SERVER=dedicated)))

TAF Pre-Establishing a Connection

sales1.example.com= (DESCRIPTION=  (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales1-server)         (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (INSTANCE_ROLE=primary)      (FAILOVER_MODE=       (BACKUP=sales2.example.com)        (TYPE=select)       (METHOD=preconnect))))sales2.example.com= (DESCRIPTION=  (ADDRESS=       (PROTOCOL=tcp)         (HOST=sales2-server)         (PORT=1521))   (CONNECT_DATA=     (SERVICE_NAME=sales.us.example.com)      (INSTANCE_ROLE=secondary)))

通过语句可以查看是否已经发生了故障转移,以及故障转移的方式

SQL> SELECT MACHINE, FAILOVER_TYPE, FAILOVER_METHOD, FAILED_OVER, COUNT(*)    FROM V$SESSION

    GROUP BY MACHINE, FAILOVER_TYPE, FAILOVER_METHOD, FAILED_OVER;
MACHINE FAILOVER_TYPE FAILOVER_M FAICOUNT(*)
---------------------------------------------------------------- ------------- ---------- --- ----------
rac1 NONE       NONE  NO     46

rac1 SESSION       BASIC YES      1

http://docs.oracle.com/cd/E11882_01/network.112/e10836/advcfg.htm#NETAG332