一般我们在搭建DG的过程中，通常都会进行switchover和failover切换演练，不过对于客户来说，他们更加注重failover切换演练，关心的东西才会关注。本章就简单介绍下failover切换介绍
1、当生产库异常宕机了，在备库上查看是否有日志GAP
1） 查看是否有日志GAP：
SQL> SELECT THREAD#, LOW_SEQUENCE#, HIGH_SEQUENCE# FROM V$ARCHIVE_GAP;
no rows selected
2） 如果有，则拷贝过来
3） 手工的注册这些日志：
SQL>  ALTER DATABASE REGISTER LOGFILE '路径';
注：如果返回的有记录，按照返回信息对应的复制归档文件到待转换的standby服务器。这一步非常重要，必须确保所有已生成的归档文件均已存在于standby 服务器，不然可能会数据不一致造成转换时报错
2、结束日志apply
SQL>  alter database recover managed standby database cancel; #停止归档日志应用
3、停止接收日志
SQL>  alter database recover managed standby database finish force; #停止日志接收，结束备库活动着的RFS进程，以便执行failover切换
4、切换数据库为primary
SQL>  alter database commit to switchover to primary with session shutdown;
5、open数据库
SQL> alter database open;
注：恢复备库：recover standby database until cancel;激活备库：alter database activate standby database;

具体操作笔记：
SQL> SELECT THREAD#, LOW_SEQUENCE#, HIGH_SEQUENCE# FROM V$ARCHIVE_GAP;

no rows selected

SQL> alter database recover managed standby database cancel;

Database altered.

SQL> alter database recover managed standby database finish force;

Database altered.

SQL> select database_role,switchover_status from v$database;

DATABASE_ROLE    SWITCHOVER_STATUS
---------------- --------------------
PHYSICAL STANDBY TO PRIMARY

SQL> alter database commit to switchover to primary with session shutdown;

Database altered.

SQL> alter database open;

Database altered.

SQL> select database_role,switchover_status from v$database;

DATABASE_ROLE    SWITCHOVER_STATUS
---------------- --------------------
PRIMARY          FAILED DESTINATION

SQL> select instance_name,status from v$instance;

INSTANCE_NAME    STATUS
---------------- ------------
beiku            OPEN

SQL> select NAME,RESETLOGS_TIME,LOG_MODE,OPEN_MODE,PROTECTION_MODE,REMOTE_ARCHIVE,DATABASE_ROLE,SWITCHOVER_STATUS,FORCE_LOGGING  from v$database;

NAME      RESETLOGS LOG_MODE     OPEN_MODE            PROTECTION_MODE      REMOTE_A DATABASE_ROLE    SWITCHOVER_STATUS    FOR
--------- --------- ------------ -------------------- -------------------- -------- ---------------- -------------------- ---
XUH       12-JUL-15 ARCHIVELOG   READ WRITE           MAXIMUM PERFORMANCE  ENABLED  PRIMARY          FAILED DESTINATION   YES

SQL>