主备库切换原因：

1. switch：用户主动切换；

2. failover：主库出现故障，强行切换；

switch切换过程：先主库再备库

主库

1. 查看switchover_status：select database_role,switchover_status from v$database;

如果switchover_status为：to standby 则：

ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY

否则用：ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY WITH SESSION SHUTDOWN;

2. 重启数据库：

shutdown immediate;

startup nomount;

3. 以备库mount

alter database mount standby database;

备库

1. 应用redo：alter database recover managed standby database disconnect from session;

2. 查看switchover_status：select database_role,switchover_status from v$database;

如果switchover_status为：to primary 则：ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY

否则用：ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY WITH SESSION SHUTDOWN;

3. 重启数据库

shutdown immediate;

startup;

failover 主备库切换

1.. 停止日志应用
alter database recover managed standby database cancel;

2. 关闭standby日志传输
alter database recover managed standby database finish force;

3. 切换到primary
alter database commit to switchover to primary with session shutdown;

做这一步的时候，若存在gap，则会报ORA-16139:Switchover: Media recovery required - standby not in limbo 错误。

做测试的时候，若先起主库再起备库，且未等待备库相关日志传输完毕，就会出现这个问题。此时需要强制切换
alter database activate physical standby database;

4. 重启数据库到open状态