【DataGuard】Oracle 11g DataGuard 新特性之 Active Standby：Real-Time Apply+Query

檢視角色、狀態和保護模式

select database_role,open_mode from v$database;
主庫處於read write狀態，備庫處於mounted狀態

物理備庫 應用日誌

alter database recover managed standby database disconnect;

物理備庫在處於日誌應用狀態時，是無法開庫進行資料查詢的。直接嘗試開庫會報錯。

直接嘗試open備庫報錯
select instance_name,status from v$instance;
alter database open;

物理備庫取消日誌應用：

alter database recover managed standby database cancel;

物理備庫開庫（read only）

alter database open;

select database_role,open_mode from v$database;

如果物理備庫從read only狀態回到日誌應用工狀態，要先關掉物理備庫，再將庫啟到mount狀態，最後重新應用日誌。

shutdown immediate;

mount startup mount;

alter database recover managed standby database disconnect from session; 

這種日誌應用方式無法實現備庫上日誌應用和資料查詢同時進行。
如果想實現Active Standby，即日誌實時應用 同時備庫可以進行資料查詢，備庫上需使用Real-Time Apply來應用日誌。

【Oracle官方文件中對Real-Time Apply介紹】

If the real-time apply feature is enabled, apply services can apply redo data as it is received, without waiting for the current standby redo log file to be archived. This results in faster switchover and failover times because the standby redo log files have been applied already to the standby database by the time the failover or switchover begins.

Use the ALTER DATABASE statement to enable the real-time apply feature, as follows:

• For physical standby databases, issue the ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE statement.

• For logical standby databases, issue the ALTER DATABASE START LOGICAL STANDBY APPLY IMMEDIATE statement.

Real-time apply requires a standby database that is configured with a standby redo log and that is in ARCHIVELOG mode.

Figure 7-1 shows a Data Guard configuration with a local destination and a standby destination. As the remote file server (RFS) process writes the redo data to standby redo log files on the standby database, apply services can recover redo from standby redo log files as they are being filled.