Data Guard Real Time Apply Tips Oracle Database Tips by Donald BurlesonDecember 9, 2015

Oracle Data Guard

Real Time Apply

Data Guard Log Apply Services can automatically apply information from redo logs to standby databases, keeping data synchronized with the primary database. In general, log apply services wait for the full archived redo log to arrive to the Oracle instance host before applying to the standby database.

Redo data is received by the remote file server process (RFS) on the standby system, where the RFS process writes the redo data to either archived redo logs or optionally to a standby redo log.

However, if a standby redo log is used,  real time apply can be enabled, which allows Data Guard to recover redo data from the current standby redo log as it is being filled by the RFS process.  This facility of real time apply is a new feature introduced in Oracle database 10g.

It has the benefit of quicker switchover, instantly updating results after a physical Oracle instance is changed to read-only with up-to-date reporting from a logical standby database.

The ALTER DATABASE statement can be used to enable the real-time apply feature. For example, in physical standby databases, issue the statement,

ALTER DATABASE RECOVER MANAGED Oracle instance USING CURRENT LOGFILE;

For logical standby databases, issue the statement,

ALTER DATABASE START LOGICAL STANDBY APPLY IMMEDIATE;

To determine if real time apply is enabled, the recovery_mode column in the v$archive_dest_status view can be queried. It will display MANAGED REAL TIME APPLY when real time apply is enabled.

Real Time Apply

Data Guard Log Apply Services can automatically apply information from redo logs to standby databases, keeping data synchronized with the primary database. In general, log apply services wait for the full archived redo log to arrive to the standby database host before applying to the standby database.

Redo data is received by the remote file server process (RFS) on the standby system, where the RFS process writes the redo data to either archived redo logs or optionally to a standby redo log. However, if you use a standby redo log, you can optionally enable real time apply, which allows Data Guard to recover redo data from the current standby redo log as it is being filled by the RFS process. This facility of real time apply is a new feature introduced in Oracle database 10g.

It has the benefit of quicker switchover, instantly updating results after you change a physical standby database to read-only with up-to-date reporting from a logical standby database.

You can use the ALTER DATABASE statement to enable the real-time apply feature. For example:

For physical standby databases, issue the statement,

ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE;

For logical standby databases, issue the statement,

ALTER DATABASE START LOGICAL STANDBY APPLY IMMEDIATE;

To determine if real time apply is enabled, you can query the recovery_mode column in the v$archive_dest_status view. It will display MANAGED REAL TIME APPLY when real time apply is enabled.

If you define a delay on a destination (with the DELAY attribute) and use Real Time Apply, the delay is ignored.

When, for any reason, the apply service is unable to keep up, the apply service will automatically go to the archive log files as needed. It will also try to catch up and go back to reading the current standby redo log file as soon as possible.