Oracle Data Guard work on two database roles Primary and Standby. In the previous article, we have seen switching the role of Primary and standby database and failover Primary role to Standby database manually.

To see Manual Switch Over Manual SwitchOver in Oracle

To see Manual Fail Over Manual Failover in Data Guard

With Oracle Data Guard we can enable fast-start failover.

To enable Fast Start Failover we need to choose standby database in advance. Fast start failover enables oracle data guard to fail over rapidly and automatically to a Previously chosen standby database. And it does not need manual intervention.

It provides higher database availability as we don’t need to manual intervene to perform failover task in case of a Primary database crash.

Now, When does Fast-Start Failover work?

Once configured, fast start failover will work in the following circumstances.

  1. Loss of connectivity between Primary Database and Observer or Primary Database and Standby Database. The observer is an Oracle Call Interface (OCI) client-side component that typically runs on a separate computer and monitors the availability of the primary database.

2. An instance is crashed in the single instance database

3. All instance crashes of RAC database

4. If the database is aborted using shutdown abort.

5. An application initiates a fast-start failover by calling the DBMS_DG.INITIATE_FS_FAILOVER function.

6. If any of the following condition occurs

  • Data file becomes offline as a result of the write error.
  • Dictionary corruption of a critical database object occurs.
  • Control file is damaged permanently because of a write error.
  • LGWR is unable to write to any member of the log group because of an I/O error
  • Archiver is unable to archive a redo log because the device is full or unavailable

Observer Software 
To manage availability of database observer should be installed on a separate computer system which should be separate then Primary Database and Standby Database server.DGMGRL contains observer software.

So to install observer software on a separate computer system we need to install DGMGRL.

  • Install the complete Oracle Client Administrator by choosing the Administrator option in the Oracle Universal Installer. This installation includes DGMGRL but it does not include the Oracle Enterprise Manager agent. This type of installation enables you to manage the observer by using DGMGRL commands but not Oracle Enterprise Manager.
  • Install the full Oracle Database software kit. This installation includes DGMGRL.

And as it name says observer observes activities between primary and standby database and it is responsible to decide fast-start failover, we need to install observer software on the machine which has the highest level of connectivity. This method could introduce a false failover, that is a fast-start failover occurs even though the primary is running and accessible locally, but if the clients can’t get to the primary it may as well be down.

Prerequisites for Fast Start Fail Over :

  1. Database Protection should be Maximum Availability or Maximum Performance till 11g.
  2. LOGXPTMODE property should contain SYNC in case of Maximum Availability or ASYNC in case of Maximum Performance.
  3. Flashback database must be enabled for Primary and for standby database which we are choosing as a target for fast-start failover.
  4. Configure the tnsnames.ora file on the observer system so that the observer is able to connect to the primary database and the preselected target standby database.
  5. Create a static service name so that the observer can automatically restart a database as part of reinstatement.

In the next article, we will see configuring fast-start failover.

Stay tuned for More articles on Oracle DataGuard

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