Oracle Data Guard ensures high availability, data protection, and disaster recovery for enterprise data. Data Guard provides a comprehensive set of services that create, maintain, manage, and monitor one or more standby databases to enable production Oracle databases to survive disasters and data corruptions

What are the differences between physical, logical, snapshot standby and ADG (or) what are different types of standby databases?

Answer :

Physical standby – in mount state, MRP will apply archives

ADG (Active Data Guard) – in READ ONLY state, MRP will apply archives

Logical standby – in READ ONLY state, LSP will run

Snapshot standby databases – Physical standby database can be converted to snapshot standby database, which will be in READ WRITE mode, can do any kind of testing, then we can convert back snapshot standby database to physical standby database and start MRP which will apply all pending archives.

If you didn't have access to the standby database and you want to find out what error has occurred in a Data Guard configuration, what view would you check in the primary database to check the error message?

Answer :

You can check the v$dataguard_status view.

select message from v$dataguard_status;

What is failover/switchover (or) what is the difference between failover and switchover?

Answer :

Switchover – This is done when both primary and standby databases are available. It is pre-planned.

Failover – This is done when the primary database is NO longer available (i.e. in a Disaster). It is not pre-planned.