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Log services in dataguard

Log Transport Services

Automatically transfers redo data from a primary to a standby database within a a data guard environment.
Logs can be transmitted either by arch or lgwr.
See also v$managed_standby.

Log Apply Services

When redo arrives on a standby database (as sent by the log transport services they are stored as archived redo logs. Log apply services applies them on the standby database, thus synchronizing the primary database with the standby database.
In the case of physical standby databases, data guard applies the redo directly while in the case of logical standby databases, it converts the redo first into SQL statements and then executes them.
The log apply service also detects missing redo logs on a standby system and tries to retrieve them from the primary or another standby database in the environment.
Initiating log apply services on a physical standby database.
See also v$managed_standby.

Role Management Services

These services are in charge to change the role of database (primary to standby or vice versa). See switchover and failover.