The software which Oracle Corporation markets as Oracle Data Guard forms an extension to the Oracle database management system. It aids in establishing and maintaining secondary "standby databases" as alternative/supplementary repositories to production "primary databases".
Oracle Corporation provides both GUI and command-line tools for managing Data Guard configurations.
Data Guard supports both physical standby and logical standby sites. It runs under most Oracle editions, but not under Oracle Express Edition (XE).
With appropriately set-up Data Guard operations, DBAs can facilitate failovers or switchovers to alternative hosts in the same or alternative locations.
Operation
A primary site which has an archived redo log to transfer invokes the RFS (remote file server) background process within the Oracle instance running on a standby host. The RFS process then performs the task of receiving archived redo logs sent from the primary database.
Alternatively, a supplementary mechanism may transfer the archived redo logs. On the standby database a Fetch Archive Log (FAL) client monitors for gaps in the sequence of received logs. If it finds a gap, it may invoke one or more Fetch Archive Log (FAL) servers to run on the primary database to forward the missing item(s).[1]
Once the archived redo logs have arrived, other processes (an ARCH (Archiver process), MRP (Managed Recovery Process), LSP (Logical Standby Process)) may set about applying their contents to the standby database..
Advantages
Data Guard provides high availability for a database system. It can also reduce the human intervention required to switch between databases.