Oracle Data Guard

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The software which Oracle Corporation markets as Oracle Data Guard forms an extension to the Oracle RDBMS. 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. Oracle Corporation makes Data Guard available only as a bundled feature included within its "Enterprise Edition" of the Oracle RDBMS.^[1]

With appropriately set-up Data Guard operations, DBAs can facilitate failovers or switchovers to alternative hosts in the same or alternative locations.

1 Configurations
2 Operation
3 Advantages
4 References

[edit] Configurations

A physical standby database attempts to replicate the exact structure and layout of its primary database across the Oracle Net network layer. It can function either in archiving ("recovery") mode or in read-only mode, but not in both modes at the same time. It can become a standalone database when required, but needs rebuilding before reverting from standalone to standby functionality.

Logical standby databases re-apply SQL transactions generated on their primary node; thus physical structures and organization. As of Oracle release 11g Data Guard may operate across different "CPU architectures, operating systems ... operating system binaries (32-bit/64-bit), and Oracle database binaries (32-bit/64-bit)."^[2] Users (under certain conditions) can write to as well as read from logical standby databases; and such databases can switch in function to and fro between primary and standby roles.

[edit] 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).^[3]

Once the archived redo logs have arrived, other processes (such as an ARCH (Archiver process), an MRP (Managed Recovery Process), and/or an LSP (Logical Standby Process)) may set about applying the log contents to the standby database..

[edit] Advantages

Data Guard provides high availability for a database system. It can also reduce the human intervention required to switch between databases at disaster-recovery ("failover") or upgrade/maintenance ("switchover") time.