Oracle Essentials [Electronic resources] : Oracle Database 10g, 3rd Edition نسخه متنی

8.5 High Availability

From an operational perspective,
OLTP systems represent a company's electronic
central nervous system, so the databases that support these systems
must be highly available. Oracle has a number of features that
contribute to high availability:

Standby database

Oracle
provides database redundancy by maintaining a copy of the primary
database on another machine, usually at another site. Redo logs from
the primary server are shipped to the standby server and applied
there to duplicate the production activity.
Oracle8i introduced the automated shipping of
redo logs to the standby site and the ability to open the standby
database for read-only access for reporting.

Oracle9i Release
2 introduced the concept of logical
standby. With a logical standby database the
changes are propagated with SQL statements, rather than redo logs,
which allows the logical standby database to be used for other
database operations.

Transparent Application Failover (TAF)

TAF is a programming interface that
enables you to automatically reconnect a user session to another
Oracle instance should the primary instance fail.

Advanced Queuing (AQ)

AQ
provides a method for asynchronous, or deferred, intersystem
communication, allowing systems to operate more independently.
Avoiding direct system dependencies can help to avoid
"cascading" failures, allowing
interconnected systems to continue to operate even if one system
fails. AQ is described in more detail in the following section.

Replication

You can use Oracle's built-in
replication
functionality to provide data redundancy. Changes made by
transactions are replicated synchronously or asynchronously to other
databases. If the primary database fails, the data is available from
the other databases. As of Oracle9i Release 2,
log-based replication and Advanced Queueing were included as part of
Streams.

Real Application Clusters

Real Application Clusters (Oracle
Parallel Server prior to Oracle9i), increased
the scalability of the Oracle database. However, by supporting
multiple instances with full access to one database, RAC also
provides the highest levels of availability for protection from the
failure of a node in a clustered or MPP environment. If one node
fails, the surviving nodes provide continued access to the database.
With Oracle Database 10g, grid computing deployment further extends
availability capabilities.

For a more detailed discussion of high availability, see Chapter 10.