Oracle Essentials: Oracle9i, Oracle8i and Oracle8, Second Edition

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 introduces the automated shipping of redo logs to the standby site and the ability to open the standby database for read-only access for reporting.

Later releases of Oracle9i will introduce 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, because if one system is down the others continue to function. AQ is described in more detail in the following section.

Replication

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