Databases Reference
In-Depth Information
Because OCI is a low-level API, writing programs with OCI requires more effort and
sophistication on the part of the developer. Fortunately, Oracle uses OCI to write client
tools and various drivers, so that applications using these tools can leverage TAF. Sup‐
port for TAF in ODBC and JDBC drivers is especially useful; it means that TAF can be
leveraged by any client application that uses these drivers to connect to Oracle. For
example, TAF can provide automatic reconnection for a third-party query tool that uses
ODBC. To implement TAF with ODBC, set up an ODBC data source that uses an Oracle
Net service name that is configured to use TAF in the Oracle Net configuration files.
ODBC uses Oracle Net and can therefore leverage the TAF feature.
TAF and various Oracle configurations
Although the TAF-Real Application Clusters combination is the most obvious combi‐
nation for high availability, TAF can be used with a single Oracle instance or with mul‐
tiple databases, each accessible from a single instance. Some possible configurations are
as follows:
• TAF can automatically reconnect clients back to their original instances for cases
in which the instance failed but the node did not. An automated monitoring system,
such as the management framework under Oracle Enterprise Manager, can detect
instance failure quickly and restart the instance. The fast-start recovery features in
Oracle enable very short crash recovery times. Users that aren't performing heads-
down data entry work can be automatically reconnected by TAF and might never
be aware that their instance failed and was restarted.
• In simple clusters, TAF can reconnect users to the instance started by simple hard‐
ware failover on the surviving node of a cluster. The reconnection cannot occur
until the alternate node has started Oracle and has performed crash recovery.
• When there are two distinct databases, each with a single instance, TAF can recon‐
nect clients to an instance that provides access to a different database running in
another data center. This clearly requires replication of the relevant data between
the two databases.
Oracle Application Continuity
Oracle Database 12
c
introduces
Application Continuity
, an infrastructure that will re‐
play an application request to the database when a recoverable error is received during
unplanned or planned outages. During lost database sessions, the client will remain
current with entered data, returned data, cached data, and variables. Where transactions
are being initiated to the database, the client may not know if a commit occurred after
it was issued or, if it was not issued or executed, whether it must resubmit the request
upon rollback of an in-flight transaction. Restored sessions where Application Con‐
tinuity is deployed include all states, cursors, variables, and the most recent transaction
if there is one.
