Oracle B32100-01 - Manual (Page 121)

Overview of High Availability Configurations
Installing in High Availability Environments: Overview 7-3
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Application independence
Some applications may not be suited to an active-active configuration. This may
include applications which rely heavily on application state or on information
stored locally. An active-passive configuration has only one instance serving
requests at any particular time.
In general, the term OracleAS Cold Failover Cluster describes clustering at the Oracle
Application Server instance level. However, if it is necessary to call out the specific
type of instances being clustered, this document will use OracleAS Cold Failover
Cluster (type) to characterize the cluster solution. For example:
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OracleAS Cold Failover Cluster (Identity Management)
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OracleAS Cold Failover Cluster (Infrastructure)
From the entry point of an Oracle Application Server system (content cache) to the
back end layer (data sources), all the tiers that are crossed by a client request can be
configured in a redundant manner either in an active-active configuration using
OracleAS Clusterss or in an active-passive configuration using OracleAS Cold Failover
Clusters.
See Chapter 8, "Installing in High Availability Environments: OracleAS Cold Failover
Cluster" for installation details.
7.1.2 OracleAS Clusters
Oracle Application Server provides an active-active model for all its components with
OracleAS Clusters. In an OracleAS Clusters, two or more Oracle Application Server
instances are configured to serve the same application workload. These instances
typically run on different nodes.
You need an external load balancer in front of the nodes. Clients direct requests to
these nodes through the load balancer, which then sends the requests to one of the
nodes for processing. The load balancer uses its own algorithm to decide which node
to send a request to.
The most common properties of an OracleAS Clusters configuration include:
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Identical instance configuration
The instances are meant to serve the same workload or application. Their identical
configuration guarantees that they deliver identical responses to the same request.
Note that some configuration properties are allowed to be instance-specific, such
as local host name information.
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Managed as a virtual single instance
Changes in configuration made to one instance usually need to be propagated to
the other instances in an active-active topology.
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Independent operation
The loss of one Oracle Application Server instance in an active-active topology
should not affect the ability of the other instances to continue to serve requests.
The advantages of an OracleAS Clusters configuration include:
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Increased availability
An active-active topology has built-in redundancy (multiple Oracle Application
Server instances run the same components). Loss of one instance can be tolerated
because other instances can continue to serve the same requests.