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About High Availability Clusters and
HADB
A highly availability cluster inSun Java System Application Server Enterprise Edition
integrates a state replication service with the clusters and load balancer created earlier,
enabling failover of HTTP sessions.
HttpSession objects and Stateful Session Bean state is stored in HADB, a
high-availability database for storing session state. This horizontally scalable state
management service can be managed independently of the application server tier. It
was designed to support up to 99.999% service and data availability with load
balancing, failover and state recovery capabilities.
Keeping state management responsibilities separated from Application Server has
significant benefits. Application Server instances spend their cycles performing as a
scalable and high performance Java™ 2 Platform, Enterprise Edition (J2EE™ platform)
containers delegating state replication to an external high availability state service.
Due to this loosely coupled architecture, application server instances can be easily
added to or deleted from a cluster. The HADB state replication service can be
independently scaled for optimum availability and performance. When an application
server instance also performs replication, the performance of J2EE applications can
suffer and can be subject to longer garbage collection pauses.
Because each HADB node requires 512 Mbytes of memory, you need 1 Gbyte of
memory to run two HADB nodes on the same machine. If you have less memory, set
up each node on a different machine. Running a two-node database on only one host
is not recommended for deployment since it is not fault tolerant.
HADB Preinstallation Steps
This procedure covers the most common preinstallation tasks. For information on
other preinstallation topics, including prerequisites for installing HADB, configuring
network redundancy, and file system support, see Chapter 2, “Installing and Setting
Up High Availability Database,” in Sun Java System Application Server Enterprise
Edition 8.1 2005Q2 High Availability Administration Guide.
The recommended system configuration values in this section are sufficient for
running up to six HADB nodes and do not take into consideration other applications
on the system that also use shared memory.
26 Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Quick Start Guide