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The High Availability Add-On for Red Hat Enterprise Linux provides continuous availability of services by eliminating single points of failure. By offering failover services between nodes within a cluster, the High Availability Add-On supports high availability for up to 16 nodes.
The High Availability Add-On also enables failover for off-the-shelf applications such as Apache, MySQL, and PostgreSQL, any of which can be coupled with resources like IP address and single-node file systems to form highly available services. The High Availability Add-On can also be easily extended to any user-specified application that is controlled by an init script per UNIX System V (SysV) standards.
When using the High Availability Add-On, a highly available service can fail over from one node to another with no apparent interruption to cluster clients. The High Availability Add-On also ensures absolute data integrity when one cluster node takes over control of a service from another cluster node. It achieves this by promptly evicting nodes from the cluster that are deemed to be faulty using a method called "fencing" that prevents data corruption. The High Availability Add-On supports several types of fencing, including both power- and storage area network (SAN) -based fencing.