Considerations For Configuring Ha Services - Red Hat ENTERPRISE LINUX 5 - ADMINISTRATION Manual

Considerations for Configuring HA Services

2.4. Considerations for Configuring HA Services
You can create a cluster to suit your needs for high availability by configuring HA (high-availability)
services. The key component for HA service management in a Red Hat cluster, rgmanager,
implements cold failover for off-the-shelf applications. In a Red Hat cluster, an application is configured
with other cluster resources to form an HA service that can fail over from one cluster node to another
with no apparent interruption to cluster clients. HA-service failover can occur if a cluster node fails or if
a cluster system administrator moves the service from one cluster node to another (for example, for a
planned outage of a cluster node).
To create an HA service, you must configure it in the cluster configuration file. An HA service
comprises cluster resources. Cluster resources are building blocks that you create and manage in the
cluster configuration file — for example, an IP address, an application initialization script, or a Red Hat
GFS shared partition.
An HA service can run on only one cluster node at a time to maintain data integrity. You can specify
failover priority in a failover domain. Specifying failover priority consists of assigning a priority level to
each node in a failover domain. The priority level determines the failover order — determining which
node that an HA service should fail over to. If you do not specify failover priority, an HA service can fail
over to any node in its failover domain. Also, you can specify if an HA service is restricted to run only
on nodes of its associated failover domain. (When associated with an unrestricted failover domain, an
HA service can start on any cluster node in the event no member of the failover domain is available.)
Figure 2.1, "Web Server Cluster Service Example"
shows an example of an HA service that is a web
server named "content-webserver". It is running in cluster node B and is in a failover domain that
consists of nodes A, B, and D. In addition, the failover domain is configured with a failover priority to
fail over to node D before node A and to restrict failover to nodes only in that failover domain. The HA
service comprises these cluster resources:
• IP address resource — IP address 10.10.10.201.
• An application resource named "httpd-content" — a web server application init script /etc/
init.d/httpd (specifying httpd).
• A file system resource — Red Hat GFS named "gfs-content-webserver".
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