Appendix D. HA Resource Behavior

This appendix describes common behavior of HA resources. It is meant to provide ancillary information that may be helpful in configuring HA services. You can configure the parameters with Luci, system-config-cluster, or by editing etc/cluster/cluster.conf. For descriptions of HA resource parameters, refer to Appendix C, HA Resource Parameters. To understand resource agents in more detail you can view them in /usr/share/cluster of any cluster node.

Note

To fully comprehend the information in this appendix, you may require detailed understanding of resource agents and the cluster configuration file, /etc/cluster/cluster.conf.
An HA service is a group of cluster resources configured into a coherent entity that provides specialized services to clients. An HA service is represented as a resource tree in the cluster configuration file, /etc/cluster/cluster.conf (in each cluster node). In the cluster configuration file, each resource tree is an XML representation that specifies each resource, its attributes, and its relationship among other resources in the resource tree (parent, child, and sibling relationships).

Note

Because an HA service consists of resources organized into a hierarchical tree, a service is sometimes referred to as a resource tree or resource group. Both phrases are synonymous with HA service.
At the root of each resource tree is a special type of resource — a service resource. Other types of resources comprise the rest of a service, determining its characteristics. Configuring an HA service consists of creating a service resource, creating subordinate cluster resources, and organizing them into a coherent entity that conforms to hierarchical restrictions of the service.
This appendix consists of the following sections:

Note

The sections that follow present examples from the cluster configuration file, /etc/cluster/cluster.conf, for illustration purposes only.

D.1. Parent, Child, and Sibling Relationships Among Resources

A cluster service is an integrated entity that runs under the control of rgmanager. All resources in a service run on the same node. From the perspective of rgmanager, a cluster service is one entity that can be started, stopped, or relocated. Within a cluster service, however, the hierarchy of the resources determines the order in which each resource is started and stopped.The hierarchical levels consist of parent, child, and sibling.
Example D.1, “Resource Hierarchy of Service foo” shows a sample resource tree of the service foo. In the example, the relationships among the resources are as follows:
  • fs:myfs (<fs name="myfs" ...>) and ip:10.1.1.2 (<ip address="10.1.1.2 .../>) are siblings.
  • fs:myfs (<fs name="myfs" ...>) is the parent of script:script_child (<script name="script_child"/>).
  • script:script_child (<script name="script_child"/>) is the child of fs:myfs (<fs name="myfs" ...>).

Example D.1. Resource Hierarchy of Service foo

<service name="foo" ...>
    <fs name="myfs" ...>
        <script name="script_child"/>
    </fs>
    <ip address="10.1.1.2" .../>
</service>

The following rules apply to parent/child relationships in a resource tree:
  • Parents are started before children.
  • Children must all stop cleanly before a parent may be stopped.
  • For a resource to be considered in good health, all its children must be in good health.