Understanding Cluster Components

Resources

Resources are hardware or software entities, such as disk groups and file systems, network interface cards (NIC), IP addresses, and applications. Controlling a resource means bringing it online (starting), taking it offline (stopping), and monitoring the resource.

Resource Dependencies

Resource dependencies determine the order in which resources are brought online or taken offline when their associated service group is brought online or taken offline. For example, a disk group must be imported before volumes in the disk group start, and volumes must start before file systems are mounted. Conversely, file systems must be unmounted before volumes stop, and volumes must stop before disk groups are deported.

In VCS terminology, resources are categorized as parents or children. Child resources must be online before parent resources can be brought online, and parent resources must be taken offline before child resources can be taken offline.

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In the preceding figure, the disk group and the network card can be brought online concurrently because they have no interdependencies. When each child resource required by the parent is brought online, the parent is brought online, and so on up the tree, until finally the application program is started. Conversely, when deactivating a service, the VCS engine, HAD, begins at the top. In this example, the application is stopped first, followed by the file system and the IP address, and so on down the tree until the application program is stopped.

Resource Categories

Different types of resources require different levels of control. In VCS there are three categories of resources:

• On-Off. VCS starts and stops On-Off resources as required. For example, VCS imports a disk group when required, and deports it when it is no longer needed.
• On-Only. VCS starts On-Only resources, but does not stop them. For example, VCS requires NFS daemons to be running to export a file system. VCS starts the daemons if required, but does not stop them if the associated service group is taken offline.
• Persistent. These resources cannot be brought online or taken offline. For example, a network interface card cannot be started or stopped, but it is required to configure an IP address. A Persistent resource has an operation value of None. VCS monitors Persistent resources to ensure their status and operation. Failure of a Persistent resource triggers a service group failover.

Service Groups

A service group is a logical grouping of resources and resource dependencies. It is a management unit that controls resource sets.

For example, a database service group may be composed of resources that manage logical network (IP) addresses, the database management software (DBMS), the underlying file systems, the logical volumes, and a set of physical disks managed by the volume manager (typically VERITAS Volume Manager in a VCS cluster).

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A single node may host any number of service groups, each providing a discrete service to networked clients. Each service group is monitored and managed independently. Independent management enables a group to be failed over automatically or manually idled for administration or maintenance without necessarily affecting other service groups. If the server crashes, all service groups on that node must be failed over elsewhere.

VCS monitors each resource in a service group and, when a failure is detected, restarts that service group. This could mean restarting it locally or moving it to another node and then restarting it. The method is determined by the type of failure incurred. In the case of local restart, the entire service group may not need to be restarted. It could be that only a single resource within the group is restarted to restore the application service.

Administrative operations are performed on resources, including starting, stopping, restarting, and monitoring at the service group level. Service group operations initiate administrative operations for all resources within the group. For example, when a service group is brought online, all resources within the group are also brought online. When a failover occurs in VCS, resources never fail over individually–the entire service group fails over. If there is more than one group defined on a server, one group may fail over without affecting the other groups on the server.

Types of Service Groups

VCS service groups fall in three main categories: failover, parallel, and hybrid.

Failover Service Groups

A failover service group runs on one system in the cluster at a time. Failover groups are used for most applications not designed to maintain data consistency when multiple copies are started, including most databases and NFS servers. VCS assures that a service group is online, partially online or in any state other than offline, such as attempting to go online or attempting to go offline.

Parallel Service Groups

A parallel service group runs simultaneously on more than one system in the cluster. It is more complex than a failover group, and requires an application that can be started safely on more than one system at a time, with no threat of data corruption.

Hybrid Service Groups

A hybrid service group is for replicated data clusters and is a combination of the two groups cited above. It behaves as a failover group within a system zone and a parallel group across system zones. It cannot fail over across system zones, and a switch operation on a hybrid group is allowed only if both systems are within the same system zone. If there are no systems within a zone to which a hybrid group can fail over, the nofailover trigger is invoked on the lowest numbered node. Hybrid service groups adhere to the same rules governing group dependencies as do parallel groups. See the Categories of Service Group Dependencies for more information.

The ClusterService Group

The ClusterService group is a special purpose service group, which contains resources required by VCS components. The group contains resources for Cluster Manager (Web Console), Notification, and the wide-area connector (WAC) process used in global clusters.

The ClusterService group can fail over to any node despite restrictions such as "frozen." It is the first service group to come online and cannot be autodisabled. The group comes online on the first node that goes in the running state. The VCS engine discourages taking the group offline manually.