In this example, a two-node cluster exports an NFS file system to clients. Both nodes are connected to shared storage, which enables them to access the directories being shared. A single service group, NFS_Group, fails over between System A and System B, as necessary.
The VCS engine, HAD, reads the configuration file, determines what agents are required to control the resources in the service group, and starts the agents. HAD uses resource dependencies to determine the order in which to bring the resources online. VCS issues online commands to the corresponding agents in the correct order.
Figure: Dependency graph for the sample NFS group shows a dependency graph for the sample NFS group.
VCS starts the agents for DiskGroup, Mount, Share, NFS, NIC, IP, and NFSRestart on all systems that are configured to run NFS_Group.
The resource dependencies are configured as follows:
The /home file system (configured as a Mount resource), requires that the disk group (configured as a DiskGroup resource) is online before you mount.
The NFS export of the home file system (Share) requires that the file system is mounted and that the NFS daemons (NFS) are running.
The high availability IP address, nfs_IP, requires that the file system (Share) is shared and that the network interface (NIC) is up.
The NFSRestart resource requires that the IP address is up.
The NFS daemons and the disk group have no child dependencies, so they can start in parallel.
The NIC resource is a persistent resource and does not require starting.
You can configure the service group to start automatically on either node in the preceding example. It then can move or fail over to the second node on command or automatically if the first node fails. On failover or relocation, to make the resources offline on the first node, VCS begins at the top of the graph. When it starts them on the second node, it begins at the bottom.