Figure 7.1 The components of a failover cluster
A ONE-NODE CLUSTER?
I stated that a cluster consists of two or more nodes, but strictly speaking, that is
not accurate. A cluster can consist of a single node, and many times you may start
with a one-node cluster. Remember, the point of a cluster is to provide high
availability of services by enabling services to move between servers if a server
fails. With a single-node cluster, if the node fails, there is nowhere for the services
to move to. Therefore, you always want at least two nodes in a cluster to provide
high-availability services.
However, some features of Failover Clustering apply even to single-node
environments, such as the ability to monitor services that run inside virtual
machines and restart the virtual machine if a service fails three times.Figure 7.1 shows an active node and a passive node. In the example, a single service is
configured in the cluster. The node the service is running on is the active node. The
node not running the service is the passive node, but it would become the active node
if the service moved to it as part of a planned move or if the existing active node failed.
While we will talk about active and passive nodes, in reality we can configure multiple
services and applications within a cluster that can be hosted on different nodes in the
cluster, and so at any time every node may be running a specific server or application.
You just need to ensure that the resources in the cluster nodes are sufficient to run