Fencing is a very important concept in computer clusters for HA (High Availability). A cluster sometimes detects that one of the nodes is behaving strangely and needs to remove it. This is called fencing and is commonly done with a STONITH resource. Fencing may be defined as a method to bring an HA cluster to a known state.
Every resource in a cluster has a state attached. For example:
resource r1 is started on alice
. In an HA cluster, such
a state implies that resource r1 is stopped on all nodes except
alice
, because the cluster must make sure that every resource
may be started on only one node. Every node must report every change
that happens to a resource. The cluster state is thus a collection of
resource states and node states.
When the state of a node or resource cannot be established with certainty, fencing comes in. Even when the cluster is not aware of what is happening on a given node, fencing can ensure that the node does not run any important resources.