Description: The AntiAffinityClassNames property is used to identify groups that should not be hosted on the same cluster node.

A group's AntiAffinityClassNames property consists of zero or more arbitrary user-defined strings. If the AntiAffinityClassNames properties of two or more groups contain at least one identical string, the groups are said to be anti-affined. By default, all groups are affined (because their AntiAffinityClassNames property is NULL).

When a group is moved and the destination node is not specified (during failover, for example), anti-affinity affects the algorithm used to determine the destination node as follows: 1. Using the preferred owner list of the group being moved, the Cluster service finds the next preferred node. 2. If the node is not hosting any group anti-affined with the group being moved, it is selected as the destination node. 3. If the next preferred available node is currently hosting a group anti-affined with the group being moved, the Cluster service moves to the next preferred available node in the preferred owner list. 4. If the only available nodes are hosting anti-affined groups, the Cluster service ignores anti-affinity and selects the next preferred available node as the destination node.

Use this property to identify groups that should not be hosted on the same node. Generate a unique string value (such as a GUID) and add it to the AntiAffinityClassNames property of each group that should be anti-affined.

Note that because of the behavior described in point 4 above, anti-affinity does not guarantee that groups will never be hosted by the same node. If you have an application that cannot support more than one instance per node under any circumstances, you need to create a resource DLL to enforce that limitation.