ActiveCluster is an API for writing cluster-based distributed systems. It is a simple API which reuses a few of the standard JMS APIs for Message, Destination, MessageListener and adds a few simple interfaces for working with clusters.
At the core abstraction ActiveCluster maintains a list of Nodes, each Node representing a member of the cluster.
A node can optionally have some local state which is replicated to other members of the cluster.
You can look at the node list whenever you like and listen to nodes joining, leaving and updating their state.
In addition the Cluster (the group of nodes) and each Node in the cluster have a Destination. The other part of the ActiveCluster API is a simplified facade to messaging - way simpler and more abstract than JMS - which allows you to send application level messages to the cluster as a whole or to individual members of the cluster.