Akka cluster is a decentralized peer-to-peer cluster with no single point of failure or single point of bottleneck. Cluster membership is communicated between nodes using a Gossip Protocol. You can listen to membership changes by registering an ordinary actor as subscriber. It will be notified with a message when for example a new member joins the cluster.

Read more in the docs for Scala or in the docs for Java. Cluster membership is released as an experimental module in Akka 2.1.