This chapter provides information about the Cluster Reconfiguration Notification Protocol (CRNP). The CRNP enables failover and scalable applications to be “cluster aware.” More specifically, the CRNP provides a mechanism that enables applications to register for, and receive subsequent asynchronous notification of, Oracle Solaris Cluster reconfiguration events. Data services that run within the cluster and applications that run outside the cluster can register for notification of events. Events are generated when membership in a cluster changes and when the state of a resource group or a resource changes.