WebLogic Server uses TWO mechanisms to determine if a given server instance is UNAVAILABLE.
Each WebLogic Server instance in a cluster uses multicast to broadcast regular "heartbeat" messages that advertise its availability. By monitoring heartbeat messages, server instances in a cluster determine when a server instance has failed. The other server instances will drop a server instance from the cluster, if they do not receive three consecutive heartbeats from that server instance.

WebLogic Server ALSO monitors socket errors to determine the availability of a server instance. For example, if server instance A has an OPEN socket to server instance B, and the socket unexpectedly closes, server A assumes that server B is OFFLINE.

WebLogic Server uses two mechanisms to determine if a given server instance is unavailable.
Each WebLogic Server instance in a cluster uses multicast to broadcast regular "heartbeat" messages that advertise its availability. By monitoring heartbeat messages, server instances in a cluster determine when a server instance has failed. The other server instances will drop a server instance from the cluster, if they do not receive three consecutive heartbeats from that server instance.

WebLogic Server also monitors socket errors to determine the availability of a server instance. For example, if server instance A has an open socket to server instance B, and the socket unexpectedly closes, server A assumes that server B is offline.