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.
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