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Chapter 7 # H HA-aware applications Applications that are aware of the availability of nodes and resources. The resources can be other application components, communication end points, processes, or groups of resources. An application that is HA-aware is designed to recover from a failure. heartbeat An IP packet that is periodically multicast through each of the two physical interfaces of each peer node. When a heartbeat is detected through a physical interface, it indicates that the node is reachable and that the physical interface is alive. Heartbeats are multicast by the `nhprobed` daemon and detected by the `nhprobed` daemon. highly available services Services that run on the master node and vice-master node only. The Reliable Boot Service and Reliable NFS are highly available services. If the master node or one of these services on the master node fails, a failover occurs. See also distributed services. HME An Ethernet card driver. horizontal scalability The ability to add nodes to a cluster to increase the processing capacity of the system. host part The second part of an Internet address. The host part of an IP address identifies a node on a given network. See also netmask and network part. hot-swap The removal and replacement of a hardware component or board without shutting down the entire system.
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