Red Hat ENTERPRISE LINUX 4 Manual page 36

Chapter 1. Red Hat Cluster Suite Overview
Figure 1.20. Components of a Running LVS Cluster
The pulse daemon runs on both the active and passive LVS routers. On the backup LVS router,
pulse sends a heartbeat to the public interface of the active router to make sure the active LVS router
is properly functioning. On the active LVS router, pulse starts the lvs daemon and responds to
heartbeat queries from the backup LVS router.
Once started, the lvs daemon calls the ipvsadm utility to configure and maintain the IPVS (IP Virtual
Server) routing table in the kernel and starts a nanny process for each configured virtual server
on each real server. Each nanny process checks the state of one configured service on one real
server, and tells the lvs daemon if the service on that real server is malfunctioning. If a malfunction is
detected, the lvs daemon instructs ipvsadm to remove that real server from the IPVS routing table.
If the backup LVS router does not receive a response from the active LVS router, it initiates failover by
calling send_arp to reassign all virtual IP addresses to the NIC hardware addresses (MAC address)
of the backup LVS router, sends a command to the active LVS router via both the public and private
network interfaces to shut down the lvs daemon on the active LVS router, and starts the lvs daemon
on the backup LVS router to accept requests for the configured virtual servers.
To an outside user accessing a hosted service (such as a website or database application), LVS
appears as one server. However, the user is actually accessing real servers behind the LVS routers.
Because there is no built-in component in LVS to share the data among real servers, you have have
two basic options:
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