LVS Cluster — A Block Diagram

Red Hat Cluster Suite: Configuring and Managing a Cluster
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# 10.6. LVS Cluster — A Block Diagram

LVS routers use a collection of programs to monitor cluster members and cluster services. Figure 10-4 illustrates how these various programs on both the active and backup LVS routers work together to manage the cluster.

Figure 10-4. Components of a Running LVS Cluster

The `pulse` daemon runs on both the active and passive LVS routers. On the backup router, `pulse` sends a heartbeat to the public interface of the active router to make sure the active router is still properly functioning. On the active 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 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 router does not receive a response from the active router, it initiates failover by calling `send_arp` to reassign all virtual IP addresses to the NIC hardware addresses (MAC address) of the backup node, sends a command to the active router via both the public and private network interfaces to shut down the `lvs` daemon on the active router, and starts the `lvs` daemon on the backup node to accept requests for the configured virtual servers.

## 10.6.1. Components of an LVS Cluster

Section 10.6.1.1 `pulse` shows a detailed list of each software component in an LVS router.

### 10.6.1.1. `pulse`

This is the controlling process which starts all other daemons related to LVS routers. At boot time, the daemon is started by the `/etc/rc.d/init.d/pulse` script. It then reads the configuration file `/etc/sysconfig/ha/lvs.cf`. On the active router, `pulse` starts the LVS daemon. On the backup router, `pulse` determines the health of the active router by executing a simple heartbeat at a user-configurable interval. If the active router fails to respond after a user-configurable interval, it initiates failover. During failover, `pulse` on the backup router instructs the `pulse` daemon on the active router to shut down all LVS services, starts the `send_arp` program to reassign the floating IP addresses to the backup router's MAC address, and starts the `lvs` daemon.

### 10.6.1.2. `lvs`

The `lvs` daemon runs on the active LVS router once called by `pulse`. It reads the configuration file `/etc/sysconfig/ha/lvs.cf`, calls the `ipvsadm` utility to build and maintain the IPVS routing table, and assigns a `nanny` process for each configured LVS service. If `nanny` reports a real server is down, `lvs` instructs the `ipvsadm` utility to remove the real server from the IPVS routing table.

### 10.6.1.3. `ipvsadm`

This service updates the IPVS routing table in the kernel. The `lvs` daemon sets up and administers an LVS cluster by calling `ipvsadm` to add, change, or delete entries in the IPVS routing table.

### 10.6.1.4. `nanny`

The `nanny` monitoring daemon runs on the active LVS router. Through this daemon, the active router determines the health of each real server and, optionally, monitors its workload. A separate process runs for each service defined on each real server.

### 10.6.1.5. `/etc/sysconfig/ha/lvs.cf`

This is the LVS cluster configuration file. Directly or indirectly, all daemons get their configuration information from this file.

### 10.6.1.6. Piranha Configuration Tool

This is the Web-based tool for monitoring, configuring, and administering an LVS cluster. This is the default tool to maintain the `/etc/sysconfig/ha/lvs.cf` LVS cluster configuration file.

### 10.6.1.7. `send_arp`

This program sends out ARP broadcasts when the floating IP address changes from one node to another during failover.

Chapter 11 Initial LVS Configuration reviews important post-installation configuration steps you should take before configuring Red Hat Enterprise Linux to be an LVS router.

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