1.8.1. Two-Tier LVS Topology

1.8.1. Two-Tier LVS Topology

Figure 1.21, “Two-Tier LVS Topology” shows a simple LVS configuration consisting of two tiers: LVS routers and real servers. The LVS-router tier consists of one active LVS router and one backup LVS router. The real-server tier consists of real servers connected to the private network. Each LVS router has two network interfaces: one connected to a public network (Internet) and one connected to a private network. A network interface connected to each network allows the LVS routers to regulate traffic between clients on the public network and the real servers on the private network. In Figure 1.21, “Two-Tier LVS Topology”, the active LVS router uses Network Address Translation (NAT) to direct traffic from the public network to real servers on the private network, which in turn provide services as requested. The real servers pass all public traffic through the active LVS router. From the perspective of clients on the public network, the LVS router appears as one entity.

Two-Tier LVS Topology

Figure 1.21. Two-Tier LVS Topology

Service requests arriving at an LVS router are addressed to a virtual IP address or VIP. This is a publicly-routable address that the administrator of the site associates with a fully-qualified domain name, such as www.example.com, and which is assigned to one or more virtual servers[1]. Note that a VIP address migrates from one LVS router to the other during a failover, thus maintaining a presence at that IP address, also known as floating IP addresses.

VIP addresses may be aliased to the same device that connects the LVS router to the public network. For instance, if eth0 is connected to the Internet, then multiple virtual servers can be aliased to eth0:1. Alternatively, each virtual server can be associated with a separate device per service. For example, HTTP traffic can be handled on eth0:1, and FTP traffic can be handled on eth0:2.

Only one LVS router is active at a time. The role of the active LVS router is to redirect service requests from virtual IP addresses to the real servers. The redirection is based on one of eight load-balancing algorithms:

Also, the active LVS router dynamically monitors the overall health of the specific services on the real servers through simple send/expect scripts. To aid in detecting the health of services that require dynamic data, such as HTTPS or SSL, you can also call external executables. If a service on a real server malfunctions, the active LVS router stops sending jobs to that server until it returns to normal operation.

The backup LVS router performs the role of a standby system. Periodically, the LVS routers exchange heartbeat messages through the primary external public interface and, in a failover situation, the private interface. Should the backup LVS router fail to receive a heartbeat message within an expected interval, it initiates a failover and assumes the role of the active LVS router. During failover, the backup LVS router takes over the VIP addresses serviced by the failed router using a technique known as ARP spoofing — where the backup LVS router announces itself as the destination for IP packets addressed to the failed node. When the failed node returns to active service, the backup LVS router assumes its backup role again.

The simple, two-tier configuration in Figure 1.21, “Two-Tier LVS Topology” is suited best for clusters serving data that does not change very frequently — such as static web pages — because the individual real servers do not automatically synchronize data among themselves.

[1] A virtual server is a service configured to listen on a specific virtual IP.

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