Red Hat ENTERPRISE LINUX AS 2.1 Installation Manual page 103

Section 6.1:A Basic LVS Configuration
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 router is to redirect service requests
from virtual IP addresses to the real servers. The redirection is based on one of eight supported load-
balancing algorithms described further in Section 6.3, LVS Scheduling Overview.
4 The active router also 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, the administrator can also call external executables. If a service
on a real server malfunctions, the active router stops sending jobs to that server until it returns to normal
operation.
The backup router performs the role of a hot-standby system. Periodically, the LVS routers exchange
"I'm alive" heartbeat messages through the primary external public interface and, in a failover sit-
uation, the private interface. Should the backup node fail to receive a heartbeat message within an
expected interval, it initiates a failover and assumes the role of the active router. During failover, the
backup 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 ad-
dressed to the failed node. When the failed node returns to active service, the backup node assumes
its hot-backup role again.
The simple, two-layered configuration used in Figure 6–1, A Basic LVS Configuration is best for clus-
ters serving data which does not change very frequently — such as static Web pages — because the
individual real servers do not automatically sync data between each node. The next section discusses
approaches to replicating data across the server pool.
6.1.1 Data Replication and Data Sharing Between Real
Servers
Since there is no built-in component in LVS clustering to share the same data between the real servers,
the administrator has two basic options:
• Synchronize the data across the real server pool.
• Add a third layer to the topology for shared data access.
The first option is better for servers which do not allow large numbers of users to upload or change data
on the real servers. If the cluster allows large numbers of users to modify data, such as an e-commerce
website, the latter option is preferable.
Configuring Real Servers to Synchronize Data
There are many ways an administrator can choose to synchronize data across the pool of real servers.
For instance, shell scripts can be employed so that if a Web engineer updates a page, the page is posted
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