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Data Replication and Data Sharing Between
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 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 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 addressed 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 1.1, "A Basic LVS Configuration"
is best for
serving data which does not change very frequently — such as static webpages — because the
individual real servers do not automatically sync data between each node.
1.1. Data Replication and Data Sharing Between Real Servers
Since there is no built-in component in LVS 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 preferred for servers that do not allow large numbers of users to upload or
change data on the real servers. If the configuration allows large numbers of users to modify
data, such as an e-commerce website, adding a third layer is preferable.
1.1.1. 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 to all of the servers simultaneously. Also, the system administrator can use
programs such as
to replicate changed data across all nodes at a set interval.
rsync
However, this type of data synchronization does not optimally function if the configuration is
overloaded with users constantly uploading files or issuing database transactions. For a
configuration with a high load, a three-tier topology is the ideal solution.
2. A Three-Tier LVS Configuration
Figure 1.2, "A Three-Tier LVS Configuration"
shows a typical three-tier LVS topology. In this
example, the active LVS router routes the requests from the Internet to the pool of real servers.
3
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