Data Replication And Data Sharing Between Real Servers; A Three-Tier Lvs Configuration - Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER ADMINISTRATION Manual

Data Replication and Data Sharing Between Real Servers

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.
Figure 1.1, "A Basic LVS Configuration"
The simple, two-layered configuration used in 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.
1.1.1. 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.
1.1.1.1. 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
rsync to replicate changed data across all nodes at a set interval.
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.

1.2. 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. Each of the real
servers then accesses a shared data source over the network.
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