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Two-Tier LVS Topology
To an outside user accessing a hosted service (such as a website or database application),
LVS appears as one server. However, the user is actually accessing real servers behind the
LVS routers.
Because there is no built-in component in LVS to share the data among real servers, you have
have two basic options:
• Synchronize the data across the real servers.
• 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 real servers allow large numbers of users to modify data,
such as an e-commerce website, adding a third layer is preferable.
There are many ways to synchronize data among real servers. For example, you can use shell
scripts to post updated web pages to the real servers simultaneously. Also, you can use
programs such as
to replicate changed data across all nodes at a set interval. However,
rsync
in environments where users frequently upload files or issue database transactions, using
scripts or the
command for data synchronization does not function optimally. Therefore,
rsync
for real servers with a high amount of uploads, database transactions, or similar traffic, a
three-tiered topology is more appropriate for data synchronization.
8.1. Two-Tier LVS Topology
Figure 1.20, "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 1.20, "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.
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