"This tutorial shows how to set up a two-node Apache web server
cluster that provides high-availability. In front of the Apache
cluster we create a load balancer that splits up incoming requests
between the two Apache nodes. Because we do not want the load
balancer to become another 'Single Point Of Failure,' we must
provide high-availability for the load balancer, too. Therefore our
load balancer will in fact consist out of two load balancer nodes
that monitor each other using heartbeat, and if one load balancer
fails, the other takes over silently.
"The advantage of using a load balancer compared to using round
robin DNS is that it takes care of the load on the web server nodes
and tries to direct requests to the node with less load, and it
also takes care of connections/sessions. Many web applications
(e.g. forum software, shopping carts, etc.) make use of sessions,
and if you are in a session on Apache node 1, you would lose that
session if suddenly node 2 served your requests. In addition to
that, if one of the Apache nodes goes down, the load balancer
realizes that and directs all incoming requests to the remaining
node which would not be possible with round robin DNS..."
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