"The ubiquitous Linux, Apache, MySQL, and PHP/Perl/Python (LAMP)
combination powers many interactive Web sites and projects. It's
not at all unusual for demand to exceed the capacity of a single
LAMP-powered server over time. You can take load off by moving your
database to a second server, but when demand exceeds a two-server
solution, it's time to think cluster.
"A LAMP cluster is not the Beowulf kind of cluster that uses
specialized message-passing software to tackle a
computation-intensive task. It does not cover high availability
features, such as automatic failover. Rather, it is a load-sharing
cluster that distributes Web requests among multiple Web and
database servers while appearing to be a single server..."