In this sense, high availability is similar to other computer system threats, such as
those related to computer security, that disrupt computer-based businesses. The solution, as
in those cases, is a combination of awareness, prevention, and a balance between cost and
risk. Risk should be measured in potential profit losses, and cost should include hardware,
software, personnel, and technical support.
This chapter focuses on affordable highly-available solutions for small Websites, where
reduced cost implies low hardware cost, no significant code development or recoding, and
little proprietary software dependencies. In particular, this chapter is specifically devoted
to explain mechanisms for automatic replication applicable to existing Web applications.
To do so, the chapter shows a quite simple architecture that may cover a gap among the
currently-existing highly-available solutions, providing an affordable true highly-available
solution. The architecture is basically made by integrating a set of inexpensive off-the-self
components, it does not require specialized hardware or software, and it does not need the
Website to be recoded (if it fits a specific class of Websites). However, it is a true highlyavailable
solution that could be installed over a cluster or span a set of physically-separated
servers.
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