Understanding the purpose of High Availability

To quote Murphy's law:

"Anything that can go wrong will go wrong."

"Anything" really includes everything in life. This is well understood by all service providers who intend to retain their customers. Customers usually aren't satisfied if the service they want is not continuous, or not available. Availability is also called uptime, and its opposite is called downtime.

Depending on the service, downtime can be more or less tolerated. For example, if a house is heated using wood or coal, the homeowner can stack up a lot of it before winter to avoid depending on the availability of shipping during the winter. However, if the house is heated using natural gas, availability is a lot more important. Uninterrupted service (there should be enough pressure in the gas pipe coming into the house) and a certain heating quality of the gas are expected from the provider.

The provider must minimize downtime as much as possible. If possible, downtime should be...