What Is High-Availability?
Availability is the period of time when a service (an application or a system) is available. It is measured from user's perspective and, in this context, a user is anyone or anything interacting with that service. It's a simple question. Is the application available to users (almost) always?
Lack of availability can be caused by many things. The application might not be running for a while because it failed or it is being upgraded. Often, an application might be up-and-running, but not available to users. It might be saturated with too many requests, dependant applications might not be available, networking might not be working, and so on and so forth.
High-availability means that an application is available almost always. Today, highly-available applications are considered those that have at least a few nines after 99%. Anything below at least one nine after the decimal (99.9%), is NOT considered highly available by today's standards. That m