High availability (HA) refers to the continuous operation of a system for higher-than-normal periods of time. While availability is objectively measured as a percentage of uptime, it’s also subjectively determined from the point of view of users – if they can’t access the system, it’s not available.
Today’s businesses typically strive for an environment that’s running 99.999% of the time. This is “five nines” availability and represents the service level agreement (SLA) to the organization.
But HA isn’t just about whether the system is up or down – it’s also about whether it’s performing well enough to be useful. After all, from a user’s perspective a system that’s running slowly is almost as bad as one that isn’t running at all.