What is High Availability (HA)?
In context of IT operations, the term High Availability refers to a system (a network, a server array or cluster, etc.) that is designed to avoid loss of service by reducing or managing failures and minimizing planned downtime.
A system is expected to be highly available when life, health, and well-being – including economic well-being – are at stake.
In information technology, system or component availability is expressed as a percentage of yearly uptime. Service Level Agreements (SLAs) generally refer these availability percentages in order to calculate billing. Using the unachievable ideal of 100% availability as a baseline, the goal of the highest levels of service availability is considered to be “five nines” – 99.999% availability.
High Availability Management
High availability can be achieved only with thorough planning and consistent monitoring.
A good starting point for high availability planning involves the identification of services that must be available for business continuity, and those that should be available.
For each level of service, from must to should, it is also worthwhile to decide how far the organization is willing to go to ensure availability. This should be based on budget, staff expertise, and overall tolerance for service outages.