ELI5: What is High availability (HA)?
High availability means a system is almost never “closed.” Think of a hospital emergency room that stays open 24/7, 365 days a year. The goal is to keep services running with as close to zero downtime as possible.
Definition
High Availability (HA) is a system design approach aimed at ensuring a system remains operational for the maximum possible time, minimizing planned and unplanned downtime. HA is measured as a percentage of uptime, commonly expressed in “nines” — the number of nines in the availability percentage. Achieving high nines requires redundant components, automatic failover, and careful design to eliminate single points of failure.
Key Details
- 99.9% (“three nines”): 8.76 hours downtime per year
- 99.99% (“four nines”): 52.6 minutes downtime per year
- 99.999% (“five nines”): 5.26 minutes downtime per year
- HA requires redundant hardware, automatic failover, load balancing, and geographic distribution
- Recovery Time Objective (RTO) defines the maximum acceptable downtime; HA systems minimize RTO
Connections
- Parent: resilience-and-redundancy — high availability is the primary goal of resilient architecture design
- See also: failover